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lt3498 1 3498fa load current (ma) 0.1 65 70 80 60 50 100 40 55 45 75 250 300 400200 100 0 15050 350 efficiency (%) power loss (mw) 3498 ta01b 11 0 load from v out2 v in = 3.6v v out2 = 16v power loss from v out2 typical application features applications description 20ma led driver and oled driver with integrated schottky in 3mm x 2mm dfn the lt ? 3498 is a dual output boost converter featuring a 2.3mhz pwm led driver and pfm oled driver. it includes an internal power switch and schottky diode for each driver. both converters can be independently shut down and modulated. this highly integrated power solution is ideal for dual display electronic devices. the 2.3mhz step-up converter is designed to drive up to six white leds in series from a li-ion cell. the device features a unique high side led current sense that enables the part to function as a one-wire current sourceone side of the led string can be returned to ground anywhere. traditional led drivers use a grounded resistor to sense led current, requiring a 2-wire connection to the led string. the pfm oled driver is a low noise boost converter that features a novel control technique.* the converter controls power delivery by varying both the peak inductor current and switch off time. this technique results in low output voltage ripple, as well as, high ef? ciency over a wide load range. the off time of the switch is not allowed to exceed a ? xed level, guaranteeing a switching frequency that stays above the audio band. li-ion to six white leds and oled/lcd bias dual output boost for dual display devices drives up to six white leds and oled/lcd bias internal power switches and schottky diodes independent dimming and shutdown 200mv high side sense on led driver allows one-wire current source wide input voltage range: 2.5v to 12v wide output voltage range: up to 32v 2.3mhz pwm frequency for led driver pfm for oled driver is non-audible over entire load range open led protection (27v maximum on cap1 pin) oled output disconnect available in 12-pin dfn package 1mm tall solution height , lt, ltc and ltm are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners.*patent pending oled ef? ciency shutdown and dimming control shutdown and control v in 4.7 f 0.47 f 16v24ma 20ma 15 h 15 h 1 f v in = 3v to 5v v out2 10 2.21m 3498 ta01 lt3498 sw1 cap1 sw2 cap2 fb2 ctrl1 led1 ctrl2 gnd2 gnd1 10 f on off on off cellular phones pdas, handheld computers digital cameras mp3 players gps receivers downloaded from: http:///
lt3498 2 3498fa pin configuration electrical characteristics absolute maximum ratings input voltage (v in ) ....................................................12v ctrl1 and ctrl2 voltage ........................................12v fb2 voltage ..............................................................2.5v v out2 voltage ...........................................................32v sw1 and sw2 voltage ..............................................32v cap1 and cap2 voltage ............................................32v led1 voltage ............................................................32v operating junction temperature range ...C40c to 85c maximum junction temperature........................... 125c storage temperature range ...................C65c to 150c (notes 1, 2) parameter conditions min typ max units minimum operating voltage 2.5 v maximum operating voltage 12 v supply current (led off, oled off) v in = 3v, v ctrl1 = 0v, v ctrl2 = 0v 81 0 a supply current (led on, oled off) v in = 3v, v ctrl1 = 3v, v ctrl2 = 0v, v cap1 = 24v, v led1 = 23v 1.6 2 ma supply current (led off, oled on) v in = 3v, v ctrl1 = 0v, v ctrl2 = 3v, v fb2 = 3v 230 280 a supply current (led on, oled on) v in = 3v, v ctrl1 = 3v, v ctrl2 = 3v, v cap1 = 24v, v led1 = 23v 1.65 2.05 ma v ctrl1 for full led current v ctrl2 for full oled brightness 1.5 v v ctrl1 or v ctrl2 to turn on i c 125 mv v ctrl1 and v ctrl2 to shut down i c 75 mv ctrl1, ctrl2 pin bias current 100 na led driver led current sense voltage (v cap C v led )v cap1 = 24v, i sw = 200ma 190 200 210 mv cap1, led1 pin bias current v cap1 = 16v, v led1 = 16v 20 30 a top view 13 ddb package 12-lead ( 3mm 2mm ) plastic dfn led1 ctrl1 gnd1gnd2 ctrl2 fb2 cap1sw1 v in sw2cap2 v out2 8 7 10 9 11 12 5 6 4 2 3 1 t jmax = 125c, ja = 160c/w exposed pad (pin 13) is gnd, must be soldered to pcb the denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c, v in = 3v, v ctrl1 = v ctrl2 = 3v. order information lead free finish tape and reel part marking package description temperature range lt3498eddb#pbf lt3498eddb#trpbf lcqf 12-lead (3mm 2mm) plastic dfn C40c to 85c consult ltc marketing for parts speci? ed with wider operating temperature ranges. consult ltc marketing for information on non-standard lead based ? nish parts. for more information on lead free part marking, go to: http://www.linear.com/leadfree/ for more information on tape and reel speci? cations, go to: http://www.linear.com/tapeandreel/ downloaded from: http:///
lt3498 3 3498fa note 1: stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. exposure to any absolute maximum rating condition for extended periods may affect device reliability and lifetime. note 2: the lt3498 is guaranteed to meet performance speci? cations from 0c to 85c. speci? cations over the C40c to 85c junction operating temperature range are assured by design, characterization and correlation with statistical process controls. parameter conditions min typ max units v cap1 , v led1 common mode minimum voltage 2.5 v switching frequency 1.8 2.3 2.8 mhz maximum duty cycle 88 90 % switch current limit 300 425 ma switch v cesat i sw = 200ma 250 mv switch leakage current v sw1 = 16v, switch off 0.1 5 a cap1 pin overvoltage protection 26 27 28 v schottky forward voltage i schottky1 = 100ma 0.8 v schottky reverse leakage v reverse1 = 20v, v ctrl1 = 0v 6 a oled driver feedback voltage (note 3) 1.18 1.215 1.25 v feedback resistor 177 182 186 k minimum switch off time after start-up 150 ns minimum switch off time during start-up (note 4) 1 s maximum switch off time v fb2 = 1.5v 15 20 30 s switch current limit 180 300 400 ma switch v cesat i sw2 = 200ma 260 mv switch leakage current v sw2 = 16v, switch off 0.1 5 a schottky forward voltage i schottky2 = 100ma 800 mv schottky reverse leakage v reverse2 = 20v 2 a pmos disconnect v cap2 C v out2 i out2 = 10ma, v cap2 = 5v 250 mv ctrl2 to fb2 offset v ctrl2 = 0.5v 8 15 mv maximum shunt current v fb2 = 1.3v 220 a electrical characteristics the denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c, v in = 3v, v ctrl1 = v ctrl2 = 3v. note 3: internal reference voltage is determined by ? nding v fb2 voltage level which causes quiescent current to increase 20a above not switching level. note 4: if ctrl2 is overriding the internal reference, start-up mode occurs when v fb2 is less then half the voltage on ctrl2. if ctrl2 is not overriding the internal reference, start-up mode occurs when v fb2 is less then half the voltage of the internal reference. downloaded from: http:///
lt3498 4 3498fa temperature (c) C50 C25 300 current limit (ma) 350 500 0 50 75 3498 g07 450 400 25 100 125 v in (v) 0 25 output clamp voltage (v) 26 27 28 29 2 468 3498 g08 10 12 t = C50c t = 150c t = 25c v in (v) 0 0 shutdown current (a) 3 6 9 12 15 2 468 3498 g01 10 12 t = C50c t = 125c t = 25c switch current (ma) 0 0 switch saturation voltage (mv) 50 150 200 250 500350 100 200 250 100 400 450 300 50 150 300 350 400 3498 g02 t = C50c t = 125c t = 25c schottky forward drop (mv) 0 schottky forward current (ma) 150 200 250 600 1000 100 50 0 200 400 800 300 350 400 3498 g03 t = C50c t = 125c t = 25c v ctrl1 (mv) 0 0 sense voltage (mv) 40 80 120 160 200 240 500 1000 1500 2000 3498 g04 t = 25ct = C50c t = 125c temperature (c) C50 C25 186 sense voltage (mv) 194 206 0 50 75 3498 g05 190 202 198 25 100 125 t = C50c cap1 voltage (v) 0 sense voltage (mv) 198 202 206 20 194 190 186 5 10 15 25 3498 g06 t = 125c t = 25c typical performance characteristics shutdown current (v ctrl1 = v ctrl2 = 0v) led switch saturation voltage (v cesat1 ) led schottky forward voltage drop sense voltage (v cap1 C v led1 ) vs v ctrl1 sense voltage (v cap1 C v led1 ) vs temperature sense voltage (v cap1 C v led1 ) vs v cap1 led current limit vs temperature open circuit output clamp voltage input current in output open circuit t a = 25c, unless otherwise speci? ed. v in (v) 2 0 input current (ma) 1 2 3 4 5 6 4681 0 12 3498 g09 t = C50c t = 150c t = 25c downloaded from: http:///
lt3498 5 3498fa temperature (c) C50 led switching frequency (mhz) 2.5 25 3498 g10 2.2 2.0 C25 0 50 1.91.8 2.62.4 2.3 2.1 75 100 125 switch current (ma) 0 0 switch saturation voltage (mv) 50 100 150 200 300 50 100 150 200 3498 g11 250 300 250 t = C50c t = 125c t = 25c schottky forward drop (mv) 0 schottky forward current (ma) 150 200 250 600 1200 1000 100 50 0 200 400 800 300 350 400 3498 g12 t = C50c t = 125c t = 25c ctrl2 voltage (mv) 0 0 v out2 voltage (v) 2 6 8 10 1000 2000 18 4 500 1500 12 14 16 3498 g13 temperature (c) C50 C25 C6 output voltage change (%) C3 6 0 50 75 3498 g14 3 0 25 100 125 load current (ma) 0 v out2 voltage change (%) C0.5 0 0.5 30 50 C1.0 C1.5 C2.0 10 20 40 1.0 1.5 2.0 3498 g15 temperature (c) C50 switching frequency (khz) 70 25 3498 g16 40 20 C25 0 50 10 0 8060 50 30 75 100 125 load current (ma) 0.1 0 switching frequency (khz) 800 1000 1200 1 10 100 3498 g17 600400 200 temperature (c) C50 peak inductor current (ma) 550 25 3498 g18 400 300 C25 0 50 250200 600500 450 350 75 100 125 typical performance characteristics led switching frequency vs temperature oled switch saturation voltage (v cesat2 ) oled schottky forward voltage drop v out2 vs v ctrl2 (v out2 = 16v) v out2 vs temperature (v out2 = 16v) v out2 load regulation oled minimum switching frequency oled switching frequencyvs load current peak inductor current t a = 25c, unless otherwise speci? ed. downloaded from: http:///
lt3498 6 3498fa typical performance characteristics led switching waveforms led transient response oled switching waveforms with no load oled switching waveforms with 4ma load oled switching waveforms with 35ma load oled switching waveforms during start-up pin functions led1 (pin 1): connection point between the anode of the highest led and the sense resistor. the led current can be programmed by: i mv r led sense 1 1 200 = ctrl1 (pin 2): dimming and shutdown pin. connect this pin below 75mv to disable the white led driver. as the pin voltage is ramped from 0v to 1.5v, the led current ramps from 0 to (i led1 = 200mv / r sense1 ). gnd1, 2 (pins 3, 4): ground. tie directly to local ground plane. gnd1 and gnd2 are connected internally. ctrl2 (pin 5): dimming and shutdown pin. connect it below 75mv to disable the low noise boost converter. as the pin voltage is ramped from 0v to 1.5v, the output ramps up to the programmed output voltage. fb2 (pin 6): feedback pin. reference voltage is 1.215v. there is an internal 182k resistor from fb2 to gnd. to achieve desired output voltage, choose r fb2 according to the following formula: r fb2 = 182 ? v out2 1.215 1 k t a = 25c, unless otherwise speci? ed. 500ns/div v sw 10v/div v cap1 50mv/div i l 100ma/ div 3498 g19 v in = 3.6v front page application 1ms/div v cap1 5v/div v ctrl1 5v/div i l 200ma/ div 3498 g20 v in = 3.6v front page application 5s/div v out2 10mv/div ac coupled s w2 voltage 10v/div inductor current 50ma/div 3498 g21 v in = 3.6v v out2 = 16v 3498 g22 2s/div v out2 10mv/div ac coupled sw2 voltage 10v/div inductor current 200ma/div v in = 3.6v v out2 = 16v 3498 g23 500ns/div sw2 voltage 10v/div v out2 10mv/div ac coupled inductor current 200ma/div v in = 3.6v v out2 = 16v 3498 g24 500s/div v out2 voltage 5v/div cap2 voltage 5v/div inductor current 100ma/div v in = 3.6v v out2 = 16v downloaded from: http:///
lt3498 7 3498fa block diagram pin functions v out2 (pin 7): drain of output disconnect pmos. place a bypass capacitor from this pin to gnd. see the applications information section. cap2 (pin 8): output of the oled driver. this pin is connected to the cathode of the internal schottky diode. place a bypass capacitor from this pin to gnd. sw2 (pin 9): switch pin. this is the collector of the in- ternal npn power switch. minimize the metal trace area connected to this pin to minimize emi. v in (pin 10): input supply pin. must be locally by- passed. sw1 (pin 11): switch pin. connect the inductor of the white led driver at this pin. minimize metal trace area at this pin to minimize emi. cap1 (pin 12): output of the white led driver. this pin is connected to the cathode of the internal schottky. connect the output capacitor to this pin and the sense resistor from this pin to the led1 pin. exposed pad (pin 13): ground. the exposed pad must be soldered to the pcb. C + C + + C + C + C ++ 6 5 1 12 8 7 11 10 9 4 2 3 ramp generator 2.3mhz oscillator switch control start-up control a2 vref r s q overvoltage protection disconnect control shunt control 3498 bd driver driver comparator sw2 cap1 led1 v in r sense1 10 v out2 cap2 r c c c c1 1f c20.47f r fb2 2.21m q2 a5 a1 a4 a3 c3 10f l2 10f sw1 gnd2 fb2 ctrl2 ctrl1 gnd1 r q1 182k c in 4.7f l1 15f downloaded from: http:///
lt3498 8 3498fa operation led driver the led portion of the lt3498 uses a constant-frequency, current mode control scheme to provide excellent line and load regulation. operation can be best understood by referring to the block diagram. at power-up, the capacitor at the cap1 pin is charged up to v in (input supply voltage) through the inductor and the internal schottky diode. if ctrl1 is pulled higher than 125mv, the bandgap reference, the start-up bias and the oscillator are turned on. at the start of each oscillator cycle, the power switch q1 is turned on. a voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the pwm comparator, a2. when this voltage exceeds the level at the negative input of a2, the pwm logic turns off the power switch. the level at the negative input of a2 is set by the error ampli? er a1, and is simply an ampli? ed version of the difference between the v cap1 and v led1 voltage and the bandgap reference. in this manner the error ampli? er, a1, sets the correct peak current level in inductor l1 to keep the output in regulation. the ctrl1 pin is used to adjust the led current. the led driver is shutdown when ctrl1 is pulled lower than 75mv. minimum output current the led driver of the lt3498 can drive a 4-led string at 2ma led current, without pulse-skipping, using the same external components shown in the application circuit on the front page of this data sheet. as current is further reduced, the device will begin skipping pulses. this will result in some low frequency ripple, although the average led current remains regulated down to zero. the photo in figure 1 details circuit operation driving four white leds at 2ma load. peak inductor current is less than 60ma and the regulator operates in discontinuous mode, meaning the inductor current reaches zero during the discharge phase. after the inductor current reaches zero, the sw1 pin exhibits ringing due to the lc tank circuit formed by the inductor in combination with the switch and the diode capacitance. this ringing is not harmful; far less spectral energy is contained in the ringing than in the switch transitions. 200ns/div v sw 10v/div i l 50ma/div 3498 f01 v in = 4.2v i led = 2ma 4 leds figure 1. switching waveforms with four white leds at 2ma load downloaded from: http:///
lt3498 9 3498fa led current (ma) 0 65 70 80 15 60 55 51 0 2 0 5045 75 efficiency (%) 3498 f02 15uh murata lqh32cn150k5315uh murata lqh2mcn150k02 15uh cooper sd3110-150 15uh toko d312c 15uh coilcraft do3314-153ml the low noise boost of the lt3498 uses a novel control scheme to provide high ef? ciency over a wide range of output current. in addition, this technique keeps the switching frequency above the audio band over all load conditions. the operation of the part can be better understood by referring to the block diagram. the part senses the output voltage by monitoring the voltage on the fb2 pin. the user sets the desired output voltage by choosing the value of the external topside feedback resistor. the part incorporates a precision 182k bottom-side feedback resistor. assuming that output voltage adjustment is not used (ctrl2 pin is tied to 1.5v, or greater), the internal reference (v ref = 1.215v) sets the voltage to which fb2 will servo during regulation. the switch control block senses the output of the ampli- ? er and adjusts the switching frequency, as well as other parameters to achieve regulation. during the start-up of the circuit, special precautions are taken to ensure that the inductor current remains under control. because the switching frequency is never allowed to fall below approximately 50khz, a minimum load must be present to prevent the output voltage from drifting too high. this minimum load is automatically generated within the part via the shunt control block. the level of this current is adaptable, removing itself when not needed to improve ef? ciency at higher load levels. the low-noise boost of the lt3498 also has an integrated schottky diode and pmos output disconnect switch. the pmos switch is turned on when the part is enabled. when the part is in shutdown, the pmos switch turns off, allowing the v out2 node to go to ground. this type of disconnect function is often required in power supplies. operation oled driver applications information led driver inductor selectiona 15h inductor is recommended for most applications for the led driver of the lt3498. although small size and high ef? ciency are major concerns, the inductor should have low core losses at 2.3mhz and low dcr (copper wire resistance). some small inductors in this category are listed in table 1. the ef? ciency comparison of different inductors is shown in figure 2. table 1: recommended inductors part l (h) max dcr ( ) current rating (ma) vendor lqh32cn150k53lqh2mcn150k02 lqh32cn100k53 lqh2mcn100k02 1515 10 10 0.58 1.60.3 1.2 300200 450 225 murata www.murata.com sd3110-150 15 0.764 380 cooper www.cooperet.com 1001as-150m (type d312c) 15 0.80 360 toko www.toko.com d03314-153ml 15 0.86 680 coilcraft www.coilcraft.com figure 2. ef? ciency comparison of different inductors capacitor selectionthe small size of ceramic capacitors makes them ideal for lt3498 led driver applications. use only x5r and x7r types, because they retain their capacitance over wider temperature ranges than other types, such as y5v or z5u. a 4.7f input capacitor and a 1f output capacitor are suf? cient for most applications. downloaded from: http:///
lt3498 10 3498fa applications information led driver figure 3. transient response with leds disconnected from output table 2: recommended ceramic capacitor manufacturers taiyo yuden (800) 368-2496 www.t-yuden.com avx (803) 448-9411 www.avxcorp.com murata (714) 852-2001 www.murata.com overvoltage protection the led driver of the lt3498 has an internal open-circuit protection circuit. in the cases of output open circuit, when the leds are disconnected from the circuit or the leds fail open-circuit, v cap1 is clamped at 27v (typ). the led driver will then switch at a very low frequency to minimize input current. the v cap1 and input current during output open-circuit are shown in the typical performance char- acteristics. figure 3 shows the transient response when the leds are disconnected. inrush current the lt3498 led driver has a built-in schottky diode. when supply voltage is applied to the v in pin, an inrush current ? ows through the inductor and schottky diode and charges up the cap1 voltage. the schottky diode for the led driver of the lt3498 can sustain a maximum current of 1a. for low dcr inductors, which are usually the case for this application, the peak inrush current can be simpli? ed as follows: = r 2? l = 1 l? c ? r 2 4?l 2 i pk = v in ? 0.6 l? ? exp ? ? 2 where l is the inductance, r is the dcr of the inductor and c is the output capacitance. table 3 gives inrush peak currents for some component selections. table 3: inrush peak currents v in (v) r ( ) l (h) c out (f) i p (a) 4.2 0.58 15 1 0.828 4.2 1.6 15 1 0.682 4.2 0.8 15 1 0.794 4.2 0.739 15 1 0.803 500s/div i l 200ma/div v cap1 10v/div 3498 f03 v in = 3.6v front page application circuit leds disconnected at this point downloaded from: http:///
lt3498 11 3498fa the led current can be set by: i led 200mv r sense1 , when v ctrl1 > 1.5v i led v ctrl1 6.25 ? r sense1 , when v ctrl1 < 1.25v feedback voltage variation versus control voltage is given in the typical performance characteristics. using a filtered pwm signal a ? ltered pwm signal can be used to control the bright- ness of the led string. the pwm signal is ? ltered (figure 4) by a rc network and fed to the ctrl1 pin.the corner frequency of r1, c1 should be much lower than the frequency of the pwm signal. r1 needs to be much smaller than the internal impedance of the ctrl1 pin which is 10m (typ). applications information led driver programming led currentthe feedback resistor (r sense1 ) and the sense voltage (v cap1 C v led1 ) control the led current. the ctrl1 pin controls the sense reference voltage as shown in the typical performance characteristics. for ctrl1 higher than 1.5v, the sense reference is 200mv, which results in full led current. to have accurate led current, precision resistors are preferred (1% is recommended). the formula and table for r sense selection are shown below. r mv i sense led 1 200 = table 4: r sense1 value selection for 200mv sense i led (ma) r sense1 ( ) 54 0 10 20 15 13.3 20 10 dimming control there are three different types of dimming control circuits. the led current can be set by modulating the ctrl1 pin with a dc voltage, a ? ltered pwm signal or directly with a pwm signal. using a dc voltage for some applications, the preferred method of brightness control is a variable dc voltage to adjust the led current. the ctrl1 pin voltage can be modulated to set the dim- ming of the led string. as the voltage on the ctrl1 pin increases from 0v to 1.5v, the led current increases from 0 to i led . as the ctrl1 pin voltage increases beyond 1.5v, it has no effect on the led current. figure 4. dimming control using a filtered pwm signal lt3498 ctrl1 c1 0.1f pwm 10khz typ 3498 f04 r1 100k downloaded from: http:///
lt3498 12 3498fa led current (ma) 50 efficiency (%) 60 70 8055 65 75 4 8 12 16 20 2 0 6 10 14 18 3498 f07 v in = 3.6v 4 leds 100hz = pwm direct pwm dimming changing the forward current ? owing in the leds not only changes the intensity of the leds, it also changes the color. the chromaticity of the leds changes with the change in forward current. many applications cannot tolerate any shift in the color of the leds. controlling the intensity of the leds with a direct pwm signal allows dimming of the leds without changing the color. in addition, direct pwm dimming offers a wider dimming range to the user. dimming the leds via a pwm signal essentially involves turning the leds on and off at the pwm frequency. the typical human eye has a limit of ~60 frames per second. by increasing the pwm frequency to ~80hz or higher, the eye will interpret that the pulsed light source is con- tinuously on. additionally, by modulating the duty cycle (amount of on-time), the intensity of the leds can be controlled. the color of the leds remains unchanged in this scheme since the led current value is either zero or a constant value. figure 5 shows a li-ion powered driver for four white leds. direct pwm dimming method requires an external nmos tied between the cathode of the lowest led in the string and ground as shown in figure 5. a simple logic level si2304 mosfet can be used since its source is con- nected to ground. the pwm signal is applied to the ctrl1 pin of the lt3498 and the gate of the mosfet. the pwm signal should traverse between 0v to 5v, to ensure proper turn-on and -off of the driver and the nmos transistor q1. when the pwm signal goes high, the leds are connected to ground and a current of i led = 200mv / r sense1 ? ows through the leds. when the pwm signal goes low, the leds are disconnected and turn off. the mosfet ensures that the leds quickly turn off without discharging the output capacitor which in turn allows the leds to turn on faster. figure 6 shows the pwm dimming waveforms for the circuit in figure 5. applications information led driver figure 7. pwm dimming ef? ciency figure 5. li-ion to four white leds with direct pwm dimming figure 6. direct pwm dimming waveforms cap1 sw1 cap2 v out2 v in lt3498 gnd1 gnd2 sw2 c in 1h r sense1 10 q1 si2304bds 3498 f05 c out1 1f v in 3v to 5v l1 15h led1 ctrl1 ctrl2 fb2 100k 0v pwm freq 5v 2ms/div i led 20ma/div i l 200ma/div pwm 5v/div 3498 f06 v in = 3v 4 leds downloaded from: http:///
lt3498 13 3498fa the time it takes for the led current to reach its pro-grammed value sets the achievable dimming range for a given pwm frequency. for example, the settling time of the led current in figure 6 is approximately 40s for a 3v input voltage. the achievable dimming range for this application and 100hz pwm frequency can be determined using the following method. example: f = 100hz, t settle = 40 s t period = 1 f = 1 100 = 0.01s dim range = t period t settle = 0.01s 40 s = 250 :1 min duty cycle = t settle t period ? 100 = 40 s 0.01s ? 100 = 0.4% duty cycle range = 100% 0.4% at 100hz the calculations show that for a 100hz signal the dimming range is 250:1. in addition, the minimum pwm duty cycle of 0.4% ensures that the led current has enough time to settle to its ? nal value. figure 8 shows the dimming range achievable for three different frequencies with a settling time of 40s. the dimming range can be further extended by changing the amplitude of the pwm signal. the height of the pwm signal sets the commanded sense voltage across the sense resistor through the ctrl1 pin. in this manner both analog dimming and direct pwm dimming extend the dimming range for a given application. the color of the leds no longer remains constant because the forward current of the led changes with the height of the ctrl1 signal. for the four led application described above, the leds can be dimmed ? rst, modulating the duty cycle of the pwm signal. once the minimum duty cycle is reached, the height of the pwm signal can be decreased below 1.5v down to 125mv. the use of both techniques together allows the average led current for the four led application to be varied from 20ma down to less than 20a. figure 9 shows the application for dimming using both analog dimming and pwm dimming. a potentiometer must be added to ensure that the gate of the nmos receives a logic-level signal, while the ctrl1 signal can be adjusted to lower amplitudes. figure 8. dimming ratio vs freqeuncy applications information led driver pwm frequency (hz) 10 pwm dimming range 100 1000 10000 100 1000 10000 3498 f08 1 10 pulsing may be isible figure 9. li-ion to four white leds with both pwm dimming and analog dimming cap1 sw1 cap2 v out2 v in lt3498 gnd1 gnd2 sw2 c in 1h r sense1 10 q1 si2304bds 3498 f09 c out1 1f v in 3v to 5v l1 15h led1 ctrl1 ctrl2 fb2 100k 0v pwm freq 5v downloaded from: http:///
lt3498 14 3498fa applications information oled driver inductor selectionseveral recommended inductors that work well with the oled driver of the lt3498 are listed in table 5, although there are many other manufacturers and devices that can be used. consult each manufacturer for more detailed information and for their entire selection of related parts. many different sizes and shapes are available. use the equations and recommendations in the next few sections to ? nd the correct inductance value for your design. table 5: recommended inductors part l (h) max dcr ( ) current rating (ma) vendor lqh32cn100k53lqh2mcn100k02 lqh32cn150k53 lqh2mcn150k02 1010 15 15 0.31.2 0.58 1.6 450225 300 200 murata www.murata.com sd3110-100sd3110-150 1015 0.5050.764 470380 cooper www.cooperet.com inductor selectionboost regulatorthe formula below calculates the appropriate inductor value to be used for the low noise boost regulator of the lt3498 (or at least provides a good starting point). this value provides a good tradeoff in inductor size and system performance. pick a standard inductor close to this value. a larger value can be used to slightly increase the available output current, but limit it to around twice the value calculated below, as too large of an inductance will decrease the output voltage ripple without providing much additional output current. a smaller value can be used (especially for systems with output voltages greater than 12v) to give a smaller physical size. inductance can be calculated as: l = (v out2 ? v in(min) + 0.5v) ? 0.66( h) where v out2 is the desired output voltage and v in(min) is the minimum input voltage. generally, a 10h or 15h inductor is a good choice. capacitor selectionthe small size and low esr of ceramic capacitors makes them suitable for most oled driver applications. x5r and x7r types are recommended because they retain their ca- pacitance over wider voltage and temperature ranges than other types such as y5v or z5u. a 4.7f input capacitor and a 10f output capacitor are suf? cient for most ap- plications for the oled driver. always use a capacitor with a suf? cient voltage rating. many capacitors rated at 10f, particularly 0805 or 0603 case sizes, have greatly reduced capacitance when bias voltages are applied. be sure to check actual capacitance at the desired output voltage. generally a 1206 size capacitor will be adequate. a 0.47f capaci- tor placed on the cap node is recommended to ? lter the inductor current while the larger 10f placed on the v out node will give excellent transient response and stability. table 6 shows a list of several capacitor manufacturers. consult the manufacturers for more detailed information and for their entire selection of related parts. table 6. recommended ceramic capacitor manufacturers manufacturer phone url taiyo yuden 408-573-4150 www.t-yuden.com avx 843-448-9411 www.avxcorp.com murata 814-237-1431 www.murata.com kemet 408-986-0424 www.kemet.com setting output voltage and the auxiliary reference input the oled driver of the lt3498 is equipped with both an internal 1.215v reference and an auxiliary reference input. this allows the user to select between using the built-in reference, and supplying an external reference voltage. the voltage at the ctrl2 pin can be adjusted while the chip is operating to alter the output voltage of the lt3498 for purposes such as display dimming or contrast adjust- ment. to use the internal 1.215v reference, the ctrl2 pin must be held higher than 1.5v. when the ctrl2 pin is held between 0v and 1.5v the oled driver will regulate the output such that the fb2 pin voltage is nearly equal to the ctrl2 pin voltage. at ctrl2 voltages close to 1.215v, h) downloaded from: http:///
lt3498 15 3498fa v in v out2 r fb2 3498 f11 lt3498 sw1 cap1 sw2 cap2 fb2 ctrl1 led1 ctrl2 gnd2 gnd1 c2 v in v out2 r fb2 lt3498 sw1 cap1 sw2 cap2 fb2 ctrl1 led1 ctrl2 gnd2 gnd1 c3 c2 a soft transition occurs between the ctrl2 pin and the internal reference. figure 10 shows this behavior. applications information oled driver to set the maximum output voltage, select the values of r fb2 according to the following equation: r fb2 = 182 ? v out2 1.215 ?1 ,k when ctrl2 is used to override the internal reference, the output voltage can be lowered from the maximum value down to nearly the input voltage level. if the volt- age source driving the ctrl2 pin is located at a distance to the lt3498, a small 0.1f capacitor may be needed to bypass the pin locally. choosing a feedback nodethe single feedback resistor may be connected to the v out2 pin or to the cap2 pin (see figure 11). regulating the v out2 pin eliminates the output offset resulting from the voltage drop across the output disconnect pmos. regu-lating the cap2 pin does not compensate for the voltage drop across the output disconnect, resulting in an output voltage v out2 that is slightly lower than the voltage set by the resistor divider. under most conditions, it is advised that the feedback resistor be tied to the v out2 pin. connecting the load to the cap2 node the ef? ciency of the converter can be improved by con- necting the load to the cap2 pin instead of the v out2 pin. the power loss in the pmos disconnect circuit is then made negligible. by connecting the feedback resistor to the v out2 pin, no quiescent current will be consumed in the feedback resistor string during shutdown since the pmos transistor will be open (see figure 12). the disadvantage of this method is that the cap2 node can- not go to ground during shutdown, but will be limited to around a diode drop below v in . loads connected to the part should only sink current. never force external power supplies onto the cap2 or v out2 pins. the larger value output capacitor should be placed on the node to which the load is connected. figure 10. ctrl2 to fb2 transfer curve figure 11. feedback connection using the cap2 pin or the v out2 pin figure 12. improved ef? ciency ctrl2 voltage (v) 0 0 fb2 voltage (v) 0.250 0.500 0.750 1.000 1.500 0.3 0.5 0.8 1.0 3498 f10 1.3 1.5 1.250 v in v out2 r fb2 i load 3498 f12 lt3498 sw1 cap1 sw2 cap2 fb2 ctrl1 led1 ctrl2 gnd2 gnd1 c3 downloaded from: http:///
lt3498 16 3498fa maximum output load current the maximum output current of a particular lt3498 circuit is a function of several circuit variables. the fol- lowing method can be helpful in predicting the maximum load current for a given circuit: step 1: calculate the peak inductor current: ii v l amps pk limit in =+ ?? ? 400 10 9 where i limit is 0.3a for the oled driver. l is the induc- tance value in henrys and v in is the input voltage to the boost circuit. step 2: calculate the inductor ripple current: i vv l amps ripple out in = + () 2 9 1 150 10 ??? ? where v out2 is the desired output voltage. if the inductor ripple current is less then the peak current, then the circuit will only operate in discontinuous conduc- tion mode. the inductor value should be increased so that i ripple < i pk . an application circuit can be designed to operate only in discontinuous mode, but the output current capability will be reduced. step 3: calculate the average input current: ii i amps in avg pk ripple () ? = 2 step 4: calculate the nominal output current: i iv v amps out nom in avg in out () () ??. = 075 2 step 5: derate output current: i out = i out(nom) ? 0.7 amps for low output voltages the output current capability will be increased. when using output disconnect (load current taken from v out2 ), these higher currents will cause the drop in the pmos switch to be higher resulting in reduced output current capability than those predicted by the preceding equations. inrush current when v in is stepped from ground to the operating voltage while the output capacitor is discharged, a higher level of inrush current will ? ow through the inductor and integrated schottky diode into the output capacitor. conditions that increase inrush current include a larger more abrupt voltage step at v in , a larger output capacitor tied to the cap2 pin, and an inductor with a low saturation current. while the internal diode is designed to handle such events, the inrush current should not be allowed to exceed 1a. for circuits that use output capacitor values within the recommended range and have input voltages of less than 5v, inrush cur- rent remains low, posing no hazard to the device. in cases where there are large steps at v in (more than 5v) and/or a large capacitor is used at the cap2 pin, inrush current should be measured to ensure safe operation. applications information oled driver downloaded from: http:///
lt3498 17 3498fa applications information led and oled driver board layout considerationsas with all switching regulators, careful attention must be paid to the pcb board layout and component placement. to prevent electromagnetic interference (emi) problems, proper layout of high frequency switching paths is essential. minimize the length and area of all traces connected to the switching node pins (sw1 and sw2). keep the sense voltage pins (cap1 and led1) away from the switching node. the fb2 connection for the feedback resistor r fb2 should be tied directly from the v out2 pin to the fb2 pin and be kept as short as possible, ensuring a clean, noise-free connection. place c out1 and c out2 next to the cap1 and cap2 pins respectively. always use a ground plane ender the switching regulator to minimize interplane coupling. recommended component placement is shown in figure 13. figure 13. recommended board layout 3498 f13 fb2 v out2 v out2 c3 r fb2 r sense1 c2 v in c in l1l2 cap1 sw1sw2 cap2 c1 ctrl1 led1 gndctrl2 gnd gnd 8 7 10 9 11 12 5 6 4 2 3 1 vias to ground plane required to improve thermal performance vias to v out2 downloaded from: http:///
lt3498 18 3498fa li-ion to two white leds and oled/lcd bias typical applications shutdown and dimming control shutdown and control v in c20.47f 16v24ma 20ma l2 10h l1 10h v in = 3v to 5v v out2 r fb2 2.21m 3498 ta02 lt3498 sw1 cap1 sw2 cap2 fb2 ctrl1 led1 ctrl2 gnd2 gnd1 c310f on off on off r sense1 10 c in 4.7 f c in , c2: x5r or x7r with sufficient voltage rating c1: taiyo yuden gmk212bj105kg c3: taiyo yuden tmk316bj106ml l1, l2: murata lqh32cn100k53 c11 f led ef? ciency v in = 3.6v, 2 leds led current (ma) 0 60 65 75 15 55 50 51 0 2 0 4540 70 efficiency (%) 3498 ta02b downloaded from: http:///
lt3498 19 3498fa typical applications shutdown and dimming control shutdown and control v in c20.47f 16v24ma 20ma l2 10h l1 10h v in = 3v to 5v v out2 r fb2 2.21m 3498 ta03 lt3498 sw1 cap1 sw2 cap2 fb2 ctrl1 led1 ctrl2 gnd2 gnd1 c310f on off on off r sense1 10 c in 4.7 f c in , c2: x5r or x7r with sufficient voltage rating c1: taiyo yuden gmk212bj105kg c3: taiyo yuden tmk316bj106ml l1, l2: murata lqh32cn100k53 c11 f led ef? ciency v in = 3.6v, 2 leds led current (ma) 0 60 65 8075 15 55 50 51 0 2 0 4540 70 efficiency (%) 3498 ta03b li-ion to two white leds and oled/lcd bias downloaded from: http:///
lt3498 20 3498fa led current (ma) 0 60 65 8075 15 55 50 51 0 2 0 45 70 efficiency (%) 3498 ta04b typical applications shutdown and dimming control shutdown and control v in c20.47f 16v24ma 20ma l2 10h l1 15h v in = 3v to 5v v out2 r fb2 2.21m 3498 ta04 lt3498 sw1 cap1 sw2 cap2 fb2 ctrl1 led1 ctrl2 gnd2 gnd1 c310f on off on off r sense1 10 c in 4.7 f c in , c2: x5r or x7r with sufficient voltage rating c1: taiyo yuden gmk212bj105kg c3: taiyo yuden tmk316bj106ml l1: murata lqh32cn150k53 l2: murata lqh32cn100k53 c11 f led ef? ciency v in = 3.6v, 3 leds li-ion to three white leds and oled/lcd bias downloaded from: http:///
lt3498 21 3498fa li-ion to four white leds and oled/lcd bias typical applications shutdown and dimming control shutdown and control v in c20.47f 16v24ma 20ma l2 10h l1 15h v in = 3v to 5v v out2 r fb2 2.21m 3498 ta05 lt3498 sw1 cap1 sw2 cap2 fb2 ctrl1 led1 ctrl2 gnd2 gnd1 c310f on off on off r sense1 10 c in 4.7 f c in , c2: x5r or x7r with sufficient voltage rating c1: taiyo yuden gmk212bj105kg c3: taiyo yuden tmk316bj106ml l1: murata lqh32cn150k53 l2: murata lqh32cn100k53 c11 f led ef? ciency v in = 3.6v, 4 leds led current (ma) 0 65 70 80 15 60 55 51 0 2 0 50 75 efficiency (%) 3498 ta05b downloaded from: http:///
lt3498 22 3498fa typical applications shutdown and dimming control shutdown and control v in c20.47f 16v24ma l2 10h l1 15h d1 v in = 3v to 5v v out2 r fb2 2.21m 3498 ta06 lt3498 sw1 cap1 sw2 cap2 fb2 ctrl1 led1 ctrl2 gnd2 gnd1 c310f on off on off c in 4.7 f c in , c2: x5r or x7r with sufficient voltage rating c1: taiyo yuden gmk212bj105kg c3: taiyo yuden tmk316bj106ml d1: central semiconductor cmdsh-3 l1: murata lqh32cn150k53 l2: murata lqh32cn100k53 r sense1 10 c1 1 f 20ma led ef? ciency, v in = 3.6v, 6 leds oled ef? ciency and power loss v in = 3.6v, v out2 = 16v li-ion to six white leds and oled/lcd bias led current (ma) 0 65 70 80 15 60 55 51 0 2 0 50 75 efficiency (%) 3498 ta06b load current (ma) 65 70 80 60 50 40 55 45 75 250 300 400200 100 0 15050 350 efficiency (%) power loss (mw) 3498 ta06c load from v out2 load from cap2power loss from v out2 power loss from cap2 0.1 10 1 100 downloaded from: http:///
lt3498 23 3498fa information furnished by linear technology corporation is believed to be accurate and reliable. however, no responsibility is assumed for its use. linear technology corporation makes no representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. package description ddb package 12-lead plastic dfn (3mm 2mm) (reference ltc dwg # 05-08-1723 rev ?) 2.00 0.10 (2 sides) note:1. drawing is not a jedec package outline 2. drawing not to scale 3. all dimensions are in millimeters 4. dimensions of exposed pad on bottom of package do not include mold flash. mold flash, if present, shall not exceed 0.15mm on any side 5. exposed pad shall be solder plated 6. shaded area is only a reference for pin 1 location on the top and bottom of package 0.40 0.10 bottom viewexposed pad 0.64 0.10 (2 sides) 0.75 0.05 r = 0.115 typ r = 0.05 typ 2.39 0.10 (2 sides) 3.00 0.10 (2 sides) 1 6 12 7 pin 1 bar top mark (see note 6) 0.200 ref 0 ? 0.05 (ddb12) dfn 0106 rev ? 0.23 0.05 0.45 bsc pin 1r = 0.20 or 0.25 45 chamfer 0.25 0.05 2.39 0.05 (2 sides) recommended solder pad pitch and dimensions apply solder mask to areas that are not soldered 0.64 0.05 (2 sides) 1.15 0.05 0.70 0.05 2.55 0.05 packageoutline 0.45 bsc downloaded from: http:///
lt3498 24 3498fa linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 2007 lt 0508 rev a printed in usa related parts typical application part number description comments lt1932 constant-current, 1.2mhz, high ef? ciency white led boost regulator v in : 1v to 10v; v out(max) = 34v; i q = 1.2ma; i sd = <1a; thinsot tm package lt1937 constant-current, 1.2mhz, high ef? ciency white led boost regulator v in : 2.5v to 10v; v out(max) = 34v; i q = 1.9a; i sd = <1a; thinsot and sc70 packages lt3463/ lt3463a dual output, boost/inverter, 250ma i sw , constant off- time, high ef? ciency step-up dc/dc converter with integrated schottky diodes v in : 2.3v to 15v; v out(max) = 40v; i q = 40a; i sd = <1a; 3mm 3mm dfn-10 package lt3465/ lt3465a constant-current, 1.2/2.7mhz, high ef? ciency white led boost regulator with integrated schottky diode v in : 2.3v to 16v; v out(max) = 40v; i q = 40a; i sd = <1a; 3mm 3mm dfn-10 package lt3466/ lt3466-1 dual constant-current, 2mhz, high ef? ciency white led boost regulator with integrated schottky diode v in : 2.3v to 16v; v out(max) = 40v; i q = 65a; i sd = <1a; 3mm 2mm dfn-8 package lt3471 dual output, boost/inverter, 1.3a isw, 1.2mhz, high ef? ciency boost-inverting dc/dc converter v in : 2.4v to 16v; v out(max) = 40v; i q = 2.5a; i sd = <1a; 3mm 3mm dfn-10 package lt3473/ lt3473a 40v, 1a , 1.2mhz micropower low noise boost converter with output disconnect v in : 2.2v to 16v; v out(max) = 36v; i q = 150a; i sd = <1a; 3mm 3mm dfn-12 package lt3491 constant-current, 2.3mhz, high ef? ciency white led boost regulator with integrated schottky diode v in : 2.5v to 12v; v out(max) = 27v; i q = 12.6a; i sd = <8a; 2mm 2mm dfn-6 and sc70 packages lt3494/ lt3494a 40v, 180ma/350ma micropower low noise boost converter with output disconnect v in : 2.3v to 16v; v out(max) = 40v; i q = 65a; i sd = <1a; 3mm 2mm dfn-8 package lt3497 dual 2.3mhz, full function led driver with integrated schottky diode and 250:1 true color pwm tm dimming v in : 2.5v to 10v; v out(max) = 32v; i q = 6ma; i sd = <12a; 3mm 2mm dfn-10 package lt3591 constant-current, 1mhz, high ef? ciency white led boost regulator with integrated schottky diode and 80:1 true color pwm dimming v in : 2.5v to 12v; v out(max) = 40v; i q = 4ma; i sd = <9a; 3mm 2mm dfn-8 package thinsot and true color pwm are trademarks of linear technology corporation output voltage ripple vs load current v out r fb2 value required (m ) maximum output current at 3v input (ma) 25 3.57 12.5 24 3.40 13.4 23 3.24 14.4 22 3.09 15.6 21 2.94 16.8 20 2.80 18.1 19 2.67 19.6 18 2.49 21.2 17 2.37 22.5 16 2.21 24.2 15 2.05 26 load current (ma) 0.1 4 5 7 10 3 2 1 100 0 1 6 v out2 peak-to-peak ripple (mv) 3498 ta06d downloaded from: http:///

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