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RT8488 1 ds8488-02 april 2011 www.richtek.com high voltage 6-ch led driver controller features z z z z z wide operation voltage range : 7v to 28v z z z z z programmable channel current z z z z z 3% current matching accuracy between channels z z z z z programmable switching frequency z z z z z easy analog and digital dimming control z z z z z programmable soft-start z z z z z automatic open channel detection z z z z z programmable output over voltage protection z z z z z under voltage lockout and thermal shutdown z z z z z 32-lead wqfn package general description the RT8488 is a 6-ch led driver controller that delivers well matched led current to each channel of led string. with external current sources, the number of leds per string is only limited by the current source and the v in / v out conditions. the current mode pwm boost type controller operates at a programmable switching frequency of up to 1mhz, with a wide v in range covering from 7v to 28v. the switch driver is designed to drive industrial grade high power mosfets. the pwm loop selects and regulates the led string with the highest voltage string to 0.7v, thus allowing voltage mismatches between the led strings. the RT8488 automatically detects and excludes any open and/or broken strings during operation from the pwm loop to prevent v out from over voltage. the led currents on all channels are simply programmed with a resistor on each channel. three convenient dimming methods are provided : 1. analog dimming is linearly controlled by an external voltage. 2. true digitally controlled pwm dimming can regulate the duty cycle of the led current. 3. for noise free pwm dimming, use an on board output clamping amplifier as a low pass filter to convert pwm dimming signals into analog dimming signals. other protection features include programmable output over voltage protection, pwm switch current limit and thermal shutdown. applications z building and street lighting z led tv backlight z led monitor backlight z industrial display backlight ordering information note : richtek products are : rohs compliant and compatible with the current require- ments of ipc/jedec j-std-020. suitable for use in snpb or pb-free soldering processes. marking information RT8488gqw : product number ymdnn : date code RT8488 gqwymdnn RT8488 package type qw : wqfn-32l 5x5 (w-type) lead plating system g : green (halogen free and pb free) pin configurations wqfn-32l 5x5 (top view) rset ss ovp en led2 led3 sense2 gate2 sense6 gate6 led6 sense5 gnd gate gbias vcc actl vc sense3 gate3 nc isense gate5 led1 dctl led4 led5 gate1 sense4 sense1 nc gate4 gnd 24 23 22 21 1 2 3 4 10 11 12 13 31 30 29 28 20 19 5 6 9 32 14 27 18 7 15 26 16 25 17 8 33
RT8488 2 ds8488-02 april 2011 www.richtek.com typical application circuit rset ovp vc ss gbias gate isense led1 RT8488 en dctl actl gnd vcc gate1 sense1 led2 gate2 sense2 led3 gate3 sense3 led4 gate4 sense4 led5 gate5 sense5 led6 gate6 sense6 v in v out chip enable l1 c in c vcc r set r1 r2 r vc c vc c ss c b m sw r sense m1 r s1 m2 r s2 m3 r s3 m4 r s4 m5 r s5 r s6 m6 d1 c out v out r3 6 x n leds * * : if v in is operated above 12v, it is recommended to keep the v cc at 12v for optimal application 12v
RT8488 3 ds8488-02 april 2011 www.richtek.com functional pin description pin no. pin name pin function 1 rset switching frequency set pin. put a resistor from rset to gnd to program the switching frequency. f sw = 310khz when r set = 40k . 2 en chip enable (active high). 3 ovp over voltage protection pin. ovp pin threshold is around 1.23v. use a resistor divider from output to gnd to program the ovp level. 4 ss soft-start pin. use a soft start cap from ss pin to gnd to program the soft start time period. around 5.5 a is sourcing out of ss pin. 5 vc loop compensation pin. 6 actl analog/pwm dimming control pin. when used in analog dimming, actl control range is from 0.5v to 1.4v. 7 dctl digital pwm dimming control pin. by adding a 0.1 f filter capacitor on the actl pin, the pwm dimming signal on the dctl pin will be averaged out and converted into analog dimming signal on the actl pin. 8, 9 nc no internal connection. 25, 22, 19 16, 13, 10 sensex source pin of external mosfetx (x = 1 to 6). the sensex pins are regulated around 225mv. connect a sense resistor from this pin to gnd. the led current is programmed by i led = 225mv / (sense resistance) when v actl is greater than 1.4v. 26, 23, 20 17, 14, 11 gatex gate pin of external mosfetx (x = 1 to 6). for led drivers. 27, 24, 21 18, 15, 12 ledx drain pin of external mosfetx (x = 1 to 6). for led drivers. short the pin to gnd if not used. 28 vcc power supply pin. for good bypass, a low esr capacitor is needed between this pin and gnd. 29 gbias internal gate driver bias voltage (around 10v) pin. need a good bypass capacitor between this pin and gnd. 30 gate gate pin of external mosfet. for the boost pwm control loop. 31, 33 (exposed pad) gnd ground pin. the exposed pad must be soldered to a large pcb and connected to gnd for maximum power dissipation. 32 isense switch current sense pin. connect a sense resistor from this pin to gnd. the switch current sense signal is used for boost current mode pwm loop control and power switch over current protection.
RT8488 4 ds8488-02 april 2011 www.richtek.com function block diagram + - pwm control osc vout regulation unit + - + - + - + - + - 10v vcc 5.5v 1.23v rset ovp vc led1 led2 led3 led4 led5 led6 ss shutdown 1.4v en 3.2v 1.4v 1.4v dctl actl gnd gbias gate isense led1 gate1 sense1 led2 gate2 sense2 led3 gate3 sense3 led4 gate4 sense4 led5 gate5 sense5 led6 gate6 sense6 + - gbias 3.2v + - gbias 3.2v + - gbias 3.2v + - gbias 3.2v + - gbias 3.2v + - gbias 3.2v + - 8a 3.2v
RT8488 5 ds8488-02 april 2011 www.richtek.com electrical characteristics parameter symbol test conditions min typ max unit overall supply current i vcc v vc 0.4v (switching off) -- 5 8 ma shutdown current i shdn v en 1.2v -- 5 -- a logic-high v ih 2 -- -- en threshold voltage logic-low v il -- -- 0.5 v en input current i en v en 3.3v -- 2 -- a led current programming sense1-sense6 threshold 6v > v gatex > 2v 214 225 236 mv sense voltage ch to ch matching (max) (min) (avg) vv 2v ? -- 1.5 3 % analog dimming actl input current i actl v actl Q 6v -- -- 10 a led current off threshold at actl v actl_off -- 0.4 -- v to be continued absolute maximum ratings (note 1) z vcc --------------------------------------------------------------------------------------------------------------------------- 3 2v z isense dc ----------------------------------------------------------------------------------------------------------------------------- -- 2v < 200ns --------------------------------------------------------------------------------------------------------------------- --- 6v z gbias ------------------------------------------------------------------------------------------------------------------------- 14v z sense1 to sense6 dc ----------------------------------------------------------------------------------------------------------------------------- -- 1v < 200ns --------------------------------------------------------------------------------------------------------------------- --- 6v z led1 - led6 (note 5) ---------------------------------------------------------------------------------------------------- 20v z dctl, actl, en, ovp --------------------------------------------------------------------------------------------------- 10v z power dissipation, p d @t a = 25 c wqfn-32l 5x5 ------------------------------------------------------------------------------------------------------------- 2.778w z package thermal resistance (note 2) wqfn-32l 5x5, ja -------------------------------------------------------------------------------------------------------- 36 c/w wqfn-32l 5x5, jc ------------------------------------------------------------------------------------------------------- 6 c/w z junction temperature ------------------------------------------------------------------------------------------------------ 150 c z storage temperature range --------------------------------------------------------------------------------------------- ? 65 c to 150 c z lead temperature (soldering, 10 sec.) -------------------------------------------------------------------------------- 260 c z esd susceptibility (note 3) hbm (human body mode) ----------------------------------------------------------------------------------------------- 2kv mm (ma chine mode) ------------------------------------------------------------------------------------------------------- 200v (v cc = 12v, no load, t a = 25 c, unless otherwise specified) recommended operating conditions (note 4) z supply voltage, v cc ------------------------------------------------------------------------------------------------------- 7v to 28v z junction temperature range --------------------------------------------------------------------------------------------- ? 40 c to 125 c
RT8488 6 ds8488-02 april 2011 www.richtek.com parameter symbol test conditions min typ max unit led current on threshold at actl v actl_on -- 1.4 -- v dctl input current i dctl v dctl Q 6v -- -- 1 a pwm boost converter switching frequency f sw r set = 40k 220 280 340 khz minimum off-time r set = 40k -- 300 -- ns vled threshold for no connection v led -- 0.1 -- v regulated v led v led highest voltage led string -- 0.7 -- v amplifier output current i vc 2.4v > v vc > 0.2v -- 30 -- a vc threshold for pwm switch off -- 0.7 -- v switch gate driver gbias voltage v gbias i gbia s = 20ma -- 10 -- v i gate = ? 20ma -- 7.7 -- gate high voltage v gate_h i gate = ? 0.1ma -- 8.2 -- v i gate = 20ma -- 0.7 -- gate low voltage v gate_l i gate = 0.1ma -- 0.4 -- v gate drive rise and fall time 1nf load at gate -- 20 -- ns led current sources gate driver i gatex = ? 2ma -- 8.1 -- gate1 to 6 high voltage v gatex_h i gatex = ? 0.1ma -- 8.3 -- v i gatex = 2ma -- 0.8 - - gate1 to 6 low voltage v gatex_l i gatex = 0.1ma -- 0.6 -- v ovp and soft-start ovp threshold v ovp -- 1.23 -- v ovp input current i ovp v ovp 1.23v -- 1 -- a soft-start pin current i ss v ss 3.2v -- 8 -- a thermal protection thermal shutdown temperature t sd -- 150 -- c thermal shutdown hysteresis t sd -- 20 -- c
RT8488 7 ds8488-02 april 2011 www.richtek.com note 1. stresses listed as the above ? absolute maximum ratings ? may cause permanent damage to the device. these are for stress ratings. functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. exposure to absolute maximum rating conditions for extended periods may remain possiblity to affect device reliability. note 2. ja is measured in natural convection at t a = 25 c on a high effective thermal conductivity four-layer test board of jedec 51-7 thermal measurement standard. the measurement case position of jc is on the exposed pad of the package. note 3. devices are esd sensitive. handling precaution is recommended. note 4. the device is not guaranteed to function outside its operating conditions. note 5. adding a series resistor of at least 20k for higher pin voltage.
RT8488 8 ds8488-02 april 2011 www.richtek.com efficiency vs. input voltage 0 10 20 30 40 50 60 70 80 90 100 8 1216202428 input voltage (v) efficiency (%) 6 x 10leds, i led = 100ma typical operating characteristics led current vs. actl pwm duty 0 50 100 150 200 250 020406080100 actl pwm duty (%) ledx current (ma) v in = 24v, r sx = 0.9 led1 led2 led3 led4 led5 led6 led current vs. input voltage 80 90 100 110 120 10 13 16 19 22 25 input voltage (v) ledx current (ma) led1 led2 led3 led4 led5 led6 r sx = 2.2 frequency vs. r set resistance 100 200 300 400 500 600 700 800 900 1000 10 20 30 40 50 60 70 80 90 100 110 r set frequency (khz) 1 (k ) v in = 24v supply current v s. input voltage 0 1 2 3 4 5 6 7 8 9 10 4 8 12 16 20 24 28 input voltage (v) supply current (ma) 1 switching frequency vs. input voltage 200 280 360 440 520 600 680 760 840 4 8 12 16 20 24 28 input voltage (v) switching frequency (khz) 1 r set = 20k r set = 30k r set = 10k
RT8488 9 ds8488-02 april 2011 www.richtek.com soft-start current vs. input voltage 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 4 8 12 16 20 24 28 input voltage (v) soft-start current ( a) shutdown current vs. input voltage 0 1 2 3 4 5 6 7 4 8 12 16 20 24 28 input voltage (v) shutdown current ( a) 1 en = 0 isense threshold voltage vs. input voltage 50 60 70 80 90 100 110 120 130 140 150 4 8 12 16 20 24 28 input voltage (v) isense threshold voltage (mv) led regulated voltage v s. input voltage 650 660 670 680 690 700 4 8 12 16 20 24 28 input voltage(v) led regulation voltage(mv ) load transient response time (250 s/div) gate (5v/div) i out (100ma/div) v out (50v/div) actl (5v/div) v in = 24v, pwm = 1khz, duty = 50% power on from en time (100 s/div) v in = 24v, i led = 400ma, c ss = 0.1 f en (5v/div) gate (5v/div) led1 (500ma/div) v out (50v/div)
RT8488 10 ds8488-02 april 2011 www.richtek.com application information the RT8488 is a 6-ch programmable current source controller for led backlight or lighting application. by detecting the minimum voltage required to drive each led string and hence to set the boost output accordingly, this topology reduces power dissipation and increases overall efficiency of the led lighting system. the individual current source channel regulates the current flow to give accurate current sinking for each led string. the external n-mosfet current source will accommodate the power dissipation difference among channels resulting from the forward voltage difference between the led strings. both digital pwm dimming signal and analog voltage signal can be used to control the led current of each channel. with high speed current source n-mosfet drivers, the RT8488 features highly accurate current matching of 3 percent, while also providing very fast turn-on and turn-off times. this allows a very narrow minimum on or off pulse, which increases dimming range and provides higher linearity. the RT8488 integrates adjustable switching frequency and soft-start, and provides the circuitry for over temperature, over voltage and current limit protection features. input uvlo the input operating voltage range of the RT8488 is 7v to 28v. an input capacitor at the vcc pin can reduce ripple voltage. it is recommended to use a ceramic 10 f or larger capacitance as the input capacitor. this ic provides an under voltage lockout (uvlo) function to enhance the stability when start-up. the uvlo rising input voltage threshold is set at 5.5v typically with a 0.7v hysteresis. power sequence refer to below figure 1 and 2. the recommended power on sequence states that the pwm signal should be ready before en and/or v in is ready. otherwise, the soft-start function will be disabled. as for power off sequence, en/ v in must be pulled low within 10ms to prevent ? hard- start ? as shown as figure 3. figure 1. power on sequence control by en en must be turned on later than vin and pwm signal en must be turned off earlier than vin and pwm signal power on sequence en v in pwm power off sequence uvlo v out soft-start no soft-start abnormal power-on sequence if pwm turns on late
RT8488 11 ds8488-02 april 2011 www.richtek.com figure 2. power on sequence control by v in figure 3. to prevent ? hard-start ? sequence soft-start the soft-start of the RT8488 can be achieved by connecting a capacitor from the ss pin to gnd. the built in soft-start circuit reduces the start up current spike and output voltage overshoot. the soft-start time is determined by the external capacitor charged by an internal 8 a constant charging current. the ss pin directly limits the rate of voltage rise on the vc pin, which in turn limits the peak switch current. the soft-start interval is set by the soft-start capacitor selection according to the equation : ss ss 3.2v t = c (s) 8a a typical value for the soft-start capacitor is 0.1 f. the soft-start pin reduces the oscillator frequency and the maximum current in the switch. the soft-start capacitor is discharged when en/uvlo falls below its threshold, during an over-temperature event, or during a gbias under voltage event. gbias regulator operation the gbias pin requires a capacitor for stable operation and also to store the charge for the large gate switching currents. choose a 10v rated low esr, x7r or x5r ceramic capacitor for best performance. the value of the en and/or v in should be pulled low once pwm pull low for over 10 ms pwm en/v in 10ms v in must be turned on later than en and pwm signal v in must be turned off earlier than en and pwm signal power on sequence en v in pwm power off sequence uvlo v out soft-start no soft-start abnormal powe r-on sequence if pwm turns on late
RT8488 12 ds8488-02 april 2011 www.richtek.com capacitor is determined primarily by the stability of the regulator rather than the gate charge of the switching n-mosfet. a 1 f capacitor will be adequate for most applications. place the capacitor close to the ic to minimize the trace length to the gbias pin and also to the ic ground. an internal current limit on the gbias protects the RT8488 from excessive on chip power dissipation. if the input voltage, v in , does not exceed 10v, then the gbias pin should be connected to the input supply. be aware that a typical 20ma current will load the gbias to shutdown. loop compensation the RT8488 uses an internal error amplifier, in which through its compensation pin (vc) the loop response is optimized for specific applications. the external inductor, output capacitor, compensation resistor, and compensation capacitor determine the loop stability. the inductor and output capacitor are chosen based on performance, size and cost. the compensation resistor and capacitor at vc are selected to optimize control loop response and stability. the compensation resistor and capacitor are connected in series from the vc pin to gnd to provide a pole and a zero for proper loop compensation. the typical compensation values for RT8488 is 1.8k and 3.3nf. led current setting the maximum current of channel 1 to 6 is programmed by placing an appropriate sense resistor at each led string. when the voltage of actl is higher than 1.4v, the led current can be calculated by the following equation : led, max sx 225mv i = (ma) r where, r sx is the resistor between external regulating n-mosfet and gnd. the actl pin should be tied to a voltage higher than 1.4v to get the full scale 225mv (typical) threshold across the sense resistor. the actl pin can also be used to dim the led current to zero, although relative accuracy decreases with the decreasing voltage sense threshold. when the actl pin voltage is less than 1.4v, the led current is : actl led sx (v 0.4) 225mv i = (ma) r ? the actl pin can also be used in conjunction with a thermistor to provide over temperature protection for the led load, or with a resistive voltage divider to v in to reduce output power and switching current when v in is low. brightness control for led applications where a wide dimming range is required, two methods are available: analog dimming and pwm dimming. the easiest method is to simply vary the dc current through the led by analog dimming. however, a better dimming method is pwm dimming, which switches the led on and off via different duty cycle to control the average led current. the pwm dimming offers several advantages over analog dimming and is more preferred by led manufacturers. one advantage is the chromaticity of the leds which remains unchanged since the led current is either zero or at the programmed current. another advantage of pwm dimming over analog dimming is that a wider dimming range is possible. the RT8488 features both analog and digital dimming control. analog dimming is linearly controlled by an external voltage (0.4v to 1.4v) at the actl pin. a very high contrast ratio is true digital pwm dimming which can be achieved by driving the actl pin with a pwm signal at a recommended pwm frequency of 100hz to 10khz. dimming frequency can be sufficiently adjusted from 100hz to 30khz. however, led current cannot be 100% proportional to the duty cycle, especially for high frequency and low duty ratio, because of physical limitation caused by internal switching frequency. referring to figure 4, the minimum dimming duty can be as low as 1% for the frequency range from 100hz to 300hz. for the dimming frequency from 300hz to 1khz, the minimum dimming duty is about 5%. if the frequency is increased from 1khz to 30khz, the minimum dimming duty will be about 10%.
RT8488 13 ds8488-02 april 2011 www.richtek.com programmable switching frequency the rset frequency adjust pin allows the user to program the switching frequency from 100khz to 1mhz in order to optimize efficiency and performance or minimize external component size. higher frequency operation yields smaller component size but increases switching losses and gate driving current, and may not allow sufficiently high or low duty cycle operation. lower frequency operation gives better performance, but is more costly with larger external component size. an external resistor from the rset pin to gnd is required do not leave this pin open. for an appropriate r set value, refer to figure 5. figure 4. led current vs. pwm dimming duty cycle figure 5. switching frequence vs r set led current vs. pwm duty cycle 0 20 40 60 80 100 120 140 160 0 20406080100 duty cycle (%) led current (ma) 200hz 1khz 3khz frequency vs. r set resistance 100 200 300 400 500 600 700 800 900 1000 10 20 30 40 50 60 70 80 90 100 110 r set frequency (khz) 1 (k ) v in = 12v led pin external resistor connection the RT8488 equips 6 channel led drivers and each channel supports numerous leds. the 6 led strings are connected from v out to pin ledx (x = 1 to 6) respectively. if one of the led channel is not used, the ledx (x = 1 to 6) pins should be connected to ground directly. in this case, there should be a current limiting resistor between external mosfet drain node and ledx pin to limit the ledx pin input current below 100 a. the formula for this resistor is rx = (v out ? | v ledx(max) | ) / 100 a ledx gatex sensex RT8488 r sx v out rx gnd figure 6. led pin external resistor connection input over current protection the resistor, r sense , between the source of the external switching n-mosfet and gnd should be selected to provide adequate switch current. the RT8488 senses the inductor current through isense pin in the switch on period. the duty cycle depends on the current sense signal summed with the internal slope compensation and compared to the vc signal. the external n-mosfet will be turned off when the current signal is larger than the vc signal. in the off period, the inductor current will descend. the external n-mosfet is turned on by the oscillator in the next beginning cycle.to drive the application without exceeding the 120mv (typical) current limit threshold on the isense pin of the RT8488. select a resistor that gives a switch current of at least 20% greater than the required led current according to : in sense out out v0.1v r = () vi ?? ?? ??
RT8488 14 ds8488-02 april 2011 www.richtek.com ( 2 out in in 2 out out (v v ) (v ) l = 2 i f v ) x 0.3 ? where v out = maximum output voltage. v in = minimum input voltage. f = operating frequency. i out = sum of current from all led strings. is the efficiency of the power converter. the boost converter operates in discontinuous conduction mode over the entire input voltage range when the l1 inductor value is less than this value l. with an inductance greater than l, the converter operates in continuous conduction mode at the minimum input voltage and may be discontinuous at higher voltages. power mosfet selection for applications operating at high input or output voltages, the power n-mosfet switch is typically chosen for drain voltage v ds rating and low gate charge. consideration of switch on resistance, r ds(on) , is usually secondary because switching losses dominate power loss. the gbias regulator on the RT8488 has a fixed current limit to protect the ic from excessive power dissipation at high v in , so the n-mosfet should be chosen such that the product of q g at 7v and the switching frequency does not exceed the gbias current limit. schottky diode selection the schottky diode, with their low forward voltage drop and fast switching speed, is necessary for the RT8488 applications. in addition, power dissipation, reverse voltage rating and pulsating peak current are important parameters of the schottky diode that must be considered. choose a suitable schottky diode whose reverse voltage rating is greater than the maximum output voltage. the diode?s average current rating must exceed the average output current. the diode conducts current only when the power switch is turned off (typically less than 50% duty cycle). if using the pwm feature for dimming, it is important to consider diode leakage, which increases with the temperature, from the output during the pwm low interval. therefore, choose the schottky diode with sufficiently low leakage current. the isense pin input to RT8488 should be a kelvin connection to the positive terminal of r sense . output over voltage protection setting the RT8488 is equipped with over voltage protection (ovp) function. when the voltage at the ovp pin exceeds a threshold of approximately 1.23v, the power switch is turned off. the power switch can be turned on once again after the voltage at the ovp pin drops below 1.23v. the output voltage can be clamped at a certain voltage level set by the following equation : out,ovp r1 v = 1.23 1 r2 ?? + ?? ?? where r1 and r2 make up the resistive voltage divider from v out to gnd with the divider center node connected to the ovp pin. as long as one string is in normal operation, the controller will automatically ignore the open strings and continue to regulate the current for the string(s) in normal operation. over temperature protection the RT8488 has an over temperature protection (otp) function to prevent overheating caused by excessive power dissipation. the otp function will shut down switching operation when the die junction temperature exceeds 150 c. the chip will automatically start to switch again once the die junction temperature starts cooling down by approximately 20 c. inductor selection the inductor for the RT8488 should have a saturation current rating appropriate to the maximum switch current. choose an inductor value based on the operating frequency, input voltage and output voltage to provide a current mode ramp during the mos switching. allow the peak to peak inductor ripple to be 30% of the output current.the following equations are useful to estimate the inductor value : out out out in in peak in out vi v v vt i = v2lv ?? ? + ?? ?? the inductor must be selected with a saturation current rating greater than the peak current provided by the following equation :
RT8488 15 ds8488-02 april 2011 www.richtek.com capacitor selection the input capacitor reduces current spikes from the input supply and minimizes noise injection to the converter. for most applications, a 10 f ceramic capacitor is sufficient. a value higher or lower may be used depending on the noise level from the input supply and the input current to the converter. in boost applications, the output capacitor is typically a ceramic capacitor selected based on the output voltage ripple requirements. the minimum value of the output capacitor, c out , is approximately given by the following equation : out out in out ripple out i(v v) c vvf ? = where v ripple is rhe output voltage ripple, for led applications, the equivalent resistance of the led is typically low and the output filter capacitor should be sized to attenuate the current ripple. use of x7r type ceramic capacitors is recommended. lower operating frequencies will require proportionately higher capacitor values. thermal considerations for continuous operation, do not exceed absolute maximum junction temperature. the maximum power dissipation depends on the thermal resistance of the ic package, pcb layout, rate of surrounding airflow, and difference between junction and ambient temperature. the maximum power dissipation can be calculated by the following formula : p d(max) = (t j(max) ? t a ) / ja where t j(max ) is the maximum junction temperature, t a is the ambient temperature, and ja is the junction to ambient thermal resistance. for recommended operating condition specifications of the RT8488, the maximum junction temperature is 125 c and t a is the ambient temperature. the junction to ambient thermal resistance, ja , is layout dependent. for wqfn- 32l 5x5 packages, the thermal resistance, ja , is 36 c / w on a standard jedec 51-7 four-layer thermal test board. the maximum power dissipation at t a = 25 c can be calculated by the following formula : p d(max) = (125 c ? 25 c ) / (36 c /w) = 2.778w for wqfn-32l 5x5 package figure 7. derating curves for RT8488 package layout consideration pcb layout is very important when designing power switching converter circuits. some recommended layout guidelines are suggested as follows : the power components l1, d1, c in , m sw and c out must be placed as close to each other as possible to reduce the ac current loop area. the pcb trace between power components must be as short and wide as possible due to large current flow through these traces during operation. place l1 and d1, which are connected to n-mosfet, as close as possible. the trace should be as short and wide as possible. the input capacitor, c vcc must be placed as close to the vcc pin as possible. place the compensation components to the vc pin as close as possible to avoid noise pick up. the maximum power dissipation depends on the operating ambient temperature for fixed t j(max) and thermal resistance, ja . for the RT8488 package, the derating curve in figure 7 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. 0.0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 0 25 50 75 100 125 ambient temperature (c) maximum power dissipation (w) 1 four-layer pcb
RT8488 16 ds8488-02 april 2011 www.richtek.com figure 8. pcb layout guide rset ss ovp en led2 led3 sense2 gate2 sense6 gate6 led6 sense5 gnd gate gbias vcc actl vc sense3 gate3 nc isense gate5 led1 dctl led4 led5 gate1 sense4 sense1 nc gate4 gnd 24 23 22 21 1 2 3 4 10 11 12 13 31 30 29 28 20 19 5 6 9 32 14 27 18 7 15 26 16 25 17 8 v out v in v out l1 d1 c in c out c vcc m1 m sw m2 m3 m4 m5 m6 r sense leds x n leds x n leds x n place the capa citors close to the input or output. v out 33 r s1 r s2 r s3 r s4 r s5 r s6
RT8488 17 ds8488-02 april 2011 www.richtek.com information that is provided by richtek technology corporation is believed to be accurate and reliable. richtek reserves the ri ght to make any change in circuit design, specification or other related things if necessary without notice at any time. no third party intellectual property infringemen t of the applications should be guaranteed by users when integrating richtek products into any application. no legal responsibility for any said applications is assumed b y richtek. richtek technology corporation headquarter 5f, no. 20, taiyuen street, chupei city hsinchu, taiwan, r.o.c. tel: (8863)5526789 fax: (8863)5526611 richtek technology corporation taipei office (marketing) 5f, no. 95, minchiuan road, hsintien city taipei county, taiwan, r.o.c. tel: (8862)86672399 fax: (8862)86672377 email: marketing@richtek.com outline dimension w-type 32l qfn 5x5 package e d 1 d2 e2 l b e a a1 a3 see detail a note : the configuration of the pin #1 identifier is optional, but must be located within the zone indicated. det ail a pin #1 id and tie bar mark options 1 1 2 2 dimensions in millimeters dimensions in inches symbol min max min max a 0.700 0.800 0.028 0.031 a1 0.000 0.050 0.000 0.002 a3 0.175 0.250 0.007 0.010 b 0.180 0.300 0.007 0.012 d 4.950 5.050 0.195 0.199 d2 3.400 3.750 0.134 0.148 e 4.950 5.050 0.195 0.199 e2 3.400 3.750 0.134 0.148 e 0.500 0.020 l 0.350 0.450 0.014 0.018

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