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1 RT9288A ds9288a-02 april 2011 www.richtek.com applications z tft lcd panels z led backlighting pwm step-up dc/dc controller for white-led driver ordering information features z z z z z v in operating range : 3v to 13.5v z z z z z fixed pwm frequency : 1mhz z z z z z 200hz to 200khz pwm dimming frequency z z z z z flexible pwm/analog dimming control z z z z z voltage mode with external compensation z z z z z soft start function z z z z z rohs compliant and 100% lead (pb)-free general description the RT9288A is a wide input operating voltage range step- up controlle r. high v oltage output and large output current are feasible by using an external n-mosfet. the RT9288A input operating range is from 3v to 13.5v. besides, it could support up to 60v output at 12v input. the RT9288A is an optimized design for wled driver applications. adjusting the output current of the RT9288A changes the brightness of the wleds. chip enable pin can be used as a digital input allowing wled brightness control with a logic-level pwm signal. functional pin description pin no. pin name pin function 1 vdd supply input voltage pin. bypass an 1uf capacitor to gnd to reduce the input noise. 2 en chip enable (active high). 3 fb feedback to error amplifier input. 4 comp output of error amplifier. connect a capacitor between the comp pin and gnd for compensation. while shutdown, this pin is pulled down by an internal resistor. 5 gnd ground pin. 6 ext output for external transistor. 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 for marking information, contact our sales representative directly or through a richtek distributor located in your area. pin configurations (top view) sot-23-6 vdd en fb ext gnd comp 4 23 5 6 RT9288A package type e : sot-23-6 lead plating system p : pb free g : green (halogen free and pb free)
2 RT9288A www.richtek.com ds9288a-02 april 2011 typical application circuit figure 1. led driver with pwm brightness control (5v j 30v) figure 2. led driver with pwm brightness control (12v j 60v) figure 3. application for constant output voltage vdd gnd ext fb comp RT9288A v in en 1 2 5 3 4 6 pwm dimming c2 100nf r3 d1 l1 10uh/1.5a c4 10uf 5v v out 30v/25ma c3 100nf r2 10 c1 r1 vdd gnd ext fb comp RT9288A v in en 1 2 5 3 4 6 pwm dimming c2 100nf r3 d1 l1 10uh/1.5a c4 10uf 12v v out 60v/25ma c3 100nf r2 10 c1 r1 vdd gnd ext fb comp RT9288A v in en 1 2 5 3 4 6 c2 100nf r4 d1 l1 10uh/1.5a c4 10uf 12v v out 24v c c 6.8nf r2 10 c1 r1 chip enable r3 r c 30k
3 RT9288A ds9288a-02 april 2011 www.richtek.com function block diagram operation soft-start and short circuit protection while power-on, the RT9288A enters soft-start cycle to reduce the in-rush current and output voltage overshoot. the internal soft-start time is 10ms for the RT9288A. the RT9288A enters shutdown and can be re-enabled by turning off-on en pin. in normal operation, if the output loading changes large enough to let error amplifier output larger than 1.8v, the short circuit timer is started. if the time duration of this condition is kept continuously to more than 10ms , the short circuit state is latched and the rt9288 enters shutdown and can be re-enabled by turning off-on en pin. dimming control for led lighting en is also used as a digital input allowing led brightness control with a logic-level pwm signal applied directly to en. the frequency range is from 200hz to 200khz, while 0% duty cycle corresponds to zero current and 100% duty cycle corresponds to full current. the error amplifier and compensation capacitor form a lowpass filter, so the pwm dimming results in dc current to the leds without any additional rc filters. the pwm signal must be applied after soft-start finished. under-voltage lock-out the under voltage lock-out circuit is adopted as a voltage detector and always monitors the supply voltage (v dd ) while en at logic high. while power-on, the chip is kept in shutdown mode till the v dd rises to higher than 2.5v (max). while power-off, the chip does not leave operating mode till v dd falls to less than 2.2v(min). pwm logic pre- regulator power on & uvlo + - + soft start/short circuit protection - + 1.8v - + error amplifier 1mhz osc pwm dimming timer bandgap reference oc vdd en fb comp gnd ext por cmp
4 RT9288A www.richtek.com ds9288a-02 april 2011 absolute maximum ratings (note 1) z supply input voltage, v dd ------------------------------------------------------------------------------------------- ? 0.3v to 16v z en, ext pins ----------------------------------------------------------------------------------------------------------- ? 0.3v to v dd + 0.3v z fb, comp pins ------------------------------------------------------------------------------------------------------- ? 0.3v to 7v z power dissipation, p d @ t a = 25 c sot-23-6 ---------------------------------------------------------------------------------------------------------------- 0.455w z package thermal resistance (note 2) sot-23-6, ja ----------------------------------------------------------------------------------------------------------- 220 c/w z lead temperature (soldering, 10 se c.) --------------------------------------------------------------------------- 260 c z junction temperature ------------------------------------------------------------------------------------------------- 150 c z storage temperature range ---------------------------------------------------------------------------------------- ? 65 c to 150 c z esd susceptibility (note 3) hbm (human body mode) ------------------------------------------------------------------------------------------ 2kv mm (ma chine mode) -------------------------------------------------------------------------------------------------- 200v recommended operating conditions (note 4) z supply input voltage, v dd ------------------------------------------------------------------------------------------- 3v to 13.5v z junction temperature range ---------------------------------------------------------------------------------------- ? 40 c to 125 c z ambient temperature range ---------------------------------------------------------------------------------------- ? 40 c to 85 c electrical characteristics to be continued (v dd = 5v, t a = 25 c, unless otherwise specified) parameter symbol test conditions min typ max unit power-on reset operating supply voltage range v dd normal operation 3 5 13.5 v under voltage lock out uvlo v dd rising 2.2 -- 2.5 v supply current in pwm mode i pwm v fb = v ref + 0.1v -- 2 -- ma shutdown current i shdn v en = 0v -- 1 10 ua sawtooth generator oscillation frequency f os c 0.8 1 1.2 mhz frequency stab ility v dd = 3v to 13.5v -- 2 10 % maximum duty cycle 85 90 95 % error amplifier trans-conductance gm -- 60 -- ua/v feedback voltage v fb -- 0.5 -- v feedback line regulation v dd = 3v to 13.5v -- 5 -- mv maximum output voltage v fb_max v comp = v fb = low -- 2.4 -- v minimum output voltage v fb_min v comp = v fb = high -- 0.05 -- v output s ource current v comp = 0.7v, v fb = low -- 20 -- ua output sink current v comp = 0.7v, v fb = high -- 20 -- ua soft start & short circuit unit soft-start ramp time 5 10 20 ms
5 RT9288A ds9288a-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 possibility to affect device reliability. note 2. ja is measured in the natural convection at t a = 25 c on a low effective thermal conductivity test board of jedec 51-3 thermal measurement standard. 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. guarantee by design. parameter symbol test conditions min typ max unit output driver on resistance (p-mosfet) r ds(on)_p -- 30 60 on resistance (n-mosfet) r ds(on)_n -- 20 40 output rising/falling time (note 5) c l = 1000pf, v fb = low -- 100 -- ns logic en pin low voltage v il -- -- 0.5 v en pin high voltage v ih 1.8 -- v dd v
6 RT9288A www.richtek.com ds9288a-02 april 2011 typical operating characteristics efficiency vs. output current 0 10 20 30 40 50 60 70 80 90 100 0 200 400 600 800 1000 output current (ma) efficiency (%) v in = 12v, v out = 15v, c out = 10uf, l = 10uh efficiency vs. output current 0 10 20 30 40 50 60 70 80 90 100 0 100 200 300 400 500 600 700 output current (ma) efficiency (%) v in = 12v, v out = 30v, c out = 10uf, l = 10uh output voltage vs. output current 14.95 14.98 15.01 15.04 15.07 15.10 0 100 200 300 400 500 600 700 800 900 100 0 output current (ma) output voltage (v) v in = 12v, v out = 15v, c out = 10uf, l = 10uh 1000 output voltage vs. output current 30.20 30.22 30.24 30.26 30.28 30.30 30.32 30.34 30.36 30.38 30.40 0 100 200 300 400 500 600 700 output current (ma) output voltage (v) v in = 12v, v out = 30v, c out = 10uf, l = 10uh output voltage vs. input voltage 15.60 15.65 15.70 15.75 15.80 15.85 15.90 15.95 16.00 16.05 16.10 16.15 3 5.1 7.2 9.3 11.4 13.5 input voltage (v) output voltage (v) v out = 15.9v, i out = 1ma, c out = 10uf, l = 10uh supply current vs. input voltage 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 3 5.1 7.2 9.3 11.4 13.5 input voltage (v) supply current (ma) duty = 50%, f = mhz
7 RT9288A ds9288a-02 april 2011 www.richtek.com v fb v s. input voltage 0.5025 0.5035 0.5045 0.5055 0.5065 0.5075 3 5.1 7.2 9.3 11.4 13.5 input voltage (v) v fb (v) t a = 25 c maximum duty vs. temperature 85 86 87 88 89 90 91 92 93 94 95 -50 -25 0 25 50 75 100 125 temperature maximum duty (%) ( c) v in = 5v frequency vs. temperature 700 800 900 1000 1100 1200 1300 -50 -25 0 25 50 75 100 125 temperature frequency (khz) v in = 5v, c out = 10uf, l = 10uh ( c) v fb vs. temperature 0.495 0.497 0.499 0.501 0.503 0.505 -50 -25 0 25 50 75 100 125 temperature v fb (v) ( c) v in = 5v frequency vs. input voltage 900 920 940 960 980 1000 1020 1040 1060 1080 1100 3 5.1 7.2 9.3 11.4 13.5 input voltage (v) frequency (khz) c out = 10uf, l = 10uh, t a = 25 c supply current vs. temperature 0 0.5 1 1.5 2 2.5 -50 -25 0 25 50 75 100 125 temperature supply current (ma) v in = 5v, duty = 50%, f = mhz ( c)
8 RT9288A www.richtek.com ds9288a-02 april 2011 v in = 5v, v out = 12v, l = 4.7uh, i out = 100ma stability time (500ns/div) v out_ac (50mv/div) v lx (10v/div) i load (0.5a/div) v in = 5v, i out = 100ma disable operating time (5ms/div) v out (10v/div) v en (5v/div) v comp (2v/div) v in = 5v, i out = 100ma power off time (5ms/div) v out (10v/div) v in (5v/div) v lx (10v/div) v in = 5v, i out = 100ma enable operating time (5ms/div) v out (10v/div) v en (5v/div) v comp (2v/div) v in = 5v, i out = 100ma power on v out (10v/div) v in (5v/div) v lx (10v/div) time (5ms/div) i led vs. duty 0 5 10 15 20 25 0 20406080100 duty (%) i led (ma) 200hz 2khz 20khz 200khz
9 RT9288A ds9288a-02 april 2011 www.richtek.com v in = 12v, l = 4.7uh, duty = 50% pwm dimming by en time (2.5ms/div) v en (5v/div) v comp (0.5v/div) v ext (10v/div) f pwm = 2khz, c comp = 100nf v in = 5v, v out = 12v, l = 4.7uh, i out = 200ma stability time (500ns/div) v out_ac (100mv/div) v lx (10v/div) i load (0.5a/div) v in = 5v, v out = 12v, l = 4.7uh, i out = 300ma stability time (500ns/div) v out_ac (100mv/div) v lx (10v/div) i load (1a/div)
10 RT9288A www.richtek.com ds9288a-02 april 2011 application information the RT9288A is a boost controller for dc to dc conversion. the main switch of the power stage can stand significant current that is greater than the internal main switch. there is no significant power dissipated in the RT9288A, therefore the thermal performance could be excellent. for the RT9288A, determine the maximum input current is the first step of the design procedure. inductor selection for the inductor selection, the inductance value depends on the maximum input current. generally the inductor ripple current range is 20% to 40% of the maximum input curr ent. take 40% as an example, the value can be calculated as follows : out out(max) in(max) in ripple in(max) vi i = v i= 0.4i where is the efficiency, i in(max) is the maximum input current and i ripple is the inductor ripple current. beside, the input peak current is the maximum input current plus half of the inductor ripple current. peak in(max) i= 1.2i note that the saturated current of inductor must be greater than i peak . the inductance value can be eventually determined as follows : 2 in out in 2 out out(max) osc (v ) (v v ) l = 0.4 (v ) i f ? where f osc is the switching frequency. consider the system performance, a shielded inductor is preferred to avoid emi issue. figure 4. the waveform of the inductor current diode selection schottky diode is a good choice for an asynchronous boost converter due to the small forward voltage. however, power dissipation, reverse voltage rating and pulsating peak current are the important parameters of schottky diode consideration. it is recommended to choose a suitable diode whose reverse voltage rating is greater than the maximum output voltage. input capacitor selection low esr ceramic capacitors are recommended for input capacitor applications. low esr will effectively reduce the input ripple voltage caused by switching operation. a 10uf is sufficient for most applications. nevertheless, this value can be decreased with lower output current requirement. another consideration is the voltage rating of input capacitor must be greater than the maximum input voltage. output capacitor selection output ripple voltage is an important index for estimating the performance. this portion consists of two parts, one is the product of (i in ? i out ) and esr of the output capacitor, another part is formed by charging and discharging process of output capacitor. refer to figure 5, evaluate v out1 by ideal energy equalization. according to the definition of q that is calculated as follows : ?? ???? +? +?? ???? ?? ???? ?? out out in l in l in out out1 out osc 11 1 q = i i i i i i 22 2 v 1 = c v vf where t s is the inverse of switching frequency and the i l is the inductor ripple current. move c out to left side to estimate the value of v out1 as : out out1 out osc di v = cf finally, the output ripple voltage can be determined as : () ?+ out out in out out osc di v = i i esr cf 0a i l i out(max) i peak i in(max ) i ripple t on (1) (2) (3) (4) (6) (7) (5)
11 RT9288A ds9288a-02 april 2011 www.richtek.com figure 5. the output ripple voltage without the contribution of esr main switch selection the RT9288A uses an n-mosfet as the main switch to achieve power conversion. the main switch stays in two states in the operation, one is the on state and the other is the off state. the potential of switching point, lx, is 0v in the on state. nevertheless, the potential of lx rises to output voltage plus the forward voltage of d 1 in the off state, this potential is the highest voltage in the boost converter. thus, the absolute v ds rating of the main switch must be greater than this voltage to prevent main switch damage in the off state or reliability problem. another key parameter of main switch is the maximum continuous drain curre nt. for a safety design, it is important to choose a maximum continuous drain current at two times the maximum input current. energy saving is the trend in recent years. therefore, design a high efficiency system is the important course. conduction loss and switching loss play important roles for the efficiency in heavy load and light load respectively. main switch with a small on resistance leads to lower conduction loss, however, it also means a greater gate capacitance. great gate capacitance prolongs rising and falling transition in lx, t 1 and t 2 . i l and v lx produce the main switching loss during t 1 and t 2 . thus, choose a main switch with proper gate capacitance could reduce switching loss. time time inductor current output current output ripple voltage (ac) (1-d)t s v out1 i l input current v th time time time t 1 i l t 2 v out + v d1 i out v in v ext v lx i l figure 6. the waveforms of ext, lx and inductor current related to the switching loss loop compensation it is easy to compensate the loop stability for the RT9288A's application in led driving. compensation network only contains a capacitor between comp pin and gnd as shown in figure 1. the best criterion to optimize the loop compensation is by inspecting the transient response and adjusting the compensation network. layout consideration the pcb layout is a very important issue for switching converter circuits design. there are some recommended layout guidelines that are shown as follows : ` the power components m1, l1, d1, c in and c out should be placed as close to the ic as possible to reduce the ac current loop. the connections between power components must be short and wide as possible due to large current stream flowing through these traces during operation. ` the function of c1 is to regulate v dd . place c1 close to pin 1 is necessary. ext vdd v in c in i l l1 d1 fb c out v out r f1 r f2 RT9288A lx m1
RT9288A 12 www.richtek.com ds9288a-02 april 2011 ` r f1 and r f2 formed a voltage divider to set correct output voltage. pin 3 is connected to the branch of voltage divider and is a very sensitive point, placed this trace short and wide as possibly and far away from the switching point to avoid perturbation. ` pin 4 is the compensation point for system stability. place the compensation components as close to pin 4 as possibly, no matter the compensation is r c or capacitance. note that, the gnd of the compensation components should be connected with pin 5. then, short it to system ground by via or trace. this will provide a clean reference for the ic. vdd en fb ext comp v in c in v out gnd c out c c gnd 2 3 6 4 1 5 c1 r1 l1 r c r f2 r f1 gnd d1 m1 gnd figure 7. sketch map of pcb layout.
RT9288A 13 ds9288a-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 inf ringement of the applications should be guaranteed by users when integrating richtek products into any application. no legal responsibility for any said applications i s assumed by 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 a a1 e b b d c h l sot-23-6 surface mount package dimensions in millimeters dimensions in inches symbol min max min max a 0.889 1.295 0.031 0.051 a1 0.000 0.152 0.000 0.006 b 1.397 1.803 0.055 0.071 b 0.250 0.560 0.010 0.022 c 2.591 2.997 0.102 0.118 d 2.692 3.099 0.106 0.122 e 0.838 1.041 0.033 0.041 h 0.080 0.254 0.003 0.010 l 0.300 0.610 0.012 0.024

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