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RT8489 ? ds8489-00 november 2012 www.richtek.com 1 copyright 2012 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. ? high voltage 4-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 adjustable switching frequency z z z z z easy analog and digital dimming control z z z z z adjustable soft-start z z z z z adjustable output over voltage protection z z z z z under voltage lockout and thermal shutdown z z z z z sop-24 package z z z z z rohs compliant and halogen free general description the RT8489 is a 4-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 an adjustable 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 RT8489 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 adjustable 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. applications z building and street lighting z led tv backlight z led monitor backlight z industrial display backlight other protection features include adjustable output over voltage protection, pwm switch current limit and thermal shutdown. simplified application circuit rset vc ss gate isense led1 RT8489 dctl actl gnd vcc gate1 sense1 led4 gate4 sense4 v in l1 c in c vcc r set r vc c vc c ss m sw r sense m1 r s1 m4 r s4 d1 c out v out r3 4 x n leds v cc
RT8489 2 ds8489-00 november 2012 www.richtek.com ? copyright 2012 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. 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. RT8489 package type s : sop-24 lead plating system g : green (halogen free and pb free) pin configurations sop-24 (top view) led1 gate1 led2 sense1 gate2 sense2 led3 vcc gbias gate gnd isense rset en gate3 ovp sense3 ss led4 vc gate4 actl sense4 dctl 6 7 8 9 10 11 12 4 5 2 3 21 22 23 18 19 20 16 13 17 24 15 14 marking information RT8489gs : product number ymdnn : date code richtek RT8489 gsymdnn functional pin description pin no. pin name pin function 1 vcc power supply input. for good bypass, a low esr capacitor is needed between this pin and gnd. 2 gbias internal gate driver bias voltage (around 10v) pin. a good bypass capacitor between this pin and gnd is needed. 3 gate gate of external mosfet. for the boost pwm control loop. 4 gnd ground. 5 isense current sense input. 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. 6 rset switching frequency set pin. put a resistor from rset to gnd to program the switching frequency. f sw = 280khz when r set = 40k . 7 en enable control input (active high). 8 ovp over voltage protection sense input. ovp pin threshold is around 1.23v. use a resistor divider from output to gnd to program the ovp level. 9 ss soft-start. connect a soft-start capacitor from ss pin to gnd to adjust the soft-start time. an internal 8 a constant current charges external capacitor during soft-start period. 10 vc loop compensation. 11 actl analog/pwm dimming control input. when used in analog dimming, actl control range is from 0.4v to 1.4v. 12 dctl digital pwm dimming control input. 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.
RT8489 3 ds8489-00 november 2012 www.richtek.com ? copyright 2012 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. pin no. pin name pin function 13, 16, 19, 22 sensex source of external mosfetx (x = 1 to 4). the sensex pins are regulated around 225mv. connect a sense resistor from this pin to gnd. the led current is set by i led = 225mv / (sense resistance) when v actl is greater than 1.4v. 14, 17, 20, 23 gatex gate drive output for external mosfetx (x = 1 to 4). 15, 18, 21, 24 ledx led string voltage sense input. connect these pins to the drain of external mosfetx (x = 1 to 4). short the pin to gnd if not used. function block diagram + - pwm control osc vout regulation unit + - + - + - 10v vcc 5.5v 1.23v rset ovp vc led1 led2 led3 led4 ss gbias gate isense led1 gate1 sense1 led2 gate2 sense2 led3 gate3 sense3 led4 gate4 sense4 gbias 3.2v + - + - gbias + - gbias 8a 3.2v + - shutdown 1.4v en + - 3.2v 1.4v 1.4v dctl actl + - gnd gbias 3.2v 3.2v 3.2v + -
RT8489 4 ds8489-00 november 2012 www.richtek.com ? copyright 2012 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. operation the internal gate driver circuit is powered from gbias pin around 10v. the gbias voltage is generated by an internal base driver to drive an external npn emitter follower. the osc block generates a programmable frequency which is set by an external resistor at rset pin for RT8489 operation. at the beginning of the oscillator cycle, the gate turns high. the vout regulation unit automatically detects the lowest sensed ledx pin feedback voltage among the 4 led strings and compares to 0.7v. if the lowest ledx pin voltage is lower than 0.7v, the vc pin (the output of the op amp in vout regulation unit) is charged high. the isense pin voltage is the triangular feedback signal of the sensed switch current (which equals inductor current ramp).the pwm comparator compares isense pin voltage to vc pin voltage. when isense pin voltage exceeds vc pin voltage, the pwm comparator resets the latch and turns off gate. if isense pin voltage exceeds vc pin voltage by the end of the switching cycle, the gate will be turned off for minimum off-time. the cycle repeats when the gate is turned on at the beginning of the next switching cycle. by this pwm closed loop control, the lowest sensed ledx pin voltage among the 4 led strings is regulated to 0.7v. as the system starts, the cap at the soft-start pin is slowly charged up by an internal current source around 8 a. the slowly rising vc pin voltage allows the pwm duty to increase gradually to achieve soft-start function. the dimming can be set by varying actl pin analog or pwm voltage signal. the internal sense threshold for the 4 current sources follows actl signal to achieve dimming control.
RT8489 5 ds8489-00 november 2012 www.richtek.com ? copyright 2012 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. 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 input voltage logic-low v il -- -- 0.5 v en input current i en v en 3.3v -- 2 -- a led current programming sense1-sense4 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 6v -- -- 10 a led current off threshold at actl v actl_off -- 0.4 -- v absolute maximum ratings (note 1) z vcc --------------------------------------------------------------------------------------------------------------------------- ? 0.3v to 32v z isense dc ----------------------------------------------------------------------------------------------------------------------------- - ? 0.3v to 2v < 20ns ---------------------------------------------------------------------------------------------------------------------- -- ? 0.3v to 6v z gbias ------------------------------------------------------------------------------------------------------------------------- ? 0.3v to 14v z sense1 to sense4 dc ----------------------------------------------------------------------------------------------------------------------------- - ? 0.3v to 1v < 20ns ---------------------------------------------------------------------------------------------------------------------- -- ? 0.3v to 6v z led1 - led4 (note 5) ---------------------------------------------------------------------------------------------------- ? 0.3v to 20v z dctl, actl, en, ovp --------------------------------------------------------------------------------------------------- ? 0.3v to 10v z power dissipation, p d @t a = 25 c sop-24 ----------------------------------------------------------------------------------------------------------------------- 1.3w z package thermal resistance (note 2) sop-24, ja ----------------------------------------------------------------------------------------------------------------- 77 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 model) ----------------------------------------------------------------------------------------------- 2kv mm (ma chine model) ------------------------------------------------------------------------------------------------------ 200v (v in = 12v, no load, t a = 25 c, unless otherwise specified) recommended operating conditions (note 4) z supply voltage, vin ------------------------------------------------------------------------------------------------------- 7v to 28v z junction temperature range --------------------------------------------------------------------------------------------- ? 40 c to 125 c z ambient temperature range --------------------------------------------------------------------------------------------- ? 40 c to 85 c
RT8489 6 ds8489-00 november 2012 www.richtek.com ? copyright 2012 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. 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 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 gbias = 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 4 high voltage v gatex_h i gatex = ? 0.1ma -- 8.3 -- v i gatex = 2ma -- 0.8 -- gate1 to 4 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 note 1. stresses beyond those listed ? absolute maximum ratings ? may cause permanent damage to the device. these are stress ratings only, and 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 may affect device reliability. note 2. ja is measured at t a = 25 c on a high effective thermal conductivity four-layer test board per jedec 51-7. note 3. devices are esd sensitive. handling precaution is recommended. note 4. the device is not guaranteed to function outside its operating conditions.
RT8489 7 ds8489-00 november 2012 www.richtek.com ? copyright 2012 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. typical application circuit * : if v in is operated above 12v, it is recommende d to keep v cc at 12v for optimal application rset ovp vc ss gbias gate isense led1 RT8489 en dctl actl gnd vcc gate1 sense1 led2 gate2 sense2 led3 gate3 sense3 led4 gate4 sense4 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 d1 c out v out r3 4 x n leds * 12v 1 6 8 10 9 7 12 11 4 2 3 5 24 23 22 21 20 19 18 17 16 15 14 13
RT8489 8 ds8489-00 november 2012 www.richtek.com ? copyright 2012 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. typical operating characteristics 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 led current vs. actl pwm duty 0 50 100 150 200 250 0 20406080100 actl pwm duty (%) ledx current (ma) v in = 24v, r sx = 0.9 led1 led2 led3 led4 efficiency vs. input voltage 0 10 20 30 40 50 60 70 80 90 100 8 1216202428 input voltage (v) efficiency (%) 4 x 10leds, i led = 100ma (k ) 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 v in = 24v 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 r sx = 2.2
RT8489 9 ds8489-00 november 2012 www.richtek.com ? copyright 2012 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. 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) 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 vs. input voltage 650 660 670 680 690 700 4 8 12 16 20 24 28 input voltage(v) led regulation voltage(mv )
RT8489 10 ds8489-00 november 2012 www.richtek.com ? copyright 2012 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. application information the RT8489 is a 4-ch programmable current source controller for led backlight or lighting application. by detecting the minimum voltage required to drive each led string and setting 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 are used to control the led current of each channel. with high speed current source n-mosfet drivers, the RT8489 features highly accurate current matching of 3 percent, while 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 RT8489 integrates adjustable switching frequency and soft-start, and provides over temperature, over voltage and current limit protection features. input uvlo the input operating voltage range of the RT8489 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 capacitor 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
RT8489 11 ds8489-00 november 2012 www.richtek.com ? copyright 2012 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. figure 2. power on sequence control by v in figure 3. to prevent ? hard-start ? sequence soft-start the soft-start of the RT8489 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 following 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
RT8489 12 ds8489-00 november 2012 www.richtek.com ? copyright 2012 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. 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 RT8489 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 RT8489 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 RT8489 is 1.8k and 3.3nf. led current setting the maximum current of channel 1 to 4 is set by placing an appropriate sense resistor for 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 easier method is to simply vary the dc current through the led by analog dimming. however, pwm dimming which switches the led on and off via different duty cycle to control the average led current is the better dimming method. 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 is that a wider dimming range is available. the RT8489 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. the pwm 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%.
RT8489 13 ds8489-00 november 2012 www.richtek.com ? copyright 2012 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. 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, and 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 RT8489 equips 4 channel led drivers and each channel supports numerous leds. the 4 led strings are connected from v out to pin ledx (x = 1 to 4) respectively. if one of the led channel is not used, the ledx (x = 1 to 4) 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 figure 6. external resistor connection for ledx pin input over current protection the resistor, r sense , between the source of the external n-mosfet (m sw ) and gnd should be selected to provide adequate switch current. the RT8489 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 RT8489. 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 ?? ?? ?? ledx gatex sensex RT8489 r sx v out rx gnd
RT8489 14 ds8489-00 november 2012 www.richtek.com ? copyright 2012 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. ( 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 RT8489 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 RT8489 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 which 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 RT8489 should be a kelvin connection to the positive terminal of r sense . output over voltage protection setting the RT8489 is equipped with over voltage protection (ovp) function. when the voltage at the ovp pin exceeds threshold value, typically 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 opened, the controller will automatically regulate the output voltage to ovp setting level. over temperature protection the RT8489 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 RT8489 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 equation is 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 :
RT8489 15 ds8489-00 november 2012 www.richtek.com ? copyright 2012 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. 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, the maximum junction temperature is 125 c. the junction to ambient thermal resistance, ja , is layout dependent. for sop-24 package, the thermal resistance, ja , is 77 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) / (77 c/w) = 1.3w for sop-24 package figure 7. derating curve of maximum power dissipation 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 . 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 0 25 50 75 100 125 ambient temperature (c) maximum power dissipation (w) 1 four-layer pcb
RT8489 16 ds8489-00 november 2012 www.richtek.com richtek technology corporation 5f, no. 20, taiyuen street, chupei city hsinchu, taiwan, r.o.c. tel: (8863)5526789 richtek products are sold by description only. richtek reserves the right to change the circuitry and/or specifications without notice at any time. customers should obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. richtek cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a richtek product. information furnish ed by richtek is believed to be accurate and reliable. however, no responsibility is assumed by richtek or its subsidiaries for its use; nor for any infringeme nts of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of r ichtek or its subsidiaries. outline dimension a b j f d c i m h 24 ? lead sop plastic package dimensions in millimeters dimensions in inches symbol min max min max a 15.189 15.596 0.598 0.614 b 7.391 7.595 0.291 0.299 c 2.362 2.642 0.093 0.104 d 0.330 0.508 0.013 0.020 f 1.194 1.346 0.047 0.053 h 0.229 0.330 0.009 0.013 i 0.102 0.305 0.004 0.012 j 10.008 10.643 0.394 0.419 m 0.381 1.270 0.015 0.050

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