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general description the max8758 includes a high-performance step-up regu- lator, a high-speed operational amplifier, and a logic- controlled, high-voltage switch-control block with pro- grammable delay. the device is optimized for thin-film transistor (tft) liquid-crystal display (lcd) applications. the step-up dc-dc regulator provides the regulated sup- ply voltage for the panel source driver ics. the converter is a high-frequency (640khz/1.2mhz), current-mode regu- lator with an integrated 14v n-channel power mosfet. the high-switching frequency allows the use of ultra-small inductors and ceramic capacitors. the current-mode con- trol architecture provides fast transient response to pulsed loads. the regulator achieves efficiencies over 85% by bootstrapping the supply rail of the internal gate driver from the step-up regulator output. the step-up regulator features undervoltage lockout (uvlo), soft-start, and internal current limit. the high-current operational amplifier is designed to drive the lcd backplane (vcom). the amplifier features high output current (150ma), fast slew rate (7.5v/?), wide bandwidth (12mhz), and rail-to-rail inputs and outputs. the max8758 is available in a 24-pin, 4mm x 4mm, thin qfn package with a maximum thickness of 0.8mm for ultra-thin lcd panels. the device operates over the -40? to +85? temperature range. applications notebook displays lcd monitors features ? 1.8v to 5.5v input voltage range ? input undervoltage lockout ? 0.5ma quiescent current ? 640khz/1.2mhz current-mode step-up regulator fast transient response high-accuracy output voltage (1.5%) built-in 14v, 2.5a, 115m mosfet high efficiency programmable soft-start current limit with lossless sensing timer-delay fault latch ? high-speed operational amplifier ?50ma output current 7.5v/? slew rate 12mhz, -3db bandwidth rail-to-rail inputs/outputs ? dual-mode, logic-controlled, high-voltage switch with programmable delay ? thermal-overload protection ? 24-pin, 4mm x 4mm, thin qfn package max8758 step-up regulator with switch control and operational amplifier for tft lcd ________________________________________________________________ maxim integrated products 1 ordering information v main to vcom backplane v gon from tcon v in v goff in freq shdn comp ldo fb lx gnd pgnd out supb gon drn ctl mode ss posb negb outb thr dlp src max8758 simplified operating circuit 19-3699; rev 1; 9/05 for pricing, delivery, and ordering information, please contact maxim/dallas direct! at 1-888-629-4642, or visit maxim? website at www.maxim-ic.com. part temp range pin-package MAX8758ETG -40? to +85? 24 thin qfn-ep* 4mm x 4mm MAX8758ETG+ -40? to +85? 24 thin qfn-ep* 4mm x 4mm * ep = exposed pad. + denotes lead-free package. pin configuration appears at end of data sheet. dualmode is a trademark of maxim integrated products, inc.
max8758 step-up regulator with switch control and operational amplifier for tft lcd 2 _______________________________________________________________________________________ absolute maximum ratings electrical characteristics (v in = v shdn = +3v, out = +10v, freq = gnd, t a = 0? to +85? , unless otherwise noted. typical values are at t a = +25?.) stresses beyond those listed under ?bsolute maximum ratings?may cause permanent damage to the device. these are stress rating s only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specificatio ns is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. in, shdn , ctl, ldo to gnd ...................................-0.3v to +6v supb, lx, out to gnd..........................................-0.3v to +14v outb, negb, posb to gnd ..................-0.3v to (supb + 0.3v) thr, dlp, mode, freq, comp, fb, ss to gnd..............................................-0.3v to v ldo + 0.3v pgnd to gnd ......................................................-0.3v to + 0.3v src to gnd ..........................................................-0.3v to + 30v gon, drn to gnd ....................................-0.3v to v src + 0.3v gon rms current rating................................................ 50ma outb rms current rating .............................................. 60ma lx rms current rating .........................................................1.6a continuous power dissipation (t a = +70?) 24-pin, 4mm x 4mm thin qfn (derate 16.9mw/? above +70?) ..........................1349.1mw operating temperature range ...........................-40? to +85? junction temperature ......................................................+150? storage temperature range .............................-65? to +160? lead temperature (soldering, 10s) .................................+300? parameter conditions min typ max units in input voltage range 1.8 5.5 v in quiescent current v in = 3v, v fb = 1.5v 27 40 ? in undervoltage lockout in rising, 200mv hysteresis, lx remains off below this level 1.3 1.75 v ldo output voltage 6v v out 13v, i ldo = 12.5ma, v fb = 1.5v (note1) 4.8 5.0 5.2 v ldo undervoltage lockout voltage ldo rising, 200mv hysteresis 2.4 2.7 3.0 v out supply voltage range (note 1) 4.5 13.0 v out overvoltage fault threshold 13.2 13.6 14.0 v out undervoltage fault threshold 1.4 v v fb = 1.5v, no load 0.5 2.0 out supply current v fb = 1.1v, no load 4 10.0 ma shutdown supply current (total of in, out, and supb) v in = v out = v supb = 3v 4 10 a thermal shutdown temperature rising, 15? hysteresis +160 ? step-up regulator freq = gnd 512 600 768 operating frequency freq = in 1020 1200 1380 khz freq = gnd 91 95 99 oscillator maximum duty cycle freq = in 88 92 96 % fb regulation voltage 1.228 1.24 1.252 v fb fault trip level falling edge 0.96 1.0 1.04 v freq = gnd 43 51 64 duration to trigger fault condition freq = in 47 55 65 ms fb load regulation 0 < i load < 200ma, transient only -1 % fb line regulation v in = 1.8v to 5.5v -0.15 -0.08 +0.15 %/v fb input bias current v fb = 1.3v 125 200 na fb transconductance i = 5? at comp 75 160 280 ? fb voltage gain fb to comp 700 v/v lx on-resistance i lx = 200ma 115 200 m
max8758 step-up regulator with switch control and operational amplifier for tft lcd _______________________________________________________________________________________ 3 electrical characteristics (continued) (v in = v shdn = +3v, out = +10v, freq = gnd, t a = 0? to +85? , unless otherwise noted. typical values are at t a = +25?.) parameter conditions min typ max units lx leakage current v lx = 13v 0.01 20 ? lx current limit 65% duty cycle 2.0 2.5 3.0 a current-sense transresistance 0.19 0.3 0.40 v/a ss source current 3.0 4.0 5.5 ? positive gate driver timing and control switches ctl input low voltage v in = 1.8v to 5.5v 0.6 v v in = 1.8v to 2.4v 1.4 ctl input high voltage v in = 2.4v to 5.5v 2.0 v ctl input leakage current v ctl = 0 or v in -1 +1 ? gon rising, v mode = 1.24v, v ctl = 0 to 3v step, no load on gon 100 ctl-to-src propagation delay gon falling, v mode = 1.24v, v ctl = 3v to 0 step, no load on gon 100 ns src input voltage v dlp = 0, v in = 3v 2500 src input current mode = dlp = ctl = ldo 150 250 ? drn input current mode = dlp = ldo, v drn = 8v, v ctl = 0 150 250 ? src-to-gon switch on-resistance dlp = ctl = ldo 15 30 drn-to-gon switch on-resistance dlp = ldo, v ctl = 0 65 130 mode switch on-resistance v dlp = 0, v in = 3v 1000 mode 1 voltage threshold mode rising 0.9 x v ldo v mode capacitor charge current (mode 2) v mode = 1.5v 40 50 62 ? mode 2 switch transition voltage threshold gon connected to drn 2.3 2.5 2.7 v mode current-source stop threshold mode rising 3.3 3.5 3.7 v dlp capacitor charge current during startup, v dlp = 1.0v 4 5 6 ? dlp turn-on threshold 2.375 2.500 2.625 v thr-to-gon voltage gain v gon = 12v, v thr = 1.2v 9.7 10.0 10.3 v/v operational amplifier supb supply range 4.5 13.0 v supb supply current buffer configuration, v posb = 4v, no load 1.0 ma input offset voltage v negb , v posb = v supb /2, t a = +25? 12 mv input bias current v negb , v posb = v supb /2 -50 +50 na input common-mode voltage range 0 v supb v
max8758 step-up regulator with switch control and operational amplifier for tft lcd 4 _______________________________________________________________________________________ electrical characteristics (continued) (v in = v shdn = +3v, out = +10v, freq = gnd, t a = 0? to +85? , unless otherwise noted. typical values are at t a = +25?.) parameter conditions min typ max units i outb = 100? v supb - 15 output voltage swing high i outb = 5ma v supb - 150 mv i outb = -100? 15 output voltage swing low i outb = -5ma 150 mv slew rate 7.5 v/? -3db bandwidth 12 mhz gain-bandwidth product 8 mhz outb shorted to v supb /2, sourcing 75 150 short-circuit current outb shorted to v supb /2, sinking 75 150 ma control inputs freq input low voltage v in = 1.8v to 5.5v 0.6 v v in = 1.8v to 2.4v 1.4 freq input high voltage v in = 2.4v to 5.5v 2.0 v freq pulldown current v freq = 1.0v 3.5 5.0 6.0 ? shdn input low voltage v in = 1.8v to 5.5v 0.6 v v in = 1.8v to 2.4v 1.4 v in = 2.4v to 3.6v 2.0 shdn input high voltage v in = 3.6v to 5.5v 2.9 v shdn input current 0.001 1.0 ? electrical characteristics (v in = v shdn = +3v, out = +10v, freq = gnd, t a = -40? to +85? , unless otherwise noted.) (note 2) parameter conditions min typ max units in input voltage range 1.8 5.5 v in quiescent current v in = 3v, v fb = 1.5v 30 ? in undervoltage lockout in rising, 200mv hysteresis, lx remains off below this level 1.75 v ldo output voltage 6v v out 13v, i ldo = 12.5ma, v fb = 1.5v (note 1) 4.8 5.2 v ldo undervoltage lockout voltage ldo rising, 200mv hysteresis 2.4 3.0 v out supply voltage range (note 1) 4.5 13.0 v v fb = 1.5v, no load 2.0 out supply current v fb = 1.1v, no load 10.0 ma step-up regulator freq = gnd 512 768 operating frequency freq = in 990 1380 khz
parameter conditions min typ max units freq = gnd 91 99 oscillator maximum duty cycle freq = in 88 96 % fb regulation voltage 1.220 1.252 v fb transconductance i = 5? at comp 75 280 ? lx on-resistance i lx = 200ma 200 m lx current limit 65% duty cycle 2.0 3.0 a positive gate driver timing and control switches src input voltage range 28 v src input current mode = dlp = ctl = ldo 250 ? drn input current mode = dlp = ldo, v drn = 8v, v ctl = 0 250 ? src-to-gon switch on-resistance dlp = ctl = ldo 30 drn-to-gon switch on-resistance dlp = ldo, v ctl = 0 130 thr-to-gon voltage gain v gon = 12v, v thr = 1.2v 9.7 10.3 v/v operational amplifier supb supply range 4.5 13.0 v supb supply current buffer configuration, v posb = 4v, no load 1.0 ma input offset voltage v negb , v posb = v supb / 2 18 mv input common-mode voltage range 0 v supb v i outb = 100? v supb - 15 output voltage swing high i outb = 5ma v supb - 150 mv i outb = -100? 15 output voltage swing low i outb = -5ma 150 mv outb shorted to v supb /2, sourcing 75 short-circuit current outb shorted to v supb /2, sinking 75 ma max8758 step-up regulator with switch control and operational amplifier for tft lcd _______________________________________________________________________________________ 5 electrical characteristics (continued) (v in = v shdn = +3v, out = +10v, freq = gnd, t a = -40? to +85? , unless otherwise noted.) (note 2) note 1: out and sup can operate down to 4.5v. ldo will be out of regulation, but ic will function correctly. note 2: -40? specs are guaranteed by design, not production tested.
max8758 step-up regulator with switch control and operational amplifier for tft lcd 6 _______________________________________________________________________________________ typical operating characteristics (circuit of figure 1, v in = 3.3v, v main = 8.5v, freq = shdn = in, t a = +25?, unless otherwise noted.) step-up regulator efficiency vs. load current (v main = 8.5v) max8758 toc01 load current (ma) efficiency (%) 100 10 55 60 65 70 75 80 85 90 95 50 11000 f osc = 1.2mhz l = 4.7 h v in = 5.5v v in = 1.8v v in = 3.3v step-up regulator efficiency vs. load current (v main = 8.5v) max8758 toc02 load current (ma) efficiency (%) 100 10 55 60 65 70 75 80 85 90 95 50 1 1000 f osc = 640khz l = 10 h v in = 5.5v v in = 1.8v v in = 3.3v output voltage (v) 8.0 8.1 8.2 8.3 8.4 8.5 8.6 7.9 step-up regulator output voltage vs. load current (v main = 8.5v) max8758 toc03 load current (ma) 100 10 1 1000 f osc = 1.2hz v in = 3.3v in quiescent current vs. supply voltage max8758 toc04 v in (v) supply current ( a) 5.0 4.5 4.0 3.5 3.0 2.5 2.0 10 20 30 40 50 0 1.5 5.5 current into in pin not switching v fb - 1.5v temperature ( c) supply current ( a) 60 35 10 -15 25 26 27 28 29 30 24 -40 85 in quiescent current vs. temperature max8758 toc05 current into in pin v in = 3.3v not switching v fb - 1.5v switching frequency vs. input voltage max8758 toc06 v in (v) switching frequency (khz) 4.5 3.5 2.5 600 800 1000 1200 400 1.5 5.5 freq = v in freq = agnd i main = 200ma step-up regulator heavy-load soft-start max8758 toc07 1ms v in 2v/div v main 5v/div i l 500mav/div step-up regulator load transient response max8758 toc08 20 s/div v main ac-coupled 200mv/div l = 4.7 h r comp = 100k c comp1 = 220pf c comp2 = 47pf 50ma 0 i main 500ma/div i l 1av/div
max8758 step-up regulator with switch control and operational amplifier for tft lcd _______________________________________________________________________________________ 7 step-up regulator pulsed load transient response max8758 toc09 20 s/div v main ac-coupled 200mv/div l = 4.7 h r comp = 100k c comp1 = 220pf c comp2 = 47pf i main 1a/div i l 1av/div timer-delay latch response to overload max8758 toc10 20ms/div v main 5v/div lx 5v/div i l 2a/div 0a 0v 0v max8758 toc11 supb supply current vs. supb voltage v supb (v) i supb (ma) 4.5 6.0 7.5 9.0 10.5 12.0 13.5 15.0 0.10 0.15 0.20 0.25 0.30 no load buffer configuration pos_ = v supb / 2 max8758 toc12 supb supply current vs. temperature temperature ( c) i supb (ma) -40 -10 20 50 70 0.10 0.15 0.20 0.25 0.30 no load buffer configuration v posb = v supb / 2 v supb = 12v v supb = 8v v supb = 5v operational amplifier frequency response for various c load max8758 toc13 frequency (hz) magnitude (db) 10k 1k -40 -30 -20 -10 0 10 -50 100 100k v sup = 8.5v a v = 1 r l = 10k 1000pf 56pf power-supply rejection ratio vs. frequency max8758 toc14 frequency (hz) psrr (db) 100k 10k 1k 100 10 20 40 60 80 100 120 0 11m v supb = 8.5v op-amp rail-to-rail input/output max8758 toc15 100 s/div v posb 5v/div v outb 5v/div op-amp load transient response max8758 toc16 1 s/div i outb 50ma/div 0 v outb 2v/div typical operating characteristics (continued) (circuit of figure 1, v in = 3.3v, v main = 8.5v, freq = shdn = in, t a = +25?, unless otherwise noted.)
max8758 step-up regulator with switch control and operational amplifier for tft lcd 8 _______________________________________________________________________________________ typical operating characteristics (continued) (circuit of figure 1, v in = 3.3v, v main = 8.5v, freq = shdn = in, t a = +25?, unless otherwise noted.) op-amp large-signal step response max8758 toc17 1 s/div v outb 2v/div op-amp small-signal step response max8758 toc18 200ns/div v posb 100mv/div ac-coupled v outb 200mv/div ac-coupled high-voltage switch control function (mode 1) max8758 toc19 400 s/div v mode v ctl v gon high-voltage switch control function (mode 2) max8758 toc20 400 s/div v mode v ctl v gon positive charge-pump output voltage vs. charge-pump load current max8758 toc21 charge-pump load current (ma) output voltage (v) 15 10 5 21 22 23 24 25 20 020 v in = 3.3v f osc = 1.2mhz -9 -8 -7 -6 -5 -10 negative charge-pump output voltage vs. load current max8758 toc22 charge-pump load current (ma) output voltage (v) 15 10 5 020 v in = 3.3v f osc = 1.2mhz
max8758 step-up regulator with switch control and operational amplifier for tft lcd _______________________________________________________________________________________ 9 pin description pin name function 1 gnd analog ground 2 gon internal high-voltage-switch common connection. gon is the output of the high-voltage-switch- control block. gon is internally pulled to pgnd through a 1k resistor in shutdown. see the high- voltage switch control section for details. 3 ctl h i g h- v ol tag e, s w i tch- c ontr ol bl ock ti m i ng p i n. s ee the h i g h- v ol tag e s w i tch c ontr ol secti on for d etai l s. 4 dlp high-voltage, switch-control block delay pin. connect a capacitor from dlp to gnd to set the delay time. a 5? current source charges c dlp . dlp is internally pulled to gnd by a resistor in shutdown. see the high-voltage switch control section for details. 5 thr gon falling regulation adjustment pin. connect thr to the center of a resistive voltage-divider between ldo or out and gnd to adjust the v gon falling regulation level. the actual regulation level is 10 x v thr . see the high-voltage switch control section for details. 6 supb operational amplifier supply input. bypass supb to gnd with a 0.1? capacitor. 7 outb operational amplifier output 8 negb operational amplifier inverting input 9 posb operational amplifier noninverting input 10 n.c. no connection. not internally connected. 11 ldo 5v internal linear regulator output. this regulator powers all internal circuitry except the operational amplifier. bypass ldo to gnd with a 0.22? or greater ceramic capacitor. 12 out internal linear regulator supply pin. out is the supply input of the internal 5v linear regulator. connect out directly to the output of the step-up regulator. 13 i.c. internally connected. make no connection to this pin. 14 ss soft-start control pin. connect a capacitor between ss and gnd to set the soft-start period of the step-up regulator. see the bootstrapping and soft-start section for details. 15 comp error amplifier compensation pin. see the step-up regulator loop compensation section for details. 16 freq frequency-select pin. connect freq to gnd for 600khz operation, and connect freq to in for 1.2mhz operation. 17 in supply pin. bypass in to gnd with a 1? ceramic capacitor. place the capacitor close to the in pin. 18 lx switching node. lx is the drain of the internal power mosfet. connect the inductor and the schottky diode to lx and minimize trace area for low emi. 19 shdn shutdown control pin. pull shdn low to turn off the step-up regulator, the operational amplifier, and the switch control block. 20 fb feedback pin. the fb regulation point is 1.24v (typ). connect fb to the center of a resistive voltage- divider between the step-up regulator output and gnd to set the step-up regulator output voltage. place the divider close to the fb pin. 21 pgnd power ground 22 mode high-voltage, switch-control block-mode selection timing-adjustment pin. see the high-voltage switch control section for details. mode is high impedance when it is connected to ldo. mode is internally pulled down by a 1k resistor during uvlo, when v dlp < 0.5 x v ldo , or in shutdown. 23 drn high-voltage, switch-control input. drn is the drain of the internal high-voltage p-channel mosfet connected to gon. 24 src high-voltage switch-control input. src is the source of the internal high-voltage p-channel mosfet.
max8758 typical operating circuit the typical operating circuit ( figure 1) of the max8758 is a power-supply solution for tft lcd panels in note- book computers. the circuit generates a +8.5v source driver supply, and approximately +22v and -7v gate driver supplies. the input voltage range for the ic is from +1.8v to +5.5v, but the figure 1 circuit is designed to run from 2.7v to 3.6v. table 1 lists some selected components and table 2 lists the contact information of component suppliers. step-up regulator with switch control and operational amplifier for tft lcd 10 ______________________________________________________________________________________ max8758 v main +8.5v/300ma to vcom backplane v gon +24v/20ma from tcon v in +1.8v to +5.5v v goff -8v/20ma in freq shdn comp ldo fb lx gnd pgnd out supb gon drn ctl mode ss posb negb outb thr dlp src c15 0.1 f c1 3.3 f 6.3v c2 3.3 f 6.3v r4 10 c6 1 f r10 100k r3 100k c7 220pf c8 33pf c9 0.22 f c10 0.022 f c11 150pf r9 20k d2 d3 d4 c3 4.7 f 10v c4 4.7 f 10v c5 4.7 f 10v l1 4.7 h c6 0.1 f c17 0.1 f c19 0.1 f c18 0.1 f r1 200k 1% r2 34.0k 1% c12 0.1 f r5 100k r6 100k r8 20.0k 1% r7 51.1k 1% c13 0.033 f c14 0.1 f d1 figure 1. typical operating circuit
detailed description the max8758 is designed primarily for tft lcd panels used in notebook computers. it contains a high-perfor- mance step-up regulator, a high-speed operational amplifier, a logic-controlled, high-voltage switch-control block with programmable delay, and an internal linear regulator for bootstrapping operation. figure 2 shows the max8758 functional block diagram. step-up regulator the step-up regulator is designed to generate the lcd source driver supply. it employs a current-mode, fixed- frequency pwm architecture to maximize loop band- width and provide fast transient response to pulsed loads typical of tft lcd panel source drivers. the inter- nal oscillator offers two pin-selectable frequency options (640khz/1.2mhz), allowing users to optimize their designs based on the specific application requirements. max8758 step-up regulator with switch control and operational amplifier for tft lcd ______________________________________________________________________________________ 11 src gon dlp mode thr ctl drn switch control supb negb outb posb gnd pgnd step-up regulator controller fb comp ss lx linear regulator and bootstrap v in max8758 shdn freq ldo in figure 2. functional diagram table 1. component list designation description c1, c2 3.3? ?0%, 6.3v x5r ceramic capacitors (0603) tdk c1608x5r0j335m c3, c4, c5 4.7? ?0%, 10v x5r ceramic capacitors (1206) tdk c3216x5r1a475m d1 3a, 30v schottky diode (m-flat) toshiba cms02 (top mark s2) d2, d3, d4 200ma, 100v dual diodes (sot23) fairchild mmbd4148se (top mark d4) l1 4.2?, 1.9a inductor sumida cdrh6d12-4r2
max8758 the internal n-channel power mosfet reduces the number of external components. the supply rail of the internal gate driver is bootstrapped to the internal linear regulator output to improve the efficiency at low-input voltages. the external-capacitor, soft-start function effectively controls inrush currents. the output voltage can be set from v in to 13v with an external resistive voltage-divider. pwm control block figure 3 is the block diagram of the step-up regulator. the regulator controls the output voltage and the power delivered to the output by modulating the duty cycle (d) of the internal power mosfet in each switching cycle. the duty cycle of the mosfet is approximated by: where v out is the output voltage of the step-up regulator. on the rising edge of the internal oscillator clock, the controller sets a flip-flop, turning on the n-channel mosfet and applying the input voltage across the inductor. the current through the inductor ramps up lin- early, storing energy in its magnetic field. a transcon- ductance error amplifier compares the fb voltage with a 1.24v (typ) reference voltage. the error amplifier changes the comp voltage by charging or discharging the comp capacitor. the comp voltage is compared with a ramp, which is the sum of the current-sense sig- nal and a slope compensation signal. once the ramp signal exceeds the comp voltage, the controller resets the flip-flop and turns off the mosfet. since the induc- tor current is continuous, a transverse potential devel- ops across the inductor that turns on the schottky diode (d1 in figure 1). the voltage across the inductor then becomes the difference between the output volt- age and the input voltage. this discharge condition forces the current through the inductor to ramp down, transferring the energy stored in the magnetic field to the output capacitor and the load. the mosfet remains off for the rest of the clock cycle. bootstrapping and soft-start the max8758 features bootstrapping operation. in nor- mal operation, the internal linear regulator supplies power to the internal circuitry. the input of the linear regulator (out) should be directly connected to the output of the step-up regulator. the step-up regulator is enabled when the input voltage at out is above 1.75v, shdn is high, and the fault latch is not set. after being enabled, the regulator starts open-loop switching to generate the supply voltage for the linear regulator with a controlled duty cycle. the internal ref- erence block turns on when the ldo voltage exceeds 2.7v (typ). when the reference voltage reaches regula- tion, the pwm controller and the current-limit circuit are enabled and the step-up regulator enters soft-start. d vv v out in out ? step-up regulator with switch control and operational amplifier for tft lcd 12 ______________________________________________________________________________________ table 2. component suppliers supplier phone fax website fairchild semiconductor 408-822-2000 408-822-2102 www.fairchildsemi.com sumida 847-545-6700 847-545-6720 www.sumida.com tdk 847-803-6100 847-390-4405 www.component.tdk.com toshiba 949-455-2000 949-859-3963 www.toshiba.com/taec soft- start current sense oscillator logic and driver clock slope comp to fault logic 1.0v 1.24v i limit lx pgnd ss fb comp freq ilim comparator pwm comparator fault comparator error amp figure 3. step-up regulator block diagram
the soft-start timing can be adjusted with an external capacitor connected between ss and gnd. after the step-up regulator is enabled, the ss pin is immediately charged to 0.5v. then the capacitor is charged at a constant current of 4? (typ). during this time, the ss voltage directly controls the peak inductor current, allowing a linear ramp from zero up to the full current limit. the maximum load current is available after the voltage on ss exceeds 1.5v. the soft-start capacitor is discharged to ground when shdn is low. the soft-start routine minimizes inrush current and voltage overshoot and ensures a well-defined startup behavior (see the step-up regulator heavy load soft-start waveform in the typical operating characteristics ). fault protection during steady-state operation, the max8758 monitors the fb voltage. if the fb voltage is below 1v (typ), the max8758 activates an internal fault timer. if there is a continuous fault for the fault-timer duration, the max8758 sets the fault latch, shutting down all the outputs. once the fault condition is removed, cycle the input voltage to clear the fault latch and reactivate the device. the fault- detection circuit is disabled during the soft-start time. the max8758 monitors the out voltage for undervoltage and overvoltage conditions. if the out voltage is below 1.4v (typ) or above 13.5v (typ), the max8758 disables the gate driver of the step-up regulator and prevents the internal mosfet from switching. the out undervoltage and overvoltage conditions do not set the fault latch. thermal-overload protection the thermal-overload protection prevents excessive power dissipation from overheating the max8758. when the junction temperature exceeds t j = +160?, a thermal sensor immediately activates the fault protec- tion, which sets the fault latch and shuts down all the outputs, allowing the device to cool down. once the device cools down by approximately 15?, cycle the input voltage or toggle shdn to clear the fault latch and restart the device. the thermal-overload protection protects the controller in the event of fault conditions. for continuous opera- tion, do not exceed the absolute maximum junction temperature rating of t j = +150?. frequency selection (freq) the freq pin selects the switching frequency. table 3 shows the switching frequency based on the freq con- nection. high-frequency (1.2mhz) operation optimizes the application for the smallest component size, trading off efficiency due to higher switching losses. low-fre- quency (600khz) operation offers the best overall efficien- cy at the expense of component size and board space. operational amplifier the max8758? operational amplifier is typically used to drive the lcd backplane (vcom) or the gamma-cor- rection-divider string. the operational amplifier features ?50ma output short-circuit current, 7.5v/? slew rate, and 12mhz bandwidth. the rail-to-rail input and output capability maximizes system flexibility. short-circuit current limit the operational amplifier limits short-circuit current to approximately ?50ma if the output is directly shorted to supb or to gnd. if the short-circuit condition persists, the junction temperature of the ic rises until it reaches the thermal shutdown threshold (+160? typ). once the junction temperature reaches the thermal shutdown threshold, an internal thermal sensor immediately sets the thermal fault latch, shutting off all the ic? outputs. the device remains inactive until the input voltage is cycled or shdn is toggled. driving pure capacitive load the operational amplifier is typically used to drive the lcd backplane (vcom) or the gamma-correction divider string. the lcd backplane consists of a distrib- uted series capacitance and resistance, a load that can be easily driven by the operational amplifier. however, if the operational amplifier is used in an application with a pure capacitive load, steps must be taken to ensure stable operation. as the operational amplifier? capacitive load increases, the amplifier? bandwidth decreases and gain peaking increases. a 5 to 50 small resistor placed between outb and the capacitive load reduces peaking but also reduces the gain. an alternative method of reducing peaking is to place a series rc network (snubber) in par- allel with the capacitive load. the rc network does not continuously load the output or reduce the gain. typical values of the resistor are between 100 and 200 and the typical value of the capacitor is 10pf. high-voltage switch control the max8758? high-voltage switch-control block ( figure 5) consists of two high-voltage, p-channel mosfets: q1, between src and gon and q2, between gon and drn. the switch-control block is enabled when v dlp exceeds v ldo /2 and then q1 and q2 are controlled by ctl and mode. there are two different modes of opera- tion (see the typical operating characteristics section.) max8758 step-up regulator with switch control and operational amplifier for tft lcd ______________________________________________________________________________________ 13 table 3. frequency selection freq switching frequency (khz) gnd 600 in 1200
max8758 activate the first mode by connecting mode to ldo. when ctl is logic high, q1 turns on and q2 turns off, connecting gon to src. when ctl is logic low, q1 turns off and q2 turns on, connecting gon to drn. gon can then be discharged through a resistor con- nected between drn and pgnd or av dd . q2 turns off and stops discharging gon when v gon reaches 10 times the voltage on thr. when v mode is less than 0.9 x v ldo , the switch control block works in the second mode. the rising edge of v ctl turns on q1 and turns off q2, connecting gon to src. an internal n-channel mosfet q3 between mode and gnd is also turned on to discharge an external capacitor between mode and gnd. the falling edge of v ctl turns off q3, and an internal 50? current source starts charging the mode capacitor. once v mode exceeds 0.5 x v ref , the switch control block turns off q1 and turns on q2, connecting gon to drn. gon can then be discharged through a resistor connected between drn and gnd or av dd . q2 turns off and stops discharging gon when v gon reaches 10 times the voltage on thr. the timing of enabling the switch control block can be adjusted with an external capacitor connected between dlp and gnd. an internal current source starts charg- ing the dlp capacitor if the input voltage is above 1.75v (typ), shdn is high, and the fault latch is not set. the voltage on dlp linearly rises because of the con- stant-charging current. when vdlp goes above 2.5v (typ), the switch control block is enabled. the switch control block is disabled and dlp is held low when the max8758 is shut down or in a fault state. linear regulator (ldo) the max8758 includes an internal 5v linear regulator. out is the input of the linear regulator and should be directly connected to the output of the step-up regulator. the input voltage range is between 4.5v and 13v. the output of the linear regulator (ldo) is set to 5v (typ). the regulator powers all the internal circuitry including the gate driver. this feature significantly improves the effi- ciency at low input voltages. bypass the ldo pin to gnd with a 0.22? or greater ceramic capacitor. design procedure step-up regulator step-up regulator inductor selection the inductance value, peak-current rating, and series resistance are factors to consider when selecting the inductor. these factors influence the converter? effi- ciency, maximum output-load capability, transient response time, and output voltage ripple. physical size and cost are also important factors to be considered. the maximum output current, input voltage, output volt- age, and switching frequency determine the inductor value. very high inductance values minimize the cur- rent ripple and, therefore, reduce the peak current, which decreases core losses in the inductor and i 2 r losses in the entire power path. however, large induc- tor values also require more energy storage and more turns of wire, which increase physical size and can increase i 2 r losses in the inductor. low inductance val- ues decrease the physical size but increase the current ripple and peak current. finding the best inductor involves choosing the best compromise between circuit efficiency, inductor size, and cost. the equations used here include a constant lir, which is the ratio of the inductor peak-to-peak ripple current to the average dc inductor current at the full-load cur- rent. the best trade-off between inductor size and cir- cuit efficiency for step-up regulators generally has an lir between 0.3 and 0.5. however, depending on the ac characteristics of the inductor core material and ratio of inductor resistance to other power-path resis- tances, the best lir can shift up or down. if the induc- tor resistance is relatively high, more ripple can be accepted to reduce the number of turns required and increase the wire diameter. if the inductor resistance is relatively low, increasing inductance to lower the peak current can decrease losses throughout the power path. if extremely thin high-resistance inductors are used, as is common for lcd panel applications, the best lir can increase to between 0.5 and 1.0. step-up regulator with switch control and operational amplifier for tft lcd 14 ______________________________________________________________________________________
max8758 step-up regulator with switch control and operational amplifier for tft lcd ______________________________________________________________________________________ 15 ref dlp mode ctl fault shdn ref_ok 0.5 x v ref 5 a ref 50 a 1k 9r r q3 q4 q1 q2 src gon drn thr q5 4r 1k 5r r figure 4. switch control
max8758 in figure 1? typical operating circuit , the lcd? gate- on and gate-off voltages are generated from two unreg- ulated charge pumps driven by the step-up regulator? lx node. the additional load on lx must therefore be considered in the inductance calculation. the effective maximum output current i main(eff) becomes the sum of the maximum load current on the step-up regulator? output plus the contributions from the positive and neg- ative charge pumps: i main(eff) = i main(max) + n neg x i neg + (n pos + 1) x i pos where i main(max) is the maximum output current, n neg is the number of negative charge-pump stages, n pos is the number of positive charge-pump stages, i neg is the negative charge-pump output current, and i pos is the positive charge-pump output current, assuming the pump source for i pos is v main . the required inductance can then be calculated as follows: where v in is the typical input voltage and typ is the expected efficiency obtained from the appropriate curve in the typical operating characteristics. choose an available inductor value from an appropriate inductor family. calculate the maximum dc input cur- rent at the minimum input voltage v in(min) using con- servation of energy and the expected efficiency at that operating point ( min ) taken from an appropriate curve in the typical operating characteristics : calculate the ripple current at that operating point and the peak current required for the inductor: the inductor? saturation current rating and the guaran- teed minimum value of the max8758? lx current limit (i lim ) should exceed i peak and the inductor? dc current rating should exceed i in(dc,max) . for good efficiency, choose an inductor with less than 0.1 series resistance. considering the typical operating circuit , the maxi- mum load current (i main(max) ) is 300ma for the step- up regulator, 20ma for the two-stage positive charge pump, and 20ma for the one-stage negative charge pump. altogether, the effective maximum output cur- rent, i main(eff) is 360ma with an 8.5v output and a typical input voltage of 3.3v. the switching frequency is set to 1.2mhz. choosing an lir of 0.4 and estimating efficiency of 85% at this operating point: using the circuit? minimum input voltage (3v) and esti- mating efficiency of 80% at that operating point: the ripple current and the peak current are: the peak-inductor current does not exceed the guaran- teed minimum value of the lx current limit in the electrical characteristics table . step-up regulator output capacitor selection the total output voltage ripple has two components: the capacitive ripple caused by the charging and discharg- ing of the output capacitance, and the ohmic ripple due to the capacitor? equivalent series resistance (esr): v ripple = v ripple(c) + v aripple(esr) and v ripple(esr) i peak x r esr where i peak is the peak inductor current (see the step- up regulator inductor selection section). for ceramic capacitors, the output voltage ripple is typically dominat- ed by v ripple(c) . the voltage rating and temperature characteristics of the output capacitor must also be con- sidered. v i c vv vf ripple c main main main in main sw () ? ? ? ? ? ? ? ? ia a a peak . . . =+ 128 04 2 148 i vvv h v mhz a ripple (. ) . . . . = ? 3853 42 85 12 04 i av v a in dc max (, ) . . . . = 036 85 308 128 l v v vv a mhz h . . . . . . . . . = ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 33 85 85 33 036 12 085 04 42 2 ii i peak in dc max ripple (, ) =+ 2 i vvv lv f ripple in min main in min main osc () () = () ? i iv v in dcmax main eff main in min min (, ) () () = l v v vv i f lir in main main in main eff osc typ () = ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 2 step-up regulator with switch control and operational amplifier for tft lcd 16 ______________________________________________________________________________________
step-up regulator input capacitor selection the input capacitor reduces the current peaks drawn from the input supply and reduces noise injection into the ic. two 10? ceramic capacitors are used in the typical applications circuit ( figure 1) because of the high source impedance seen in typical lab setups. actual applications usually have much lower source impedance since the step-up regulator often runs directly from the output of another regulated supply. typically, the input capacitance can be reduced below the values used in the typical applications circuit . step-up regulator rectifier diode the max8758? high switching frequency demands a high-speed rectifier. schottky diodes are recommend- ed for most applications because of their fast recovery time and low forward voltage. in general, a 2a schottky diode complements the internal mosfet well. step-up regulator output voltage selection the output voltage of the step-up regulator can be adjusted by connecting a resistive voltage-divider from the output (v out ) to gnd with the center tap connect- ed to fb (see figure 1). select r2 in the 10k to 50k range. calculate r1 with the following equation: where v fb , the step-up regulator? feedback set point, is 1.25v. place r1 and r2 close to the ic. step-up regulator loop compensation choose r comp (r3 in figure 1) to set the high-frequen- cy integrator gain for fast transient response. choose c comp (c7 in figure 1) to set the integrator zero to maintain loop stability. for low-esr output capacitors, use the following equa- tions to obtain stable performance and good transient response: to further optimize transient response, vary r comp in 20% steps and c comp in 50% steps while observing transient-response waveforms. place c comp2 (c8 in figure 1) from comp to gnd to add an additional high-frequency pole. usec comp2 between 10pf and 47pf. step-up regulator soft-start capacitor the soft-start capacitor should be large enough that it does not reach final value before the output has reached regulation. calculate the soft-start capacitor (c ss ) value using: where c main is the total output capacitance, v main is the maximum output voltage, and i inrush is the peak inrush current allowed, i main is the maximum output current, and v in is the minimum input voltage. the load must wait for the soft-start cycle to finish before drawing a significant amount of load current. the duration after which the load can begin to draw maximum load current is: t max = 6.77 x 10 5 x c ss charge pumps selecting the number of charge-pump stages for highest efficiency, always choose the lowest num- ber of charge-pump stages that meet the output volt- age requirement. the number of positive charge-pump stages is given by: where n pos is the number of positive-charge-pump stages, v gon is the positive-charge-pump output, v main is the main step-up regulator output, and v d is the forward voltage drop of the charge-pump diode. the number of negative charge-pump stages is given by: where n neg is the number of negative-charge-pump stages, v goff is the negative charge-pump output, v main is the main step-up regulator output, and v d is the forward voltage drop of the charge-pump diode. n v vv neg goff main d = ? ? 2 n vv vv pos gon main main d = ? ? 2 cc vvv vi i v ss main main in main in inrush main main = ? ? ? ? ? ? ? ? ? 21 10 6 2 c vc ir comp main main main max comp () 10 r vv c li comp in main main main max () 315 rr v v main fb 12 1 = ? ? ? ? ? ? ? max8758 step-up regulator with switch control and operational amplifier for tft lcd ______________________________________________________________________________________ 17
max8758 charge-pump flying capacitors increasing the flying capacitor (c6, c17, c18) value lowers the effective source impedance and increases the output-current capability. increasing the capaci- tance indefinitely has a negligible effect on output-cur- rent capability because the diode impedance places a lower limit on the source impedance. ceramic capaci- tors of 0.1? or greater work well in most applications that require output currents in the order of 10ma to 20ma. the flying capacitor? voltage rating must exceed the following: v c > n x v main where n is the stage number in which the flying capaci- tor appears, and v main is the output voltage of the main step-up regulator. charge-pump output capacitor increasing the output capacitance or decreasing the esr reduces the output voltage ripple and the peak-to- peak voltage during load transients. with ceramic capacitors, the output voltage ripple is dominated by the capacitance value. use the following equation to approximate the required capacitor value: where c main_cp is the output capacitor of the charge pump, i load_cp is the load current of the charge pump, and v ripple_cp is the peak-to-peak value of the output ripple. the charge-pump output capacitor is typically also the input capacitor for a linear regulator. often, its value must be increased to maintain the linear regulator? stability. charge-pump rectifier diodes use low-cost, silicon-switching diodes with a current rating equal to or greater than two times the average charge-pump input current. if it helps avoid an extra stage, some or all of the diodes can be replaced with schottky diodes with equivalent current ratings. pc board layout and grounding careful pc board layout is important for proper operation. use the following guidelines for good pc board layout: 1) minimize the area of high-current loops by placing the step-up regulator? inductor, diode, and output capacitors near its input capacitors, its lx, and pgnd pin. the high-current input loop goes from the positive terminal of the input capacitor to the inductor, to the ic? lx pin, out of pgnd, and to the input capacitor? negative terminal. the high- current output loop is from the positive terminal of the input capacitor to the inductor, to the output diode (d1), to the positive terminal of the output capacitors, reconnecting between the output capacitor and input capacitor ground terminals. connect these loop components with short, wide connections. avoid using vias in the high-current paths. if vias are unavoidable, use many vias in parallel to reduce resistance and inductance. 2) create a power ground island (pgnd) for the step-up regulator, consisting of the input and out- put capacitor grounds and the pgnd pin. maximizing the width of the power ground traces improves efficiency and reduces output voltage ripple and noise spikes. create an analog ground plane (gnd) consisting of the gnd pin, the feed- back-divider ground connection, the comp and dlp capacitor ground connections, and the device? exposed backside pad. connect the pgnd and gnd islands by connecting the two ground pins directly to the exposed backside pad. make no other connections between these sepa- rate ground planes. 3) place the feedback voltage-divider resistors as close to the feedback pin as possible. the divider? center trace should be kept short. placing the resistors far away causes the fb trace to become antennas that can pick up switching noise. care should be taken to avoid running the feedback trace near lx. 4) place the in pin bypass capacitor as close to the device as possible. the ground connection of the in bypass capacitor should be connected directly to the gnd pin with a wide trace. 5) minimize the length and maximize the width of the traces between the output capacitors and the load for best transient responses. 6) minimize the size of the lx node while keeping it wide and short. keep the lx node away from feedback node (fb) and analog ground. use dc traces as shield if necessary. refer to the max8758 evaluation kit for an example of proper board layout. c i fv main cp load cp osc ripple cp _ _ _ 2 step-up regulator with switch control and operational amplifier for tft lcd 18 ______________________________________________________________________________________
max8758 step-up regulator with switch control and operational amplifier for tft lcd ______________________________________________________________________________________ 19 chip information transistor count: 3208 process: bicmos pin configuration max8758 thin qfn 4mm x 4mm top view 2 gon 1 gnd 3 ctl 4 dlp 5 thr 6 supb 24 src 23 drn 22 mode 21 pgnd 20 fb 19 shdn 18 lx 17 in 16 freq 15 comp 14 ss 13 i.c. 11 ldo 10 n.c. 12 out 9 posb 8 negb 7 outb
max8758 step-up regulator with switch control and operational amplifier for tft lcd maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a maxim product. no circu it patent licenses are implied. maxim reserves the right to change the circuitry and specifications without notice at any time. 20 ____________________maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 2005 maxim integrated products printed usa is a registered trademark of maxim integrated products, inc. package information (the package drawing(s) in this data sheet may not reflect the most current specifications. for the latest package outline info rmation, go to www.maxim-ic.com/packages .) qfn thin.eps d2 (nd-1) x e e d c pin # 1 i.d. (ne-1) x e e/2 e 0.08 c 0.10 c a a1 a3 detail a e2/2 e2 0.10 m c a b pin # 1 i.d. b 0.35x45 d/2 d2/2 l c l c e e l c c l k l l detail b l l1 e xxxxx marking h 1 2 21-0140 package outline, 16, 20, 28, 32, 40l thin qfn, 5x5x0.8mm -drawing not to scale- l e/2 common dimensions 3.35 3.15 t2855-1 3.25 3.35 3.15 3.25 max. 3.20 exposed pad variations 3.00 t2055-2 3.10 d2 nom. min. 3.20 3.00 3.10 min. e2 nom. max. ne nd pkg. codes 1. dimensioning & tolerancing conform to asme y14.5m-1994. 2. all dimensions are in millimeters. angles are in degrees. 3. n is the total number of terminals. 4. the terminal #1 identifier and terminal numbering convention shall conform to jesd 95-1 spp-012. details of terminal #1 identifier are optional, but must be located within the zone indicated. the terminal #1 identifier may be either a mold or marked feature. 5. dimension b applies to metallized terminal and is measured between 0.25 mm and 0.30 mm from terminal tip. 6. nd and ne refer to the number of terminals on each d and e side respectively. 7. depopulation is possible in a symmetrical fashion. 8. coplanarity applies to the exposed heat sink slug as well as the terminals. 9. drawing conforms to jedec mo220, except exposed pad dimension for t2855-1, t2855-3, and t2855-6. notes: symbol pkg. n l1 e e d b a3 a a1 k 10. warpage shall not exceed 0.10 mm. jedec t1655-1 3.20 3.00 3.10 3.00 3.10 3.20 0.70 0.80 0.75 4.90 4.90 0.25 0.25 0 -- 4 whhb 4 16 0.35 0.30 5.10 5.10 5.00 0.80 bsc. 5.00 0.05 0.20 ref. 0.02 min. max. nom. 16l 5x5 3.10 t3255-2 3.00 3.20 3.00 3.10 3.20 2.70 t2855-2 2.60 2.60 2.80 2.70 2.80 l 0.30 0.50 0.40 -- - -- - whhc 20 5 5 5.00 5.00 0.30 0.55 0.65 bsc. 0.45 0.25 4.90 4.90 0.25 0.65 - - 5.10 5.10 0.35 20l 5x5 0.20 ref. 0.75 0.02 nom. 0 0.70 min. 0.05 0.80 max. -- - whhd-1 28 7 7 5.00 5.00 0.25 0.55 0.50 bsc. 0.45 0.25 4.90 4.90 0.20 0.65 - - 5.10 5.10 0.30 28l 5x5 0.20 ref. 0.75 0.02 nom. 0 0.70 min. 0.05 0.80 max. -- - whhd-2 32 8 8 5.00 5.00 0.40 0.50 bsc. 0.30 0.25 4.90 4.90 0.50 - - 5.10 5.10 32l 5x5 0.20 ref. 0.75 0.02 nom. 0 0.70 min. 0.05 0.80 max. 0.20 0.25 0.30 down bonds allowed no yes 3.10 3.00 3.20 3.10 3.00 3.20 t2055-3 3.10 3.00 3.20 3.10 3.00 3.20 t2055-4 t2855-3 3.15 3.25 3.35 3.15 3.25 3.35 t2855-6 3.15 3.25 3.35 3.15 3.25 3.35 t2855-4 2.60 2.70 2.80 2.60 2.70 2.80 t2855-5 2.60 2.70 2.80 2.60 2.70 2.80 t2855-7 2.60 2.70 2.80 2.60 2.70 2.80 3.20 3.00 3.10 t3255-3 3.20 3.00 3.10 3.20 3.00 3.10 t3255-4 3.20 3.00 3.10 no no no no no no no no yes yes yes yes 3.20 3.00 t1655-2 3.10 3.00 3.10 3.20 yes no 3.20 3.10 3.00 3.10 t1655n-1 3.00 3.20 3.35 3.15 t2055-5 3.25 3.15 3.25 3.35 yes 3.35 3.15 t2855n-1 3.25 3.15 3.25 3.35 no 3.35 3.15 t2855-8 3.25 3.15 3.25 3.35 yes 3.20 3.10 t3255n-1 3.00 no 3.20 3.10 3.00 l 0.40 0.40 ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** see common dimensions table 0.15 11. marking is for package orientation reference only. h 2 2 21-0140 package outline, 16, 20, 28, 32, 40l thin qfn, 5x5x0.8mm -drawing not to scale- 12. number of leads shown are for reference only. 3.30 t4055-1 3.20 3.40 3.20 3.30 3.40 ** yes 0.05 00.02 0.60 0.40 0.50 10 ----- 0.30 40 10 0.40 0.50 5.10 4.90 5.00 0.25 0.35 0.45 0.40 bsc. 0.15 4.90 0.25 0.20 5.00 5.10 0.20 ref. 0.70 min. 0.75 0.80 nom. 40l 5x5 max. 13. lead centerlines to be at true position as defined by basic dimension "e", 0.05.

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