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AAT1235 da ta sheet high efficiency white led drivers for backlight and ke ypad 1 skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202110a ? skyworks proprietary information ? products and product information are subject to change without notice. ? june 27, 2012 general description the AAT1235 is a highly integrated, high efficiency power solution for white led backlight and keypad backlights in mobile/portable devices. it is based on a switching boost converter which steps up the single-cell lithium-ion/poly - mer battery voltage to drive 5 strings of series-connected white leds with precision current regulation. the AAT1235 is capable of driving a total of four leds per channel. the boost converter can produce an output drive of up to 24v at 100ma. the high switching frequency (up to 2mhz) provides fast response to load transients and allows the use of small external components. a fully integrated control circuit simplifies the design and reduc - es total solution size. skyworks advanced simple serial control? (as 2 cwire?) serial digital input is used to individually turn each out - put sink on/off and adjust the led current by group. unlike conventional pulse width modulation (pwm) con - trol of led brightness, the AAT1235 drives the leds with constant, non-pulsating current. a similar device is also available with an i 2 c two-wire interface; please see the aat1236 datasheet. the AAT1235 is available in a pb-free, thermally-enhanced 16-pin 3x4mm tdfn package and is specified for opera - tion over the -40c to +85c temperature range. features? input supply voltage range: 2.7v to 5.5v ? maximum boost output drive: up to 24v at 100ma ? up to 85% efficient operation ? up to 2mhz switching frequency with small inductor ? user-programmable full-scale led current, up to 30ma ? single-wire as 2 cwire serial interface ? five addressable registers ? independent led current control by group ? backlight group b1-b2, 16 settings ? auxiliary group a1-a3, 16 settings ? independent led on/off control ? fast, 1mhz serial interface ? non-pulsating, high-performance led current drive for uniform illumination ? 10% absolute accuracy ? 2% channel-to-channel matching ? over-voltage and over-temperature protection ? automatic soft-start minimizes large inrush current at startup ? available in 3x4mm tdfn34-16 package applications ? digital still cameras (dscs) ? keypad backlight ? large panel displays ? mobile handsets ? personal media players ? pdas and notebook pcs ? white led backlight typical application en/set agnd vin ld 1 =2 .2 h c r2187k ? r312.1k ? r1 22.6k ? c in 2.2f out 2.2 f input 2.7v . v aat123 s 1 upt2v gnd rset 2 a1 a2 a3 as 2 ccnt ov kt led p rgled lin in downloaded from: http:///
AAT1235 da ta sheet high efficiency white led drivers for backlight and ke ypad 2 skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202110a ? skyworks proprietary information ? products and product information are subject to change without notice. ? june 27, 2012 pin descriptions pin # symbol function 1 vin input supply for the converter. connect a 2.2f or larger ceramic capacitor from vin to gnd. 2 ov boost output over voltage detect pin. use resistor divider to set the circuits external over-voltage protection. see applications information for details. 3 en/set as 2 cwire control or enable pin. 4 b1 backlight current sink 1. connect the cathode of the last led in the string to b1. 5 b2 backlight current sink 2. connect the cathode of the last led in the string to b2. 6 rset led current set resistor. a 22.6k w resistor from rset to agnd sets the maximum led current in a1-a3 and b1-b2 to 20ma. 7 in input bias supply for the internal circuitry. connect in to vin directly at the AAT1235. 8 gnd ground for the boost converter. connect gnd to agnd at a single point as close to the AAT1235 as practical. 9 sw boost converter switching node. a 2.2h inductor, connected between sw and lin, sets the boost converter's switching frequency. 10, 11 n/c not connected. 12 agnd ground pin. connect agnd to gnd at a single point as close to the AAT1235 as practical. 13 a3 auxiliary current sink 3. connect the cathode of the last led in the string to a3. 14 a2 auxiliary current sink 2. connect the cathode of the last led in the string to a2. 15 a1 auxiliary current sink 1. connect the cathode of the last led in the string to a1. 16 lin switched power input. connect lin to the external power inductor. ep exposed paddle (bottom) connected internally to sw. connect to sw or leave loating. pin configuration tdfn34-16 (top view) en/set b1b2 vin ov 3 rset in gnd a2a3 agnd lina1 n/c n/c sw 4 5 1 2 6 7 8 14 13 12 16 15 11 10 9 downloaded from: http:///
AAT1235 da ta sheet high efficiency white led drivers for backlight and ke ypad 3 skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202110a ? skyworks proprietary information ? products and product information are subject to change without notice. ? june 27, 2012 1. stresses above those listed in absolute maximum ratings may cause permanent damage to the device. functional operation at conditions other than the operating conditions specified is not implied. only one absolute maximum rating should be applied at any one time. 2. mounted on an fr4 circuit board. 3. derate 20mwc above 40c ambient temperature. absolute maximum ratings 1 t a = 25c unless otherwise noted. symbol description value units vin, in input voltage -0.3 to 6.0 v sw switching node 28 v en/set, bx, ax, rset, ov, lin maximum rating v in + 0.3 v t s storage temperature range -65 to 150 c t j operating temperature range -40 to 150 c t lead maximum soldering temperature (at leads, 10 sec) 300 c thermal information 2 symbol description value units q ja thermal resistance 50 c/w p d maximum power dissipation 3 2 w downloaded from: http:///
AAT1235 da ta sheet high efficiency white led drivers for backlight and ke ypad 4 skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202110a ? skyworks proprietary information ? products and product information are subject to change without notice. ? june 27, 2012 1. the AAT1235 is guaranteed to meet performance specifications over the -40c to +85c operating temperature range is assured by design, characterization, and correlation with statistical process controls. electrical characteristics 1 v in = 3.6v; c in = 2.2f;t a = -40c to +85c, unless otherwise noted. typical values are at t a = 25c. symbol description conditions min typ max units power supply v in input voltage range 2.7 5.5 v v out(max) maximum output voltage 24 v v uvlo uvlo threshold v in rising 2.7 v hysteresis 150 mv v in falling 1.8 v i cc operating current (no switching) b1 = b2 = a1 = a2 = a3 = 1.2v, 2ma setting, r set = 226k w 300 a i shdn(max) in pin shutdown current en = gnd 1.0 a i ox maximum continuous output current v o = 24v 100 ma i dx current sink accuracy r set = 22.6k w 18 20 22 ma i dx-matching current matching between any sink channels r set = 22.6k w , a1 = a2 = a3 = b1 = b2 = 0.4v 2 5 % t ss soft-start time from enable to output regulation; v fb = 300mv 300 s v ov ovp threshold voltage v out rising 1.1 1.2 1.3 v ovp threshold hysteresis 100 mv r ds(on)n low side switch on resistance i out = 100ma 80 m w r ds(on)in input disconnect switch i out = 100ma 200 m w i set current set ratio i sink /i rset , v rset = 0.6v 760 a/a i limit input switch current limit 1.2 a en/set input v en(l) enable threshold low 0.4 v v en(h) enable threshold high 1.4 v t en/set lo en/set low time 0.3 75 s t en/set hi en/set high time 75 s t off en/set off timeout 500 s t lat en/set latch timeout 500 s i en/set en/set input leakage v en/set = v in = 5v -1 1 a thermal protection t j-th t j thermal shutdown threshold 140 c t j-hys t j thermal shutdown hysteresis 15 c downloaded from: http:///
AAT1235 da ta sheet high efficiency white led drivers for backlight and ke ypad 5 skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202110a ? skyworks proprietary information ? products and product information are subject to change without notice. ? june 27, 2012 typical characteristics efficiency vs. led current (group b on; group a off) output led current (ma) efficiency (%) 76 77 78 79 80 81 82 83 1.6 3.9 6.2 8.5 10.8 13.1 15.4 17.7 20 v in = 4.2v v in = 3.6v v in = 5v efficiency vs. led current (group b off; group a on) output led current (ma) efficiency (%) 77 78 79 80 81 82 83 84 1.6 3.9 6.2 8.5 10.8 13.1 15.4 17.7 20 v in = 4.2v v in = 3.6v v in = 5v efficiency vs. led current (group a and b on) output led current (ma) efficiency (%) 79 80 81 82 83 84 85 86 1.6 3.9 6.2 8.5 10.8 13.1 15.4 17.7 20 v in = 5v v in = 4.2v v in = 3.6v led current accuracy vs. supply voltage supply voltage (v) accuracy (%) -3 -2 -1 0 1 2 3 2.7 3.1 3.4 3.8 4.1 4.5 4.8 5.2 5.5 i b1, b2, a1, a2, a3 led current vs. supply voltage supply voltage (v) led current (ma) 18.4 18.6 18.8 19.0 19.2 19.4 19.6 19.8 20.0 2.7 3.1 3.4 3.8 4.1 4.5 4.8 5.2 5.5 i b2 i b1 i a3 i a2 i a1 shutdown current vs. supply voltage and temperature supply voltage (v) shutdown current (a) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 85 c 25 c -40 c downloaded from: http:///
AAT1235 da ta sheet high efficiency white led drivers for backlight and ke ypad 6 skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202110a ? skyworks proprietary information ? products and product information are subject to change without notice. ? june 27, 2012 typical characteristics led current vs. temperature (all channels = 20ma) temperature ( c) led current (ma) 19.0 19.2 19.4 19.6 19.8 20.0 20.2 20.4 20.6 20.8 21.0 21.2 -40 -15 10 35 60 85 i a1 i a2 i a3 i b1 i b2 led current accuracy vs. temperature (all channels = 20ma) temperature ( c) led current accuracy (%) -6 -5 -4 -3 -2 -1 0 1 2 3 4 -40 -15 10 35 60 85 i a2 i a3 i b1 i b2, a1 shutdown operation (all channels) time (50s/div) enable (v) i group_a (ma) i group_b (ma) i inductor (a) 0 50 0 50 0 0.5 0 5 output ripple (all channels = 20ma) time (200ns/div) output voltage (top) (v) switching node (middle) (v) inductor current (bottom) (a) 13.5 14.0 14.5 0.0 0.5 1.0 16v 0v output ripple (all channels = 10ma) time (200ns/div) output voltage (top) (v) switching node (middle) (v) inductor current (bottom) (a) 12.5 13.0 13.5 0.0 0.5 14v 0v switching frequency vs. supply voltage and temperature supply voltage (v) switching frequency (mhz) 0.0 0.5 1.0 1.5 2.0 2.5 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 -40 c 25 c +85 c downloaded from: http:///
AAT1235 da ta sheet high efficiency white led drivers for backlight and ke ypad 7 skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202110a ? skyworks proprietary information ? products and product information are subject to change without notice. ? june 27, 2012 typical characteristics line transient (all channels = 20ma) time (50s/div) input voltage (top) (v) output voltage (bottom) (v) 3.0 3.5 4.0 4.5 13.8 13.9 14.0 14.1 14.2 enable threshold low vs. supply voltage and temperature supply voltage (v) enable threshold low (v) 0.6 0.7 0.8 0.9 1.0 1.1 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 +85 c -40 c 25 c enable threshold high vs. supply voltage and temperature supply voltage (v) enable threshold high (v) 0.7 0.8 0.9 1.0 1.1 1.2 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5. 5 +85c -40c 25c input disconnect switch resistance vs. supply voltage and temperature supply voltage (v) r ds(on)in (m ) 140 160 180 200 220 240 260 280 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 +120 c +100 c +85 c +25 c low side switch on resistance vs. supply voltage and temperature supply voltage (v) r ds(on)n (m ) 40 60 80 100 120 140 160 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6. 0 +120 c +100 c +25 c +85 c soft start operation (all channels = 20ma) time (200s/div) en/set (v) v out (v) i inductor (a) 0 10 20 0 0.5 0 2 downloaded from: http:///
AAT1235 da ta sheet high efficiency white led drivers for backlight and ke ypad 8 skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202110a ? skyworks proprietary information ? products and product information are subject to change without notice. ? june 27, 2012 typical characteristics en/set off timeout vs. supply voltage and temperature supply voltage (v) en/set off timeout (t off ) (s) -40 c +85 c +25 c 50 100 150 200 250 300 350 400 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 en/set latch timeout vs. supply voltage and temperature supply voltage (v) en/set latch timeout (t lat ) (s) -40 c +85 c +25 c 50 100 150 200 250 300 350 400 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 transition of led current (group b = 1.8ma; group a = 20ma to 1.8ma) time (250s/div) en/set (top) (v) i a1 (middle) (a) i b1 (bottom) (a) 0 2 4 0 0.02 0 transition of led current (group b = 1.8ma to 20ma; group a = 20ma) time (250s/div) en/set (top) (v) i a1 (middle) (a) i b1 (bottom) (a) 0 2 4 00 0.02 0.02 transition of led current (group b = 20ma; group a = 20ma to 1.8ma) time (250s/div) en/set (top) (v) i a1 (middle) (a) i b1 (bottom) (a) 0 2 4 0 0.020 0.02 downloaded from: http:///
AAT1235 da ta sheet high efficiency white led drivers for backlight and ke ypad 9 skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202110a ? skyworks proprietary information ? products and product information are subject to change without notice. ? june 27, 2012 functional description the AAT1235 consists of a controller for the step-up switching converter and its power switch, and five regu - lated current sinks each programmable at 16 levels into two groups, which can be turned on/off individually. an external schottky diode, a power inductor, an output capacitor, and a resistor divider are required to complete the solution. the AAT1235s boost controller is designed to deliver 100ma up to 24v. the aat1236 is capable of driving a total of five channels divided into two groups with four white leds connected in series at each channel. the output load current can be programmed by the cur - rent sink magnitudes. as 2 cwire programming allows independent control of two current sink groups (a1 to a3 and b1 to b2) and control on/off with a different con - figuration on each channel. unused sink channel(s) must be connected to agnd to ensure proper function of the AAT1235. control loop the AAT1235 provides the benefits of current mode con - trol with a simple hysteretic output current loop provid - ing exceptional stability and fast response with minimal design effort. the device maintains exceptional constant current regulation, transient response, and cycle-by- cycle current limit without additional compensation com - ponents. the AAT1235 modulates the power mosfet switching current to maintain the programmed sink current through each channel. the sink voltage at each channel is monitored and the controller provides direct feedback in order to maintain the desired led current. the switching cycle initiates when the n-channel mosfet is turned on and current ramps up in the inductor. the on interval is terminated when the inductor current reaches the programmed peak current level. during the off interval, the input current decays until the lower threshold, or zero inductor current, is reached. the lower current is equal to the peak current minus a preset functional block diagram boost converter control vin lin in sw as 2 cwire control rom v (a1, a2, a3) v ref v (b1, b2) v ref b1 a3 a2 a1 b2 a rom m crrent astent rse an ov n ense a a a a downloaded from: http:///
AAT1235 da ta sheet high efficiency white led drivers for backlight and ke ypad 10 skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202110a ? skyworks proprietary information ? products and product information are subject to change without notice. ? june 27, 2012 hysteresis threshold, which determines the inductor rip - ple current. peak current is adjusted by the controller until the desired led output current level is met. the magnitude of the feedback error signal determines the average input current. therefore, the AAT1235 con - troller implements a programmed current source con - nected to the output capacitor, parallel with the led channels. there is no right-half plane zero, and loop stability is achieved with no additional compensation components. the controller responds by increasing the peak inductor current, resulting in higher average cur - rent in the inductor and led channels. under light load conditions, the inductor off interval current goes below zero and the boost converter enters discontinuous mode operation. further reduction in the load current results in a corresponding reduction in the switching frequency. the AAT1235 provides a pulsed frequency operation which reduces switching losses and maintains high efficiency under light load conditions. operating frequency varies with changes in the input volt - age, output voltage, and inductor size. once the boost converter has reached continuous mode, further increas - es in the led current will not significantly change the operating frequency. a small 2.2h (20%) inductor is selected to maintain high frequency switching (up to 2mhz) and high efficiency operation for outputs up to 24v. soft start / enable the input disconnect switch is activated when a valid supply voltage is present and the en/set pin is strobed high. slew rate control on the input disconnect switch ensures minimal inrush current as the output voltage is charged to the input voltage, prior to switching of the n-channel power mosfet. a monotonic turn-on is guar - anteed by the built-in soft-start circuitry, which elimi - nates output current overshoot across the full input volt - age range and all load conditions. current limit and over-temperature protection the switching of the n-channel mosfet terminates when a current limit of 1.5a (typical) is exceeded. this mini - mizes power dissipation and component stresses under overload and short-circuit conditions. switching resumes when the current decays below the current limit. thermal protection disables the AAT1235 when internal power dissipation becomes excessive, as it disables both mosfets. the junction over-temperature threshold is 140c with 15c of temperature hysteresis. the output voltage automatically recovers when the over-tempera - ture fault condition is removed. over-voltage protection over-voltage protection prevents damage to the AAT1235 during open-circuit causing high output voltage condi - tions. an over-voltage event is defined as a condition where the voltage on the ov pin exceeds the over-volt - age threshold limit (v ov = 1.2v typical). when the volt - age on the ov pin has reached the threshold limit, the converter stops switching and the output voltage decays. switching resumes when the voltage on the ov pin drops below the lower hysteresis limit, maintaining an average output voltage between the upper and lower ov thresh - olds multiplied by the resistor divider scaling factor. under-voltage lockout internal bias of all circuits is controlled via the vin input. under-voltage lockout (uvlo) guarantees sufficient v in bias and proper operation of all internal circuitry prior to soft start. constant current output level settings and as 2 cwire serial interface the led current sink level of each group and the on/off status of each channel is controlled by skyworks as 2 cwire serial digital input. since each current sink is programmable, no pwm or additional control circuitry is needed to control led brightness. this feature greatly reduces the burden on a microcontroller or system ic to manage led or display brightness, allowing the user to set it and forget it. with its high-speed serial interface (1mhz data rate), the input sink current can be changed quickly and easily. also the non-pulsating led current reduces system noise and improves led reliability. as 2 cwire relies on the number of rising edges of the en/ set pin to address and load the registers. as2cwire latches data or address after the en/set pin has been held high for time t lat (500s). address or data is dif - ferentiated by the number of en/set rising edges. since the data registers are 4 bits each, the differentiating number of pulses is 24 or 16, so that address 0 is signi - fied by 17 rising edges, address 1 by 18 rising edges, and so forth. data is set to any number of rising edges between 1 and including 16. a typical write protocol is a burst of en/set rising edges, signifying a particular address, followed by a pause with en/set held high for the t lat timeout period, a burst of rising edges signifying downloaded from: http:///
AAT1235 da ta sheet high efficiency white led drivers for backlight and ke ypad 11 skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202110a ? skyworks proprietary information ? products and product information are subject to change without notice. ? june 27, 2012 data, and a t lat timeout for the data registers. once an address is set, then multiple writes to that address are allowed where only data is issued. when en/set is held low for an amount of time longer than t off (500s), the AAT1235 enters into shutdown mode and draws less than 1a from the input. data and address registers are cleared (reset to 0) during shutdown. address en/set rising edges data register 0 17 b1-b2 current 1 18 a1-a3 current 2 19 b1-b2 on/off 3 20 a1-a3 on/off 4 21 a1-a3, b1-b2 on table 1: as 2 cwire serial interface addressing. address 0 and 1: led brightness control outputs a1, a2, a3, b1, b2 are each capable of sinking up to 30ma. the maximum current can be programmed by an external resistor at the rset pin. it is suggested to connect up to four white leds in series for each chan - nel and to keep the same number of leds in each chan - nel. outputs b1 and b2 are intended to drive the white leds for the backlight in a phone, while a1:a3 can supply the keypad leds. for large displays, all five outputs can be used. data all outputs (%) 1 100 2 84 3 71 4 60 5 51 6 43 7 35 8 31 9 26 10 21 11 18 12 15 13 13.5 14 12.0 15 10.5 16 9.0 table 2: address 0 and 1 for current setting as percentage of the maximum level set by r set . address 2: led group b on/off control data b1 b2 1 off off 2 off on 3 on off 4 on on table 3: address 2 for group b on/off state. address 3: led group a on/off control data a1 a2 a3 1 off off off 2 off off on 3 off on off 4 off on on 5 on off off 6 on off on 7 on on off 8 on on on table 4: address 3 for group a on/off state. address 4: led groups a & b on control b2 b1 a1 a2 a3 on on on on on table 5: address 4 for turning group a and b all on. address 4 turns on all channels with the current pro - grammed by addresses 0 and 1. no data needs to be provided. downloaded from: http:///
AAT1235 da ta sheet high efficiency white led drivers for backlight and ke ypad 12 skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202110a ? skyworks proprietary information ? products and product information are subject to change without notice. ? june 27, 2012 application information channel disable tie all unused channels to agnd. on start-up, these channels will be automatically disabled. led selection although the AAT1235 is specifically designed to drive white leds, the device can also be used to drive most types of leds with forward voltages ranging between 2.0v and 4.7v. since the a1, a2, a3, and b1, b2 input current sinks are matched with low voltage dependence, the led-to-led brightness will be matched regardless of the individual led forward voltage (v f ) levels. in some instances, it may be necessary to drive high-v f type leds. the low dropout (~0.1v @ 20ma i led ) current sinks in the AAT1235 make it capable of driving leds with forward voltages as high as 4.7v from an input sup - ply as low as 3.0v. led outputs a1-a3 and b1-b2 can be combined to drive high-current leds without complica - tion, making the AAT1235 a perfect application for large lcd display backlighting and keypad led applications. constant current setting the led current is controlled by the r set resistor. for maximum accuracy, a 1% tolerance resistor is recom - mended. table 6 shows the r set resistor value for AAT1235 for various led full-scale current levels. i led (ma) r set (k w ) 30 14.7 25 17.4 20 22.6 15 29.4 10 44.2 5 93.1 table 6: maximum led current and r set resistor values (1% resistor tolerance). maximum led current per channel versus r set value is shown in figure 1. led current (ma) r set (k ) 0 5 10 15 20 25 30 35 10 36 62 88 114 140 166 192 218 244 270 figure 1: led current vs. r set values. as 2 cwire serial interface timing address data 1 en/set 2 17 18 address data reg 1 data reg 2 1 2 . . . n 16 01 00 n t hi t lo t lat t lat downloaded from: http:///
AAT1235 da ta sheet high efficiency white led drivers for backlight and ke ypad 13 skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202110a ? skyworks proprietary information ? products and product information are subject to change without notice. ? june 27, 2012 over-voltage protection the over-voltage protection circuit consists of a resistor network connected from the output voltage to the ov pin (see figure 2). this over voltage protection circuit prevents damage to the device when one of the five channels has an open led circuit. the AAT1235 contin - ues to operate; however, the led current in the remain - ing channels is no longer regulated and the actual led current will be determined by the externally programmed over-voltage protection threshold, the inductor value, and the switching frequency. the resistor divider can be selected such that the over- voltage threshold occurs prior to the output reaching 24v (v out(max) ). the value of r3 should be selected from 10k w to 20k w to minimize switching losses without degrading noise immunity: r 2 = r 3 - 1 v out(protection) v ov r2 r3 c out vout AAT1235 ov gnd figure 2: over-voltage protection circuit. if four leds are connected in series on one channel, the total v f from the wleds could be as high as 18.8v. therefore, using r3 = 12.1k w and setting v out(protection) = 20v is recommended. selecting a 1% resistor, this results in r2 = 187k w (rounded to the nearest standard 1% value). it is always recommended to use the same number of wleds on each channel and set the appropriate over- voltage protection. failure to do so may cause any one of the (5) sink pins to exceed the absolute maximum rating voltage and permanently damage the device in case the channel is disconnected (open circuit failure). examples of over voltage settings for various strings of series-connected leds are shown in table 7. led brightness control the AAT1235 uses as 2 cwire programming to control led brightness. the output current of the AAT1235 can be changed successively to brighten or dim the leds in smooth transitions (i.e., to fade in or fade out) or in dis - crete steps, giving the user complete programmability and real-time control of led brightness. selecting the schottky diode to ensure minimum forward voltage drop and no recov - ery, high voltage schottky diodes are recommended for the AAT1235 boost converter. the output diode is select - ed to maintain acceptable efficiency and reasonable operating junction temperature under full load operating conditions. forward voltage (v f ) and package thermal resistance ( q ja ) are the dominant factors in selecting a diode. the diodes non-repetitive peak forward surge current rating (i fsm ) should be considered for high pulsed load applications, such as camera flash. i fsm rating drops with increasing conduction period. manufacturers data - sheets should be reviewed carefully to verify reliability under peak loading conditions. the diodes published current rating may not reflect actual operating condi - tions and should be used only as a comparative measure between similarly rated devices. 20v rated schottky diodes are recommended for output voltages less than 15v, while 30v rated schottky diodes are recommended for output voltages higher than 15v. number of wleds on each channel total maximum v f (v) v out(protection) (v) r3 = 12.1k w r2 (k w ) 4 18.8 20 187 3 14.1 15 140 2 9.4 10 88.7 table 7: over-voltage protection settings. downloaded from: http:///
AAT1235 da ta sheet high efficiency white led drivers for backlight and ke ypad 14 skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202110a ? skyworks proprietary information ? products and product information are subject to change without notice. ? june 27, 2012 estimating schottky diode power dissipation the switching period is divided between on and off time intervals: = t on + t off 1 f s during the on time, the n-channel power mosfet is conducting and storing energy in the boost inductor. during the off time, the n-channel power mosfet is not conducting. stored energy is transferred from the input battery and boost inductor to the output load through the output diode. duty cycle is defined as the on time divided by the total switching interval: t on t on + t off d = = t on ? f s the maximum duty cycle can be estimated from the relationship for a continuous mode boost converter. maximum duty cycle (d max ) is the duty cycle at minimum input voltage (v in(min) ): v out - v in(min) v out d max = the average diode current during the off time can be estimated: i out 1 - d max i avg(off) = the v f of the schottky diode can be estimated from the average current during the off time. the average diode current is equal to the output current: i avg(tot) = i out the average output current multiplied by the forward diode voltage determines the loss of the output diode: p loss(diode) = i avg(tot) v f = i out v f for continuous led currents, the diode junction tem - perature can be estimated: t j(diode) = t am b + ja p loss(diode) 1 2 3 jp6 led1 rtn vin ag nd 12 nc 11 nc 10 sw 9 gn d 8 rs et 6 b2 5 b1 4 en /s et 3 ov 2 vin 1 a3 13 a2 14 in 7 a1 15 lin 16 led2 led3 led4 led6 led7 led8 led9 0 jp3 0 jp4 0 jp1 0 jp2 0 jp5 0 jp6 0 jp7 vout d1 mbr0530t1 sw_node c22.2f 25v jp80 jp90 jp100 d11d12 d13 d14 d16d17 d18 d19 d21d22 d23 d24 1 2.2 u1t1235tdfn34 1 2 .2todenr4018t2r2m c1080510v 2.2f 7rrm21br7122501 c2080525v 2.2f 7rrm21br7122573 d124osrmwm673oee r312.1 r122.6 c12.2f ese r2187 figure 3: a AAT1235-based high efficiency white led driver schematic. downloaded from: http:///
AAT1235 da ta sheet high efficiency white led drivers for backlight and ke ypad 15 skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202110a ? skyworks proprietary information ? products and product information are subject to change without notice. ? june 27, 2012 external schottky diode junction temperature should be below 110oc, and may vary depending on application and/or system guidelines. the diode q ja can be mini - mized with additional metal pcb area on the cathode. however, adding additional heat-sinking metal around the anode may degrade emi performance. the reverse leakage current of the rectifier must be considered to maintain low quiescent (input) current and high effi - ciency under light load. the rectifier reverse current increases dramatically at elevated temperatures. selecting the boost inductor the AAT1235 controller utilizes hysteretic control and the switching frequency varies with output load and input voltage. the value of the inductor determines the maxi - mum switching frequency of the boost converter. increased output inductance decreases the switching frequency, resulting in higher peak currents and increased output voltage ripple. to maintain 2mhz maximum switching frequency and stable operation, an output inductor selected between 1.5h and 2.7h is recom - mended. a better estimate of d max is possible once v f is known: (v out + v f - v in(min) ) (v out + v f ) d max = where v f is the schottky diode forward voltage. if not known or not provided by the manufacturer, a starting value of 0.5v can be used. manufacturers specifications list both the inductor dc current rating, which is a thermal limitation, and peak inductor current rating, which is determined by the satu - ration characteristics. measurements at full load and high ambient temperature should be performed to ensure that the inductor does not saturate or exhibit excessive temperature rise. the output inductor (l) is selected to avoid saturation at minimum input voltage and maximum output load con - ditions. peak current may be estimated using the follow - ing equation, assuming continuous conduction mode. worst-case peak current occurs at minimum input volt - age (maximum duty cycle) and maximum load. switching frequency (f s ) can be estimated at 500khz with a 2.2h inductor: i out (1 - d max ) d max v in(min) (2 f s l) i peak = + at light load and low output voltage, the controller reduc - es the operating frequency to maintain maximum operat - ing efficiency. as a result, further reduction in output load does not reduce the peak current. minimum peak current can be estimated between 0.5a and 0.75a. at high load and high output voltages, the switching frequency is somewhat diminished, resulting in higher i peak . bench measurements are recommended to confirm actual i peak and to ensure that the inductor does not saturate at maximum led current and minimum input supply voltage. the rms current flowing through the boost inductor is equal to the dc plus ac ripple components. under worst-case rms conditions, the current waveform is critically continuous. the resulting rms calculation yields worst-case inductor loss. the rms current value should be compared against the inductor manufacturers temperature rise, or thermal derating, guidelines: i peak i rms = 3 for a given inductor type, smaller inductor size leads to an increase in dcr winding resistance and, in most cases, increased thermal impedance. winding resistance degrades boost converter efficiency and increases the inductors operating temperature: p loss(inductor) = i rms 2 dcr manufacturer part number rated i f(av) current (a) 1 rated voltage (v) thermal resistance ( q ja , c/w) 1 case diodes, inc. b0520ws 0.50 20 426 sod-323 on semi mbr130lsft 1.00 30 325 sod-123 on semi mbr0530t 0.50 30 206 sod-123 table 8: typical surface mount schottky rectifiers for various output loads. (select t j < 110c in application circuit). downloaded from: http:///
AAT1235 da ta sheet high efficiency white led drivers for backlight and ke ypad 16 skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202110a ? skyworks proprietary information ? products and product information are subject to change without notice. ? june 27, 2012 to ensure high reliability, the inductor case temperature should not exceed 100oc. in some cases, pcb heatsink - ing applied to the lin node (non-switching) can improve the inductors thermal capability. however, as in the case of adding extra metal around the schottkys anode, add - ing extra pcb metal around the AAT1235s sw pin for heatsinking may degrade emi performance. shielded inductors provide decreased emi and may be required in noise sensitive applications. unshielded chip inductors provide significant space savings at a reduced cost compared to shielded (wound and gapped) induc - tors. in general, chip-type inductors have increased winding resistance (dcr) when compared to shielded, wound varieties. selecting the boost capacitors the high output ripple inherent in the boost converter necessitates the use of low impedance output filtering. multi-layer ceramic (mlc) capacitors provide small size and adequate capacitance, low parasitic equivalent series resistance (esr) and equivalent series inductance (esl), and are well suited for use with the AAT1235 boost regulator. mlc capacitors of type x7r or x5r are recommended to ensure good capacitance stability over the full operating temperature range. the output capacitor is selected to maintain the output load without significant voltage droop ( d v out ) during the power switch on interval, when the output diode is not conducting. a ceramic output capacitor between 2.2f and 4.7f is recommended (see table 8). typically, 25v rated capacitors are required for the 24v maximum boost output. ceramic capacitors selected as small as 0805 are available which meet these requirements. mlc capacitors exhibit significant capacitance reduction with applied voltage. output ripple measurements should confirm that output voltage droop and operating stability are within acceptable limits. voltage derating can minimize this factor, but results may vary with pack - age size and among specific manufacturers. output capacitor size can be estimated at a switching frequency (f s ) of 500khz (worst case): i out d max f s ? v out c out = to maintain stable operation at full load, the output capacitor should be selected to maintain d v out between 100mv and 200mv. the boost converter input current flows during both on and off switching intervals. the input ripple current is less than the output ripple and, as a result, less input capacitance is required. manufacturer part number inductance (h) max dc i sat current (a) dcr ( w ) size (mm) lxwxh type sumida cdrh4d22/hp-2r2 2.2 2.50 35 5.0x5.0x2.4 shielded sumida cdr4d11/hp-2r4 2.4 1.70 105 4.8x4.8x1.2 shielded sumida cdrh4d18-2r2 2.2 1.32 75 5.0x5.0x2.0 shielded murata lqh662n2r2m03 2.2 3.30 19 6.3x6.3x4.7 shielded murata lqh55dn2r2m03 2.2 3.20 29 5.0x5.7x4.7 non-shielded taiyo yuden nr4018t2r2 2.2 2.70 60 4.0x4.0x1.8 shielded taiyo yuden nr3015t2r2 2.2 1.48 60 3.0x3.0x1.5 shielded coiltronics sd3814-2r2 2.2 1.90 77 3.8x3.8x1.4 shielded coiltronics sd3114-2r2 2.2 1.48 86 3.1x3.1x1.4 shielded coiltronics sd3112-2r2 2.2 1.12 140 3.1x3.1x1.2 shielded table 9: typical surface mount inductors for various output loads (select i peak < i sat ). manufacturer part number value (f) voltage rating temp co case size murata grm188r60j225ke19 2.2 6.3 x5r 0603 murata grm21br71a225ka01 2.2 10 x7r 0805 murata grm219r61e225ka12 2.2 25 x5r 0805 murata grm21br71e225ka73l 2.2 25 x7r 0805 murata grm21br61e475ka12 4.7 25 x5r 0805 table 10: recommended ceramic capacitors. downloaded from: http:///
AAT1235 da ta sheet high efficiency white led drivers for backlight and ke ypad 17 skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202110a ? skyworks proprietary information ? products and product information are subject to change without notice. ? june 27, 2012 pcb layout guidelines boost converter performance can be adversely affected by poor layout. possible impact includes high input and output voltage ripple, poor emi performance, and reduced operating efficiency. every attempt should be made to optimize the layout in order to minimize para - sitic pcb effects (stray resistance, capacitance, and inductance) and emi coupling from the high frequency sw node. a suggested pcb layout for the AAT1235 boost converter is shown in figures 4 and 5. the following pcb layout guidelines should be considered: 1. minimize the distance from capacitor c1 and c2s negative terminals to the gnd pins. this is espe - cially true with output capacitor c2, which conducts high ripple current from the output diode back to the gnd pins. 2. minimize the distance between l1 to d1 and switch - ing pin sw; minimize the size of the pcb area con - nected to the sw pin. 3. maintain a ground plane and connect to the ic gnd pin(s) as well as the gnd connections of c1 and c2. 4. consider additional pcb metal on d1s cathode to maximize heatsinking capability. this may be neces - sary when using a diode with a high v f and/or ther - mal resistance. 5. do not connect the exposed paddle (bottom of the die) to either agnd or gnd because it is connected internally to sw. connect the exposed paddle to the sw pin or leave floating. figure 4: AAT1235 evaluation board figure 5: AAT1235 evaluation board top side layout. bottom side layout. downloaded from: http:///
AAT1235 da ta sheet high efficiency white led drivers for backlight and ke ypad 18 skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202110a ? skyworks proprietary information ? products and product information are subject to change without notice. ? june 27, 2012 figure 6: exploded view of AAT1235 evaluation board top side layout detailing plated through vias. downloaded from: http:///
AAT1235 da ta sheet high efficiency white led drivers for backlight and ke ypad 19 skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202110a ? skyworks proprietary information ? products and product information are subject to change without notice. ? june 27, 2012 1. xyy = assembly and date code. 2. sample stock is generally held on part numbers listed in bold . 3. the leadless package family, which includes qfn, tqfn, dfn, tdfn and stdfn, has exposed copper (unplated) at the end of the lead terminals due to the manufacturing process. a solder fillet at the exposed copper edge cannot be guaranteed and is not required to ensure a proper bottom solder connection. ordering information package marking 1 part number (tape and reel) 2 tdfn34-16 tkxyy AAT1235irn-t1 skyworks green? products are compliant with all applicable legislation and are halogen-free. for additional information, refer to skyworks de?nition of green? , document number sq04-0074. package information 3 tdfn34-16 3.000 0.050 1.600 0.050 0.050 0.050 0.229 0.051 (4x) 0.850 max 4.000 0.050 3.300 0.050 index area detail "a" top ie otto ie ide ie 0.350 0.100 0.230 0.050 0.450 0.050 detail "a" in 1 indiator (optional) 0.3 all dimensions in millimeters. downloaded from: http:///
AAT1235 da ta sheet high efficiency white led drivers for backlight and ke ypad 20 skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202110a ? skyworks proprietary information ? products and product information are subject to change without notice. ? june 27, 2012 copyright ? 2012 skyworks solutions, inc. all rights reserved. information in this document is provided in connection with skyworks solutions, inc. (skyworks) pr oducts or services. these materials, including the information contained herein, are provided by sky works as a service to its customers and may be used for informational purposes only by the customer. skyworks a ssumes no responsibility for errors or omissions in these materials or the information contained her ein. sky- works may change its documentation, products, services, speciications or product descriptions at any time, without notice. skyworks makes no commitment to update the materials or information and shall have no responsibility whatsoever for conlicts, incompatibilities, or other dificulties arising from any fut ure changes. no license, whether express, implied, by estoppel or otherwise, is granted to any intellectual prope rty rights by this document. skyworks assumes no liability for any materials, products or informatio n provided here- under, including the sale, distribution, reproduction or use of skyworks products, information or ma terials, except as may be provided in skyworks terms and conditions of sale. the materials, products and information are provided as is without warranty of any kind, whether e xpress, implied, statutory, or otherwise, including fitness for a particular purpose or use, merchantability, performance, quality or non-infringement of any intellectual proper ty right; all such warranties are hereby expressly disclaimed. skyworks does not warrant the accuracy or completeness of the information, text, graphics or other items contained within these materials. skyworks shall not be liable for any damages, in- cluding but not limited to any special, indirect, incidental, statutory, or consequential damages, i ncluding without limitation, lost revenues or lost profits that may result from the use of the materials or information, whether or not the recipient of materials has been advised of the possibility of such damage. skyworks products are not intended for use in medical, lifesaving or life-sustaining applications, o r other equipment in which the failure of the skyworks products could lead to personal injury, death , physical or en- vironmental damage. skyworks customers using or selling skyworks products for use in such applicatio ns do so at their own risk and agree to fully indemnify skyworks for any damages resulting from such improper use or sale. customers are responsible for their products and applications using skyworks products, which may dev iate from published speciications as a result of design defects, errors, or operation of products ou tside of pub- lished parameters or design speciications. customers should include design and operating safeguards to minimize these and other risks. skyworks assumes no liability for applications assistance, custom er product design, or damage to any equipment resulting from the use of skyworks products outside of stated pub lished speciications or parameters. skyworks, the skyworks symbol, and breakthrough simplicity are trademarks or registered trademarks of skyworks solutions, inc., in the united states and other countries. third-party brands and names are for identiication purposes only, and are the property of their respective owners. additional information , including relevant terms and conditions, posted at www.skyworksinc.com, are incorporated by refere nce. downloaded from: http:///

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