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april 2009 rev 3 1/47 47 led7707 6-rows 85 ma leds driver with boost regulator for lcd panels backlight features boost section ? 4.5 v to 36 v input voltage range ? internal power mosfet ? internal +5 v ldo for device supply ? up to 36 v output voltage ? constant frequency peak current-mode control ? 250 khz to 1 mhz adjustable switching frequency ? external synchronization for multi-device application ? pulse-skip power saving mode at light load ? programmable soft-start ? programmable ovp protection ? stable with ceramic output capacitors ? thermal shutdown backlight driver section ? six rows with 85 ma maximum current capability (adjustable) ? rows disable option ? less than 10 s minimum dimming on-time ? 2 % current matching between rows ? led failure (open and short-circuit) detection applications lcd monitors and tv panels pdas panel backlight gps panel backlight description the led7707 consists of a high efficiency monolithic boost converter and six controlled current generators (rows) specifically designed to supply leds arrays used in the backlighting of lcd panels. the device can manage an output voltage up to 36 v (i.e. 10 white leds per row). the generators can be externally programmed to sink up to 85 ma and can be dimmed via a pwm signal (1 % dimming duty-cycle at 1 khz can be managed). the device allows to detect and manage the open and shorted led faults and to let unused rows floating. basic protections (output over-voltage, internal mosfet over-current and thermal shutdown) are provided. vfqfpn-24 4x4 table 1. device summary order codes package packaging led7707 vfqfpn-24 4x4 (exposed pad) tu b e LED7707TR tape and reel www.st.com
contents led7707 2/47 contents 1 typical application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1 connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2 pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3 electrical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.1 maximum rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.2 thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4 electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5 operation description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.1 boost section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 5.1.1 functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 5.1.2 enable function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5.1.3 soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 5.1.4 over-voltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 5.1.5 switching frequency selection and synchronization . . . . . . . . . . . . . . . 14 5.1.6 slope compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 5.1.7 boost current limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.1.8 thermal protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.2 backlight driver section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 5.2.1 current generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 5.2.2 pwm dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 5.3 fault management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 5.3.1 fault pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 5.3.2 mode pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 5.3.3 open led fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 5.3.4 shorted led fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
led7707 contents 3/47 6 application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 6.1 system stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 6.1.1 loop compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 6.2 thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 6.3 component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 6.3.1 inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 6.3.2 capacitors selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 6.3.3 flywheel diode selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 6.4 design example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 6.4.1 switching frequency setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 6.4.2 row current setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 6.4.3 inductor choice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 6.4.4 output capacitor choice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 6.4.5 input capacitor choice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 6.4.6 over-voltage protection divider setting . . . . . . . . . . . . . . . . . . . . . . . . . 35 6.4.7 compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 6.4.8 boost current limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 6.4.9 power dissipation estimate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 6.5 layout consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 7 electrical characteristi cs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 8 package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 9 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
typical application circuit led7707 4/47 1 typical application circuit figure 1. application circuit row1 row2 row3 row4 row5 row6 ldo5 bilim rilim ss sgnd slope vin lx ovsel swf avcc fault en sync mode pgnd dim vin v out comp +5v up to 10 wleds per row enable dimming fault led7707 l mlcc sync output faults management selection ovp selection switching frequency selection slope compensation rows current selection internal mos ocp am00579v1
led7707 pin settings 5/47 2 pin settings 2.1 connections figure 2. pin connection (through top view) fault led7706 1 6 7 12 13 18 19 24 vin slope sgnd row1 row2 lx dim en sync ss row3 row4 row5 row6 pgnd ovsel comp rilim fsw bilim mode avcc ldo5
pin settings led7707 6/47 2.2 pin description table 2. pin functions n pin function 1comp error amplifier output. a simple rc series between this pin and ground is needed to compensate the loop of the boost regulator. 2rilim output generators current limit setting. the output current of the rows can be programmed connecting a resistor to sgnd. 3 bilim boost converter current limit setting. the internal mosfet current limit can be programmed connecting a resistor to sgnd. 4fsw switching frequency selection and external sync input. a resistor to sgnd is used to set the desired switching frequency. the pin can also be used as external synchroni zation input. see section 5.1.5 on page 14 for details. 5mode current generators fault management selector. it allows to detect and manage leds failures. see section 5.3.2 on page 22 for details. 6 avcc + 5 v analog supply. connect to ldo5 through a simple rc filter. 7ldo5 + 5 v ldo output and power section supply. bypass to sgnd with a 1 f ceramic capacitor. 8 vin input voltage. connect to the main supply rail. 9slope slope compensation setting. a resist or between the output of the boost converter and this pin is needed to avoid sub-harmonic instability. refer to section 6.1 on page 25 for details. 10 sgnd signal ground. supply return for the analog circuitry and the current generators. 11 row1 row driver output #1. 12 row2 row driver output #2. 13 row3 row driver output #3. 14 row4 row driver output #4. 15 row5 row driver output #5. 16 row6 row driver output #6. 17 pgnd power ground. source of the internal power mosfet. 18 ovsel over-voltage selection. used to set the desired 0 v threshold by an external divider. see section 5.1.4 on page 14 for details. 19 lx switching node. drain of the internal power mosfet. 20 dim dimming input. used to externally set the brightness by using a pwm signal. 21 en enable input. when low, the device is turned off. if tied high or left open, the device is turned on and a soft-start sequence takes place. 22 fault fault signal output. open drain output. the pin goes low when a fault condition is detected (see section 5.3.1 on page 22 for details). 23 sync synchronization output. used as external synchronization output. 24 ss soft-start. connect a capacitor to sgnd to set the desired soft-start duration.
led7707 electrical data 7/47 3 electrical data 3.1 maximum rating 3.2 thermal data table 3. absolute maximum ratings (1) 1. stresses beyond those listed under ?absolute maximum ratings? may cause permanent damage to the device. exposure to absolute maxi mum rated conditions for extended perio ds may affect device reliability. symbol parameter value unit v avcc avcc to sgnd -0.3 to 6 v v ldo5 ldo5 to sgnd -0.3 to 6 pgnd to sgnd -0.3 to 0.3 v in vin to pgnd -0.3 to 40 v lx lx to sgnd -0.3 to 40 lx to pgnd -0.3 to 40 rilim, bilim, sync, ovsel, ss to sgnd -0.3 to v avcc + 0.3 en, dim, sw, mode, fault to sgnd -0.3 to 6 rowx to pgnd/ sgnd -0.3 to 40 slope to vin v in - 0.3 to v in + 6 slope to sgnd -0.3 to 40 internal switch maximum rms current (flowing through lx node) 2.0 a p tot power dissipation @ t a = 25 c 2.3 (2) 2. power dissipation referred to the device mounted on the demonstration board des cribed in section 5.5 w maximum withstanding voltage range test condition: cdf-aec-q100-002- ?human body model? acceptance criteria: ?normal performance? 1000 v table 4. thermal data symbol parameter value unit r thja thermal resistance junction to ambient 42 c/w t stg storage temperature range -50 to 150 c t j junction operating temperature range -40 to 150 c
electrical characteristics led7707 8/47 4 electrical characteristics v in = 12 v; t j = 25 c and ldo5 connected to avcc if not otherwise specified (a) a. specification referred to t j from 0 c to +85 c. specific ation over the 0 to +85 c t j range are assured by design, characterization and statistical correlation. table 5. electrical characteristics symbol parameter test co ndition min. typ. max. unit supply section v in input voltage range 4.5 36 v v bst boost section output voltage 36 v v ldo5 ldo output and ic supply voltage en high i ldo5 = 0 ma 4.4 5 5.5 v v avcc i in,q operating quiescent current r rilim = 51 k , r bilim = 220 k , r slope = 680 k dim tied to sgnd. 1ma i in,shdn operating current in shutdown en low 20 30 a v uvlo,on ldo5 under voltage lock out upper threshold 4.0 4.3 v v uvlo,off ldo5 under voltage lock out lower threshold 3.5 3.7 ldo linear regulator line regulation 6 v v in 36 v, i ldo5 = 30 ma 30 mv ldo dropout voltage i ldo5 = 10 ma (-10 % drop) 80 120 ldo maximum output current v ldo5 > v uvlo,on 25 40 60 ma v ldo5 < v uvlo,off 20 30 boost section t on,min minimum switching on-time 200 ns f sw default switching frequency fsw connected to avcc 570 660 770 khz minimum fsw sync frequency 220 fsw sync input low level 240 mv fsw sync input high level 350 fsw sync input hysteresis 30 fsw sync min on time 270 % sync output duty-cycle fsw connected to avcc (internal oscillator selected) 34 40 % sync output high level i sync = 10 a v avcc -20v mv sync output low level i sync = -10 a 20
led7707 electrical characteristics 9/47 symbol parameter test co ndition min. typ. max. unit power switch k b lx current coefficient r bilim = 600 k 1 ? 10 6 1.2 ? 10 6 1.4 ? 10 6 v r ds(on) internal mosfet on-resistance 280 500 m oc and ov protections v th,ovp over-voltage protection reference threshold 1.145 v soft-start and power management en, turn-on threshold 1.6 v en, turn-off threshold 0.8 dim, high level threshold 1.3 dim, low level threshold 0.8 en, pull-up current 2.5 a ss, charge current 4 5 6 ss, end-of-startup threshold 1.8 2.4 2.6 v ss, reduced switching frequency release threshold 0.8 current generators section k r current generators gain 1850 v k r (1) current generators gain accuracy 2.0 % v ifb feedback regulation voltage 700 750 mv v rowx, fault led short circuit detection threshold mode tied to sgnd 4.0 v v fault, low fault pin low-level voltage i fault,sink = 4 ma 250 380 mv thermal shutdown t shdn thermal shutdown turn-off temperature 150 c thermal shutdown hysteresis 30 1. i row = k r / r rilim , i row /i row k r /k r + r rilim /r rilim table 5. electrical characteristics (continued)
operation description led7707 10/47 5 operation description the device can be divided into two sections: the boost section and the backlight driver section. these sections are described in the next paragraphs. figure 3 provides an overview of the internal blocks of the device. figure 3. simplified block diagram ldo5 bilim comp sync fsw rilim en fault row4 pgnd lx row6 dim vin row1 slope avcc row3 control logic row2 + _ thermal shutdown row5 ovsel + _ sgnd ext sync detector osc ovp boost_en 1.2v 4v mode boost_en 0.7v min voltage selector uvlo uvlo detector +5v ldo ramp generator current sense 1.172v soft start ovp ctrl1 ctrl6 v row1 v row2 v row5 v row4 v row3 v row6 ctrl2 v th,flt ctrl5 ctrl3 ctrl4 ctrl2 uvlo ctrl3 ctrl4 ctrl5 ctrl6 current generator 2 current generator 3 current generator 4 current generator 5 current generator 6 current generator 1 current limit + + logic boost control logic zcd + _ g m + _ i to v i to v ss mode 2 prot_en prot_en ldo5 bilim comp sync fsw rilim en fault row4 pgnd lx row6 dim vin row1 slope avcc row3 control logic row2 + _ thermal shutdown row5 ovsel + _ + _ sgnd ext sync detector osc ovp boost_en 1.2v 4v mode boost_en 0.7v min voltage selector uvlo uvlo detector +5v ldo ramp generator current sense 1.172v soft start ovp ctrl1 ctrl6 v row1 v row2 v row5 v row4 v row3 v row6 ctrl2 v th,flt ctrl5 ctrl3 ctrl4 ctrl2 uvlo ctrl3 ctrl4 ctrl5 ctrl6 current generator 2 current generator 3 current generator 4 current generator 5 current generator 6 current generator 1 current limit + + logic boost control logic zcd + _ g m + _ i to v i to v ss mode 2 prot_en prot_en
led7707 operation description 11/47 5.1 boost section 5.1.1 functional description the led7707 is a monolithic leds driver for the backlight of lcd panels and it consists of a boost converter and six pwm-dimmable current generators. the boost section is based on a constant switching frequency, peak current-mode architecture. the boost output voltage is controlled such that the lowest row's voltage, referred to sgnd, is equal to an internal reference voltage (700 mv typ.). the input voltage range is from 4.5 v up to 36 v. in addition, the led7707 has an internal ldo that supplies the internal circuitry of the device and is capable to deliver up to 40 ma. the input of the ldo is the vin pin. the ldo5 pin is the ldo output and the supply for the power mosfet driver at the same time. the avcc pin is the supply for the anal og circuitry and should be connected to the ldo output through a simple rc filter in order to improve the noise rejection. figure 4. avcc filtering two loops are involved in regulating the current sunk by the generators. the main loop is related to the boost regulator and uses a constant frequency peak current- mode architecture to regulate the power rail that supplies the leds ( figure 5 ), while an internal current loop regulates the same current (flowing through the leds) at each row according to the set value (rilim pin). figure 5. main loop and current loop diagram v in 0.7v pwm minimum voltage drop selector comp lx rowx sgnd rilim e/a slope error amplifier am00582v1
operation description led7707 12/47 a dedicated circuit automatically selects the lowest voltage drop among all the rows and provides this voltage to the main loop that, in turn, regulates the output voltage. in fact, once the reference generator has been detected, the error amplifier compares its voltage drop to the internal reference voltage and varies the comp output. the voltage at the comp pin determines the inductor peak current at each switching cycle. the output voltage of the boost regulator is thus determined by the total forward voltage of the leds strings (see figure 6 ): equation 1 where the first term represents the highest total forward voltage drop over n active rows and the second is the voltage drop across the leading generator (700 mv typ.). the device continues to monitor the voltage drop across all the rows and automatically switches to the current generator having the lowest voltage drop. figure 6. calculation of the output voltage of the boost regulator 5.1.2 enable function the led7707 is enabled by the en pin. this pin is active high and, when forced to sgnd, the device is turned off. this pin is connected to a permanently active 2.5 a current source; when sudden device turn-on at power-up is required, this pin must be left floating or connected to a delay capacitor. starting from an on state, when the led7707 is turned off, it quickly discharges the soft-start capacitor and turns off the power-mosfet, the current generators and the ldo. the power consumption is thus reduced to 20 a only. in applications where the dimming signal is used to turn on and off the device, the en pin can be connected to the dim pin as shown in figure 7 . mv 700 ) v ( max v j , f m 1 j n 1 i bst leds rows + = = = 700 mv i led row with the highest voltage drop across leds v boost v in max v f leading generator boost controller current generators section am00583v1
led7707 operation description 13/47 figure 7. external sync waveforms 5.1.3 soft-start the soft-start function is requ ired to perform a correct start- up of the system, controlling the inrush current required to charge the output capa citor and to avoid output voltage overshoot. the soft-start duration is set connecting an external capacitor between the ss pin and ground. this capacitor is charged with a 5 a (typ.) constant current, forcing the voltage on the ss pin to ramp up. when this voltage increases from zero to nearly 1.2 v, the current limit of the power mosfet is proportionally released from zero to its final value. however, because of the limited minimum on-time of the sw itching section, the in ductor might saturate due to current runaway. to solve this problem the switching frequency is reduced to one half of the nominal value at the beginning of the soft-start phase. the nominal switching frequency is restored after the ss pin voltage has crossed 0.8 v. figure 8. soft-start sequence waveforms in case of floating rows during the soft-start phase the floating rows detection is also performed. in presence of one or more floating rows, the voltage across the involved current generator drops to zero. this voltage becomes the inverting input of the error amplifier through the minimum voltage drop selector (see figure 5 ). as a consequence the error amplifier is unbalanced and the loop dim en sgnd led7707 220k 100n bas69 am00584v1 am00585v1 avcc t ss ss pin voltage t protections turn active 1.2v 0.8v current limit en pin voltage 100% output voltage ovp 95% of ovp floating rows detection 2.4v nominal switching frequency release avcc t ss ss pin voltage t protections turn active 1.2v 0.8v current limit en pin voltage 100% output voltage ovp 95% of ovp floating rows detection 2.4v nominal switching frequency release
operation description led7707 14/47 reacts by increasing the output voltage. when it reaches the floating row detection (frd) threshold (which coincides with the ovp threshold, see section 5.1.4 ), the floating rows are managed according to ta b l e 6 (see section 5.3 on page 21 ). after the ss voltage reaches a 2.4 v threshold, the start-up finishes and all the protections turn active. the soft-start capacitor c ss can be calculated according to equation 2. equation 2 where i ss = 5 a and t ss is the desired soft-start duration. 5.1.4 over-voltage protection an adjustable over-voltage prot ection is available. it can be set feeding the ovsel pin with a partition of the output voltage. the voltage of the central tap of the divider is thus compared to a fixed 1.145 v threshold. when the voltag e of the ovsel pin exceeds the ov threshold, the switching activity is suspend ed. it is resumed as ovsel returns below the ov threshold. a 10 mv hysteresis is provided. no device turn-off is performed. normally, the value of the high-side resistors of the divider is in the order of 100 k to reduce the output capacitor discharge when the boost converter is off (d uring the off phase of the dimming cycle), whereas the low-side resistor can be calculated as: equation 3 an additional filtering capacitor c f (typically in the 100 pf-330 pf range) may be required to improve noise rejection at the ovsel pin (see figure 9 ). figure 9. ovp threshold setting 5.1.5 switching frequency sel ection and synchronization the switching frequency of the boost converter can be set in the 250 khz-1 mhz range by connecting the fsw pin to ground through a resistor. calculation of the setting resistor is made using equation 4 and should not exceed the 100 k -400 k range. 4 . 2 t i c ss ss ss ? ? v 145 . 1 v 4 v v 145 . 1 r r max , out 1 2 ? + ? = am00586v1 am00586v1 lx ovsel sgnd led7707 v out v in r 1 r 2 c f c out
led7707 operation description 15/47 equation 4 in addition, when the fsw pin is tied to av cc, the led7707 uses a default 660 khz fixed switching frequency, allowing to save a resistor in minimum component-count applications. figure 10. multiple device synchronization the fsw pin can also be used as synchronizat ion input, allowing the led7707 to operate both as master or slave device. if a clock signal with a 220 khz minimum frequency is applied to this pin, the device locks synchroni zed. the signal provided to the fsw pin must cross the 270 mv threshold in order to be recognized. the minimum pulse width which allows the synchronizing pulses to be detecte d is 270 ns. an internal time-out allows synchronization as long as the external clock frequency is greater than 220 khz. keeping the fsw pin voltage lower than 270 mv for more than 4.5 s results in a stop of the device switching activity. normal operation is resumed as soon as fsw rises above the mentioned threshold and the soft-start sequence is repeated. the sync pin is a synchronization output and provides a 35 % (typ.) duty-cycle clock when the led7707 is used as master or a replica of the fsw pin when used as slave. it is used to connect multiple devices in a daisy-chain conf iguration or to synchronize other switching converters running in the system with the led7707 (master operation). when an external synchronization clock is applied to the fsw pin, the internal oscillator is over-driven: each switching cycle begins at the rising edge of clock, while the slope compensation ( figure 11 ) ramp starts at the falling edge of the same signal. thus, to prevent sub-harm onic instability (see section 5.1.6 ), the external synchronization clock is required to have a 40 % maximum duty-cycle when the boost converter is work ing in continuous-conduction mode (ccm) in order to assure that the slope compensation is effective (starts with duty-cycle lower than 40%) 5 . 2 f r sw fsw = sync sgnd led7707 avcc r fsw fsw sync sgnd led7707 slave fsw master sync out sync am00587v1
operation description led7707 16/47 figure 11. external sync waveforms 5.1.6 slope compensation the constant frequency, peak current-mode topology has the advantage of very easy loop compensation with output ceramic caps (reduced cost and size of the application) and fast transient response. in addition, the intrin sic peak-current measurement simplifies the current limit protection, avoiding un desired saturation of the inductor. on the other side, this topolo gy has a drawback: there is an inherent open loop instability when operating with a duty-ratio greater than 0.5. this phenomenon is known as ?sub- harmonic instability? and can be avoided by adding an external ramp to the one coming from the sensed current. this compensating technique, based on the additional ramp, is called ?slope compensation?. in figure 12 , where the switching duty-cycle is higher than 0.5, the small perturbation i l dies away in subsequent cycles thanks to the slope compensation and the system reverts to a stable situation. the slope pin allows to properly set the amount of slope compensation connecting a simple resistor r slope between the slope pin and the output. the compensation ramp starts at 35% (typ.) of each switching period and its slope is given by the following equation: equation 5 where k s = 5.8 ? 10 10 s -1 , v be = 2 v (typ.) and s e is the slope ramp in [a/s]. to avoid sub-harmonic instability, the compen sating slope should be at least half the slope of the inductor current during the off-phase when the duty-cycle is greater than 50%. the value of r slope can be calculated according to equation 6. slave sync pin voltage 270mv threshold fsw pin voltage (ext. sync) slave lx pin voltage 270ns minimum am00588v1 ? ? ? ? ? ? ? ? ? ? = slope be in out s e r v v v k s
led7707 operation description 17/47 equation 6 figure 12. effect of slope compensation on small inductor current perturbation (d > 0.5) ) v v ( ) v v v ( l k 2 r in out be in out s slope ? ? ? ? ? ? am00589v1 t sw programmed inductor peak current with slope compensation (s e ) inductor current (ccm) t i boost, peak 0.35 t sw i l inductor current perturbation t sw programmed inductor peak current with slope compensation (s e ) inductor current (ccm) t i boost, peak 0.35 t sw i l inductor current perturbation
operation description led7707 18/47 5.1.7 boost current limit the design of the external components, especially the inductor and the flywheel diode, must be optimized in terms of size relying on the programmable peak current limit. the led7707 improves the reliability of the final application giving the way to limit the maximum current flowing into the critical components. a simple resistor connected between the bilim pin and ground sets the desired value. the voltage at the bilim pin is internally fixed to 1.23 v and the current limit is proportional to the curren t flowing through the se tting resistor, according to the following equation: equation 7 where the maximum allowed cu rrent limit is 5 a, resulting in a minimum setting resistor r bilim > 240 k . the maximum guaranteed rms curr ent in the power switch is 2 a. in a boost converter the rms current through the internal mosfet depends on both the input and output voltages, according to equations 8a (dcm) and 8b (ccm). the current limitation works by clamping the comp pin voltage proportionally to r bilim . peak inductor current is limited to th e above threshold decreased by the slope compensation contribution. equation 8 a equation 8 b 5.1.8 thermal protection in order to avoid damage due to high junction te mperature, a thermal shutdown protection is implemented. when the junction temperature rises above 150 c (typ.), the device turns off both the control logic and the boost converter and holds the fault pin low. the ldo is kept alive and normal operation is automatically resumed after the junction temperature has been reduced by 30 c. bilim b peak , boost r k i = k b 1.2 10 6 v ? = 3 d l f d v i sw in rms , mos ? ? = () () () ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? + ? = 3 2 sw out out 2 out rms , mos d 1 d l f i v 12 1 d 1 d i i
led7707 operation description 19/47 5.2 backlight driver section 5.2.1 current generators the led7707 is a leds driver with six channels (rows); each row is able to drive multiple leds in series (max. 36 v) and to sink up to 85 ma maximum current, allowing to manage different kinds of leds. the leds current can be set by connecting an external resistor (r rilim ) between the rilim pin and ground. the voltage across the rilim pin is internally set to 1.23 v and the rows current is proportional to the rilim current according to the following equation: equation 9 where k r = 1850 v. the graph in figure 13 better shows the relationship between i row and r rilim and helps to choose the correct value of the resistor to set the desired row current. figure 13. row current vs r rilim the maximum current mismatch between the rows is 2 % @ i rowx = 60 ma. the led7707 allows parallelism different rows if required by the application. if the maximum current provided by a single row (85 ma) is not enough for the load, two or more current generators can be connected together, as shown in figure 14 . to keep the parallelism generators stable, the row current should be higher than 40 ma.the connection between channels in parallel must be done as close as possible to the device in order to minimize parasitic inductance. rilim r rowx r k i = am00590v1
operation description led7707 20/47 figure 14. rows parallelism for higher current row0 row1 row2 row3 row4 row5 vcc bilim rilim ss sgnd slope vin lx ovsel swf avcc fault en sync mode pgnd dim v in comp high current wleds enable dimming fault led7707 sync output faults management selection am00591v1
led7707 operation description 21/47 5.2.2 pwm dimming the brightness control of the leds is perfor med by a pulse-width modulation of the rows current. when a pwm signal is applied to the dim pin, the current generators are turned on and off mirroring the dim pin behavior. actually, the minimum dimming duty-cycle depends on the dimming frequency. the real limit to the pwm dimming is the minimum on-time that can be managed for the current generators; this minimum on-time is approximately 10 s. thus, the minimum dimming duty-cycle depends on the dimming frequency according to the following formula: equation 10 for example, at a dimming frequency of 1 khz, 1% of dimming duty-cycle can be managed. during the off-phase of the pwm signal the boost converter is paused and the current generators are turned off. the output voltage can be considered almost constant because of the relatively slow discharge of the output capacitor. during the start-up sequence (see section 5.1.3 on page 13 ) the dimming duty-cycle is forced to 100% to detect floating rows regardless of the applied dimming signal. figure 15. pwm dimming waveforms 5.3 fault management the main loop keeps the row having the lowest voltage drop regulated to about 700 mv. this value slightly depends on the voltage acro ss the remaining active rows. after the soft- start sequence, all protections turn active and the voltage across the active current generators is monitored to detect shorted leds. dim min , dim f s 10 d ? = ? 10s minimum on-time 10s minimum on-time
operation description led7707 22/47 5.3.1 fault pin the fault pin is an open-collect or output, (with 4 ma current capability) active low, which gives information regarding faulty conditions eventually detected. this pin can be used either to drive a status led or to warn the host system. the fault pin status is strictly related to the mode pin setting (see ta bl e 6 for details). 5.3.2 mode pin the mode pin is a digital input and can be connected to avcc or sgnd in order to choose the desired fault detection and management. the led7707 can manage a faulty condition in two different ways, according to the application needs. ta bl e 6 summarizes how the device detects and handles the internal prot ections related to the boost section (over- current, over-temperature and over-voltage) and to the current generators section (open and shorted leds). 5.3.3 open led fault in case a row is not connected or a led fails open, the device has two different behaviors according to the mode pin status. if the mode pin is high (i.e. connected to avcc), the fault pin is set high as soon as the device recognizes the event; the open row is excluded from the control loop and the device continues to work properly with the remaining rows. thus, if less than six rows are used in th e application, the mode pin must be set high. connecting the mode pin to sgnd, the led7707 behaves in a different manner: as soon as an open row is detected the fault pin is tied low and the device is turned off. the internal logic latches this status: to restore the normal operation, the device must be restarted by toggling the en pin or performi ng a power-on reset (por occurs when the voltage at the ldo5 pin falls below the lower uvlo threshold and subsequently rises above the upper one). table 6. faults management summary fault mode to gnd mode to vcc internal mosfet over-current fault pin high power mos turned off output over-voltage fault pin low device turned off, latched condition thermal shutdown fault pin low. device turned off. automatic restart after 30 c temperature drop. led short circuit fault pin low, device turned off (100s masking time), latched condition (vth = 4.0 v) - open row(s) fault pin low device turned off at first occurrence, latched condition fault pin high faulty row(s) disconnected.
led7707 operation description 23/47 figure 16 shows an example of open channel detection in case of mode connected to avcc. at the point marked as ?1? in figure 16 , the row opens (row current drops to zero). from this point on the output voltage is increased as l ong as the output voltage reaches the floating row detection threshold (see section 5.1.3 on page 13 ). then (point marked as ?2?) the faulty row is disconnected and the device keeps on working only with the remaining rows. figure 16. open channel detection (mode to avcc) 5.3.4 shorted led fault when a led is shorted, the voltage across th e related current generator increases of an amount equal to the missing voltage drop of the faulty led. since the feedback voltage on each active generator is constantly compared with a fault threshold v th,fault , the device detects the faulty condition and acts according to the mode pin status. a 100 s masking time is introduced to support esd capacitors eventually connected across the leds strings. if the mode pin is low, the fault threshold is v th,fault = 4.0 v. when the voltage across a row is higher than this threshold for more than 100 s, the fault pin is set low and the device is turned off. the internal logic latches this status until the en pin is toggled or a por is performed. in case the mode pin is connected to avcc, the led short-circuit protection is disabled. the led7707 simply keeps on regulating the se t current without affecting the fault pin. despite the higher power dissipation, this option is useful to avoid undesired triggering of the shorted-led protection simply due to t he high voltage drop spread across the leds. figure 17 shows an example of shorted led detect ion in case mode is connected to gnd. ? 1 2 1 1 2 2
operation description led7707 24/47 at the point marked as ?1? in figure 17 one led fails becoming a short-circuit. the voltage across the current generator of the channel where the failed led is connected increases by an amount equal to the forward voltage of the faulty led. since the voltage across the current generator is above the threshold (4 v), the device is turned off and the fault pin is set low (point ?2?). note that, once a new dimming cycle starts (point ?3?), the device waits the masking time (approximately 100 s) and then sets the fault pin low and turns off. figure 17. shorted led detection (mode to gnd) ? 1 2 masking time 3 1 1 2 2 masking time 3 3
led7707 application information 25/47 6 application information 6.1 system stability the boost section of the led7707 is a fixe d frequency, current-mode converter. during normal operation, a minimum voltage selection circuit compares all the voltage drops across the active current generators and provides the minimum one to the error amplifier. the output voltage of the error amplifier determines the inductor peak current in order to keep its inverting input equal to the reference voltage (700 mv typ). the compensation network consists of a simple rc series (r comp - c comp ) between the comp pin and ground. the calculation of r comp and c comp is fundamental to achieve optimal loop stability and dynamic performance of the boost converter and is strictly related to the operating conditions. 6.1.1 loop compensation the compensation network can be quickly calculated using equations 11 to 16. once both r comp and c comp have been determined, a fine-tuning phase may be required in order to get the optimal dynamic performance from the application. the first parameter to be fixed is the switching frequency. normally, a high switching frequency allows reducing the size of the inductor and positively affects the dynamic response of the converter (wider bandwidth) bu t increases the switching losses. for most of applications, the fixed value (660 khz) represents a good trad e-off between power dissipation and dynamic response, allowing to save an external resistor at the same time. in low-profile applications, the inductor value is often kept low to reduce the number of turns; an inductor value in the 4.7 h-15 h range is a good starting choice. in order to avoid instability du e to interaction between the dc -dc converter's loop and the current generators' loop, the bandwidth of the boost should not exceed the bandwidth of the current generators. a unity-gain frequency (f u ) in the order of 30-40 khz is acceptable. also, take care not to exceed the ccm- mode right half-plane zero (rhpz). equation 11 equation 12 equation 13 a sw u f 2 . 0 f ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? = ? ? out min , in v v m =
application information led7707 26/47 equation 13b where v in,min is the minimum input voltage and i out is the overall output current. note that, the lower the inductor value (and the higher the switching frequency), the higher the bandwidth can be achieved. the output capacit or is directly involved in the loop of the boost converter and must be large enough to avoid excessive output voltage drop in case of a sudden line transition from the maximum to the minimum input voltages. however a more significant requirement concerns the output voltage ripple. the output capacitor should be chosen in accordance with the following expression: equation 14 where v out, max is the maximum acceptable output voltage ripple, i l, peak is the peak inductor current, t off is the off-time of the switching cyc le (for an extensive explanation see section 6.4.4 on page 33 ). once the output capacitor has been chosen, the r comp can be calculated as: equation 15 where gm = 2.7 s and g ea = 375 s equation 15 places the loop bandwidth at f u . then, the c comp capacitor is determined to place the frequency of the compensation zero 5 times lower than the loop bandwidth: equation 16 where f z = f u /5. in most of the applications an experimental approach is also very valid to compensate the circuit. a simple technique to optimize different applications is to choose c comp = 4.7 nf and to replace r comp with a 10 k trimmer adjusting its value to properly damp the output transient response. insufficient damping will re sult in excessive ringing at the output and poor phase margin. figure 18 (a and b) give an example of compensation adjustment for a typical application. out out i v r = ( ) max , out off out peak , l out v 2 t i i c ? ? ? > m g g c f 2 r ea m u comp ? ? ? ? = comp z comp r f 2 1 c ? ? =
led7707 application information 27/47 figure 18. poor phase margin (a) and properly damped (b) load transient responses figure 19. load transient response measurement set-up a) b) row1 row2 row3 row4 row5 row6 vcc bilim rilim ss sgnd slope vin lx ovsel fsw avcc fault en sync mode pgnd dim v in = 12v v boost in c comp +5v up to 10 wleds per row led7707 6.8 h 2 x 4.7 f mlcc 100ma 500hz r l = v bst am00592v1
application information led7707 28/47 6.2 thermal considerations in order to prevent the device from exceeding t he thermal shutdown thres hold (150 c), it is important to estimate the junction te mperature through the following equation: equation 17 where t a is the ambient temperature, r th,ja is the equivalent thermal resistance junction to ambient and p d,tot is the power dissipated by the device. the r th,ja measured on the application de monstration board (described in section 6.5 ) is 42 c/w. the p d,tot has several contributions, listed below. a) conduction losses due to the r ds(on) of the internal power switch, equal to: equation 18 where d is defined as: equation 19 and d dim is the duty cycle of the pwm dimming signal. b) switching losses due to the power mosfet turn on and off, calculated as: equation 20 where t r and t f are the power mosfet rise time and fall time respectively. c) current generators losses. this contribution is strictly related to the leds used in the application. only the contribution of the leading current generator (?master? current generator) can be pr edicted, regardless of the leds forward voltage: equation 21 where i row is the current flowing th rough the row, whereas v ifb is the voltage across the master current generator (typically 700 mv). the voltages across the other current generato rs depend on the spread of the leds forward voltage. the worst case for power dissipation (maximum forward voltage leds in the master row, minimum forward voltage leds in all other rows) can be estimated as: tot , d ja , th a j p r t t ? + = ? ? ? = out in v v 1 d ? = dim f r sw in out sw , d d 2 ) t t ( f i v p ? + ? ? ? = dim ifb row master , gen d v i p ? ? =
led7707 application information 29/47 equation 22 where n rows is the number of active rows, v f,leds is the spread of the leds forward voltage and n leds is the number of leds per row. d) ldo losses, due to the dissipation of the 5 v linear regulator: equation 23 the led7707 is housed in a 24 leads 4x4-vfqfpn package with exposed pad that allows good thermal performance. however it is also important to design properly the demonstration board layout in order to assure correct heat dissipation. figure 20 shows a picture of the led7707 application demonstration board taken using an infrared camera. the chip temperature, in those application conditions, is kept below 50 c. figure 20. demonstration board thermographic analysis ( ) ( ) dim leds leds , f ifb rows row gen d n v v 1 n i p ? ? + ? ? ? = ( ) ldo ldo in ldo , d i v v p ? ? = ? 64c 50c v in = 12v i row = 60ma v out = 30v f sw = 660khz d dim = 100% t amb = 25c 64c 50c v in = 12v i row = 60ma v out = 30v f sw = 660khz d dim = 100% t amb = 25c
application information led7707 30/47 6.3 component selection 6.3.1 inductor selection being the led7707 mostly dedicated to backlight ing, real-estate applications dictate severe constrain in selecting the optimal inductor. the inductor choice must take into account different parameters like conduc tion losses (dcr), core losses (ferrite or iron-powder), saturation current and magnetic-flux shielding (core shape and technology). the switching frequency of the led7707 can be set in the 200 khz-1 mhz range, allowing a wide selecting room for the inductance value. low switching frequencies takes to high inductance value, resulting in significant dcr and size. on the other hand, high switching frequencies result in si gnificant core losses. the suggested range is 4.7-22 h, even if the best trade off between the different loss contributions varies from manufacturer to manufacturer. a 6.8 h inductor has been experimentally fo und as the most suitable for applications running at a 660 khz switching frequency. 6.3.2 capacitors selection the input and output capacitors should have ve ry low esr (ceramic capacitors) in order to minimize the ripple voltage. the boost converter of the led7707 has been designed to support ceramic capacitors. the required capacitance depends on the programmed led current and the minimum dimming frequency (the boost converter is off when the dim pin is low and the output capacitor is slowly discharged). considering the worst case (i.e. 200 hz dimming frequency and 85 ma/channel), two 4.4 f mlccs are suitable for almost all applications. particular care must be taken when selecting the rated voltage and the dielectric type of the outp ut capacitors: 50 v rated mlcc may show a significant capacitance drop when biased, especially in case of y5v dielectric. as in most of boost converters, the input capacito r is less critical, altho ugh it is necessary to reduce the switching noise on the supply rail. the input capacitor is also important for the internal ldo of the led7707 and must be kept as close as possible to the chip. the rated voltage of the input capacitor can be chosen ac cording to the supply voltage range; a 10 f x5r mlcc is recommended. table 7. recommended inductors manufacturer part number description size coilcraft lps6235-682mlc 6.8 h , 75 m , 2.7 a 6x6 mm coilcraft xpl7030-682ml 6.8 h, 60 m , 5.8 a 7x7 mm wurth 7440650068 6.8 h, 33 m , 3.6 a 10x10 mm table 8. recommended capacitors manufacturer part number description package notes taiyo yuden umk325bj106km-t ceramic, 35v, x5r, 20 % smd 1210 c in murata grm31cr71h225ka88b ceramic, 50v, x7r, 20 % smd 1206 c out
led7707 application information 31/47 6.3.3 flywheel diode selection the flywheel diode must be a schottky type to minimize the losses. this component is subject to an average current equal to the ou tput one and must sustain a reverse voltage equal to the maximum output rail voltage. considering all the channels sinking 75 ma each (i.e. 450 ma output current) and the maximum output voltage (36 v), the stp1l40m (i f,ave = 1 a, v r = 40 v) diode is a good choice. smaller diodes can be used in applications involving lower output voltage and/or lower output current. 6.4 design example in order to help the design of an application using the led7707, in this section a simple step-by-step design example is provided. a possible application could be the led backlight in a 17? lcd panel using the led7707. here below the possible application conditions are listed: v in = 12 10 % 4 strings of 42 white leds (60 ma) each (arranged in 6 rows, 7leds per row) v f, l e d s = 3.5 v 200 mv 6.4.1 switching frequency setting to reduce the number of the external components, the default switching frequency is selected (660 khz typ.) by connecting the fsw pin to avcc pin. however, in case a different switching frequency is required, a resistor from fsw pin and ground can be connected, according to the equation (5) in section 4.1.5. 6.4.2 row current setting considering the equation 9 in section 5.2.1 , the r rilim resistor can be calculated as: equation 24 the closest standard commercial value is 30 k . the actual value of the row current will be a little lower (61.7 ma). 6.4.3 inductor choice the boost section, as all dc-dc converters , can work in ccm (continuous conduction mode) or in dcm (discontinuous conduction mode) depending on load current, input and output voltage and other parameter s, among which the inductor value. in a boost converter it is usually preferable to work in dcm. once the load, the input and output voltage, and the switching frequency are fixed, the inductor value defining the boundary between dcm and ccm operation can be calculated as: = = = k 83 . 30 ma 60 v 1850 i k r row r rilim
application information led7707 32/47 equation 25 where d is the duty-cycle defined as: equation 26 whereas r 0 is: equation 27 and equation 28 the output voltage in the above calculations is considered as the maximum value (led with the maximum forward voltage connected to the leading generator): equation 29 considering the input voltage range, the lower l b will be at the lower input voltage. hence the condition to assure the dcm operation becomes: equation 30 an inductor value of 4.7 h could be a suitable value, considering also a margin from the boundary condition. it is important to highlight that the inductor choice involves not only the value itself but the saturation current (higher than the current limit, see section 6.4.4 ), the rated rms current (the compliance with the saturation current might be not enough; also the thermal performances must be taken in to account), the dcr (which affects the efficiency) and the size (in some application might be a strict requirement). however the dcr can?t be reduced keeping the size small. hence a trade off between these two requirements must be achieved according to the application. ( ) sw 2 0 b f 2 d 1 d r l ? ? ? ? = ? ? ? = = = ? = v 2 . 13 v @ 50 . 0 v 8 . 10 v @ 59 . 0 v v 1 d max , in min , in out in = = 74 i v r out out 0 ma 360 i 6 i row out = ? = = + ? = ( ) = <
led7707 application information 33/47 6.4.4 output capacitor choice the choice of the output capacitor is mainly affected by the desired output voltage ripple. since the voltage across the leds can be considered almost constant, this ripple is transferred across the current generators, affecting their dynamic response. the output ripple can be estimated as (neglecting the contribution of esr of c out , very low in case of mlcc): equation 31 where i l, peak is the inductor peak current (see figure 21 ) calculated as: equation 32 whereas d, working in dcm, is: equation 33 defining m as: equation 34 figure 21. inductor current in dcm operation ( ) ` c 2 t i i v out off out peak , l out ? ? ? = ? ? ? = = = ? ? = v 2 . 13 v @ a 762 . 1 v 8 . 10 v @ a 915 . 1 l f d v i max , in min , in sw in peak , l ? ? ? = = = ? ? ? ? = v 2 . 13 v @ 414 . 0 v 8 . 10 v @ 55 . 0 r ) 1 m ( m l f 2 d max , in min , in 0 sw ? ? ? = = = v 2 . 13 v @ 015 . 2 v 8 . 10 v @ 463 . 2 v v m max , in min , in in out i l, peak i l t t sw = 1/f sw t on t off am00593v1
application information led7707 34/47 t off can be calculated as: equation 35 defining d 2 as: equation 36 the worst case for the output voltage ri pple is when input voltage is lower (v in,min = 10.8 v). a simple way to select the c out value is fixing a maximum voltage ripple. in order to affect as less as possible the cu rrent generators, it woul d be better to fix the maximum ripple lower than the typical voltage across the generators. for example considering v out lower than 70 mv (i.e. the 10 % of the voltage across the leading generator), the required capacitance is: equation 37 a margin from the calculated value should be taken into account because of the capacitance drop due to the app lied voltage when mlccs are used. one 10 f mlcc (or two 4.7 f mlccs) can be a good choice for this application. in case a dimming duty cycle different from 100% is used, a further contribution to the capacitor discharge (during the off time of the dimming cycle) should be considered. 6.4.5 input capacitor choice the input capacitor of a boost converter is less critical than the output capacitor, due to the fact that the inductor is in series with the input, and hence, the input current waveform is continuous. a low esr capacitor is always recommended. a capacitor of 10 f is tentatively a goo d choice for most of the applications. ? ? ? = = = ? = v 2 . 13 v @ ns 2 . 618 v 8 . 10 v @ ns 7 . 569 d t t max , in min , in 2 sw off () ? ? ? = = = ? ? ? ? ? = v 2 . 13 v @ 408 . 0 v 8 . 10 v @ 376 . 0 1 m r m l f 2 d max , in min , in 0 sw 2 ( ) = ? ? ? >
led7707 application information 35/47 6.4.6 over-voltage protection divider setting the over-voltage protection (ovp) divider provides a partition of the output voltage to the ovsel pin. the ovp divider setting not only fixes the ovp threshol d, but also the open- channel detection threshold. the proper ovp divider setting can be calculated by the equation (3): equation 38 where v out, max is the maximum output voltage consi dering the worst case (all leds with the maximum v f = v f, m a x = 3.7 v on the same row): equation 39 r 1 can be chosen is in the order of hundreds of kilo-ohms to reduce the leakage current in the resistor divider. for example, setting r 1 = 510 k leads to r 2 = 21.89 k . the closest standard commercial value is r 2 = 22 k . 6.4.7 compensation network for the compensation network, the suggestions provided in section 6.1 are always valid. in this condition, tentatively the following value of r3 and c8 (see figure 24 ) are usually a good choice for the loop stability: r3 = 2.4 k c8 = 4.7 nf 6.4.8 boost current limit the boost current limit is set to protect the in ternal power switch against excessive current. the slope compensation may reduce the programmed current limit. hence, to take into account this effect, as a rule of thumb, the current limit can be set as twice as much the maximum inductor peak current (see section 6.4.4 ): i boost, peak > 3.83 a therefore, using equation (7) and choosing i boost, peak = 4 a, r bilim will be: equation 40 v 145 . 1 v 4 v v 145 . 1 r r max , out 1 2 ? + ? = v 6 . 26 mv 700 v nled v max , f ovp , out = + ? = = = k 300 i k r peak , boost b bilim
application information led7707 36/47 6.4.9 power dissipation estimate as explained in section 5.2, there are several contributions to the total power dissipation. neglecting the power dissipated by the ldo (surel y less significant compared with the other contributions), equation (18), (20), (21) and (22) help to estimate the overall power dissipation. before starting the power dissipation estimate it is important to highlight that the following calculations are considering the worst case (t he actual value of the dissipated power would require measurements). therefore the power dissipation is estimated according to the following assumptions: 1. minimum input voltage (10.8 v), which leads to maximum input current (and also d will have the higher value, see section 6.4.4 ); 2. maximum r ds(on) of the internal power mosfet; 3. leds in the row of the leading generator will have the maximum forward voltage, whereas all other leds in the other rows will have the minimum forward voltage. 4. 100 % dimming signal duty cycle is considered. the conduction and switching losses on the internal power switch can be calculated as: equation 41 equation 42 where t r = t f = 15 ns the power dissipation related to the current generators is given by: equation 43 equation 44 equation 45 the junction temperature can be estimated by equation (18) considering t a = 25 c: equation 46 mw 216 d d i r p dim 2 in dson cond , d = ? ? ? = mw 233 d 2 ) t t ( f i v p dim f r sw in out sw , d = ? + ? ? ? = mw 42 d v i p dim ifb row master , gen = ? ? = () ( ) mw 630 d n v v 1 n i p dim leds leds , f ifb rows row gen = ? ? + ? ? ? = w 12 . 1 p p p p p gen master , gen sw , d cond , d tot , d = + + + ? c 72 p r t t tot , d ja , th amb j = ? + =
led7707 application information 37/47 in order to estimate also the efficiency, other contributions to the power dissipation must be added to p d, tot (which represents only the power dissipated by the device), that is: equation 47 where v f, diode = 0.4 v equation 48 where dcr = 80 m (typical dcr of the recommended inductors). therefore the total dissipated power is: equation 49 considering the input power as the result of in put voltage multiplied by the input current, the estimated efficiency is: equation 50 note: it is important to remind that the previous ca lculations consider the worst case, especially for the power dissipated on the current generators. statistical analysis (confirmed by bench measurements) shows that the series connection of more leds on each channel leads to compensation effects. the hypothesis 3 above mentioned is thus rather unlikely. therefore p gen is significantly lower and the overall efficiency is typically around 90 %. mw 133 d i v p 2 in diode , f diode , diss = ? ? = = ? ? ? = w 316 . 1 p p p p ind , diss diode , diss tot , d total , diss = + + = = ? =
application information led7707 38/47 6.5 layout consideration 1. a careful pcb layout is important for proper operation. in this section some guidelines are provided in order to achieve a good layout. 2. the device has two different ground pins: signal ground (sgnd) and power ground (pgnd). the pgnd pin handles the switching current related to the boost section; for this reason the pcb traces should be kept as short as possible and with adequate width. 3. the signal ground is the return for the device supply and the current generators and can be connected to the thermal pad. 4. the heat dissipation area (adequate to th e application conditions) should be placed backside respect to the device and with the lowest thermal impedance possible (i.e. pcb traces in the backside should be avoi ded). the dissipation area is thermally and electrically connected to the thermal pad by several vias (nine vias are recommended). 5. the signal and power grounds must be conne cted together in a single point as close as possible to the pgnd pin to reduce ground loops. 6. the r-c components of the compensation network should be placed as close as possible to the comp pin in order to avoid noise issue an d instability of the compensation. 7. noise sensitive signals (i.e. feedbacks and compensation) should be routed as short as possible to minimize noise collection. the led7707 pinout makes it easy to separate power components (e.g. inductor, diode) from signal ones. 8. the lx switching node should have and adequate width for high efficiency. 9. the critical power path inductor-lx-pgnd must be as short as possible by mounting the inductor, the diode and cout as close as possible each other. 10. the capacitors of the compensated divi der connected to the ovsel pin should be placed as close as poss ible to the ovsel pin. 11. in order to assure good performance in terms of row current accuracy/mismatch, the pcb traces from the rows pins to the leds should have similar length and width. 12. the capacitors of the filter connected to ldo5 and vin pins should be mounted as close as possible to the mentioned pins figure 22 and figure 23 shows the demonstration board layout (top view and bottom view respectively).
led7707 application information 39/47 figure 22. demonstration board layout (top view) figure 23. demonstration board layout (bottom view) figure 24 shows the led7707 demonstration bo ard application circuit, whereas ta b l e 9 lists the used components and their value.
application information led7707 40/47 figure 24. led7707 demonstration board schematic table 9. led7707 demonstration board component list component description package part number mfr value c1 ceramic, 35 v x5r, 20 % smd 1210 umk325bj106km-t taiyo yuden 10 f c2,c3 ceramic, 50 v x7r, 20 % smd 1206 grm31cr71h475ka88b murata 4.7 f c4 smd 1206 grm31cr71h225ka88b n.m. c5 ceramic, 25 v x5r, 20 % smd 0603 standard 1 f c6 100 nf c7 3.3 nf c8 4.7 nf c9 n.m. c10 220 pf c11 4.7 nf c12 n.m. c13 15 pf r1 chip resistor 0.1 w, 1 % smd 0603 standard 510 k r2 16 k r3 2.4 k r4 4.7 r5 330 k r6 24 k r7 chip resistor 0.1 w, 1 % smd 0603 standard 360 k
led7707 application information 41/47 component description package part number mfr value r8 680 k r9, r10 100 k r11 1.2 k r12 n.m. r13 n.m. l1 6u8, 60 m , 5.8 a 7x7 mm xpl7030-682ml coilcraft 6.8 f d1 schottky, 40 v, 1 a do216-aa stps1l40m st stps1l40m d2 red led, 3 ma smd 0603 standard d3 signal schottky sod-523 bas69 n.m. u1 integrated circuit qfn4x4 led7707 st led7707 j2 pcb pad jumper j8 header 8 sil 8 standard sw1, sw2 jumper 3 sil 3 standard sw3 push button 6x6 mm fsm4jsmat tyco table 9. led7707 demonstration board component list (continued)
electrical characteristics led7707 42/47 7 electrical characteristics figure 25. efficiency versus dim duty cycle, v in = 12 v, 6 rows, 10 white leds (60 ma) in series, f sw = 660 khz figure 26. efficiency versus dim duty cycle, v in = 18 v, 6 rows, 10 white leds (60 ma) in series, f sw = 660 khz figure 27. efficiency versus dim duty cycle, v in = 24 v, 6 rows, 10 white leds (60 ma) in series, f sw = 825 khz figure 28. efficiency versus dim duty cycle, v in = 24 v, 6 rows, 10 white leds (60 ma) in series, f sw = 825 khz
led7707 electrical characteristics 43/47 figure 29. soft-start waveforms (en, ss, and v out monitored) figure 30. boost section switching signals (lx, sync and inductor current monitored), v in = 12 v, 10 leds figure 31. dimming waveforms (f dim = 200 hz) figure 32. dimming waveforms (f dim = 1 khz)
package mechanical data led7707 44/47 8 package mechanical data in order to meet environmental requirements, st offers these devices in different grades of ecopack ? packages, depending on their level of environmental compliance. ecopack ? specifications, grade definitions and product status are available at: www.st.com . ecopack is an st trademark.
led7707 package mechanical data 45/47 table 10. vfqfpn-24 4 mm x 4 mm mechanical data dim. mm min typ max a 0.80 0.90 1.00 a1 0.00 0.02 0.05 a3 0.20 b 0.18 0.25 0.30 d 3.85 4.00 4.15 d2 2.40 2.50 2.60 e 3.85 4.00 4.15 e2 2.40 2.50 2.60 e0.50 l 0.30 0.40 0.50 ddd 0.08 figure 33. package dimensions
revision history led7707 46/47 9 revision history table 11. document revision history date revision changes 18-sep-2008 1 initial release 20-oct-2008 2 updated ta bl e 3 and ta b l e 5 removed table 4 10-apr-2009 3 updated ta bl e 4 , ta b l e 5 , figure 3 , figure 4 , figure 8 , figure 9 and ta bl e 9
led7707 47/47 please read carefully: information in this document is provided solely in connection with st products. stmicroelectronics nv and its subsidiaries (?st ?) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described he rein at any time, without notice. all st products are sold pursuant to st?s terms and conditions of sale. purchasers are solely responsible for the choice, selection and use of the st products and services described herein, and st as sumes no liability whatsoever relating to the choice, selection or use of the st products and services described herein. no license, express or implied, by estoppel or otherwise, to any intellectual property rights is granted under this document. i f any part of this document refers to any third party products or services it shall not be deemed a license grant by st for the use of such third party products or services, or any intellectual property contained therein or considered as a warranty covering the use in any manner whatsoev er of such third party products or services or any intellectual property contained therein. unless otherwise set forth in st?s terms and conditions of sale st disclaims any express or implied warranty with respect to the use and/or sale of st products including without limitation implied warranties of merchantability, fitness for a parti cular purpose (and their equivalents under the laws of any jurisdiction), or infringement of any patent, copyright or other intellectual property right. unless expressly approved in writing by an authorized st representative, st products are not recommended, authorized or warranted for use in milita ry, air craft, space, life saving, or life sustaining applications, nor in products or systems where failure or malfunction may result in personal injury, death, or severe property or environmental damage. st products which are not specified as "automotive grade" may only be used in automotive applications at user?s own risk. resale of st products with provisions different from the statements and/or technical features set forth in this document shall immediately void any warranty granted by st for the st product or service described herein and shall not create or extend in any manner whatsoev er, any liability of st. st and the st logo are trademarks or registered trademarks of st in various countries. information in this document supersedes and replaces all information previously supplied. the st logo is a registered trademark of stmicroelectronics. all other names are the property of their respective owners. ? 2009 stmicroelectronics - all rights reserved stmicroelectronics group of companies australia - belgium - brazil - canada - china - czech republic - finland - france - germany - hong kong - india - israel - ital y - japan - malaysia - malta - morocco - singapore - spain - sweden - switzerland - united kingdom - united states of america www.st.com

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