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evaluation kit follows data sheet ecb and lcd display bias supply with accurate output voltage and temperature compensation general description the MAX1729 micropower step-up/step-down dc-dc converter is ideally suited for electrically controlled bire- fringence (ecb) and liquid-crystal-display (lcd) bias- supply generation. it provides step-up/step-down voltage conversion and reduces output ripple by using a step-up dc-dc converter followed by a linear regula- tor. this architecture permits a physically smaller induc- tor than those used in competing sepic and flyback topologies. this device features low quiescent current (67? typical). a logic-controlled shutdown mode fur- ther reduces quiescent current to 0.4? typical. the MAX1729 features an input that dynamically adjusts the output voltage to control display color or contrast. it offers two feedback modes: internal and external. internal feedback mode allows output volt- ages between 2.5v and 16v, and is specifically designed to hold temperature drift to ?1ppm/?. external feedback mode allows the MAX1729 output voltage range to be tailored for various displays. an on-chip temperature sensor with a positive tempera- ture coefficient provides compensation for lcd/ecb display temperature characteristics. in internal feed- back mode, the buffered temperature sensor output is read and used to adjust the output voltage via a digital control signal. external feedback mode features an additional compensation method in which the tempera- ture output is summed directly into the feedback net- work to provide first-order negative temperature compensation of the output voltage. the MAX1729 is available in the space-saving 10-pin ?ax package. applications ecb display bias & color adjustment lcd display bias & contrast adjustment cellular phones personal digital assistants features ? high-accuracy reference voltage (?%) ? ?1ppm/? output voltage drift ? on-chip temperature sensor output ? accurate voltage and temperature provide: consistent ecb colors consistent lcd gray-scale contrast ? +2.7v to +5.5v input voltage range ? output voltage range +2.5v to +16v in internal feedback mode programmable in external feedback mode ? dynamic control of the output voltage ? 67? supply current ? 0.4? shutdown current ? 10-pin ?ax package (1.09mm max height) ? evaluation kit available (MAX1729evkit) MAX1729 ________________________________________________________________ maxim integrated products 1 ps out fb comp lx tc ctlin v out +2.5v to +16v digital pwm controller gnd in v in 2.7v to 5.5v MAX1729 ref adc typical operating circuit 19-1406; rev 1; 1/99 part MAX1729eub -40? to +85? temp. range pin-package 10 ?ax ordering information pin configuration 1 2 3 4 5 10 9 8 7 6 gnd lx ps out comp ref tc in MAX1729 m max top view ctlin fb for free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. for small orders, phone 1-800-835-8769.
MAX1729 ecb and lcd display bias supply with accurate output voltage and temperature compensation 2 _______________________________________________________________________________________ absolute maximum ratings electrical characteristics (v in = +3v, ctlin = in, fb = gnd, t a = -40? to +85?, unless otherwise noted. typical values are at t a = +25?.) (note 1) 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 to gnd .................................................................-0.3v to +6v lx, ps, out to gnd...............................................-0.3v to +20v ctlin, fb, ref, comp, tc to gnd ...........-0.3v to (v in + 0.3v) lx to ps ..................................................................-20v to +1.0v lx, ps, out current ...........................................................60ma continuous power dissipation (t a = +70?) 10-pin ?ax (derate 5.6mw/? above +70?) ..........444mw operating temperature range ...........................-40? to +85? junction temperature ......................................................+150? storage temperature range .............................-65? to +160? lead temperature (soldering, 10sec) .............................+300? conditions v 2.7 5.5 v in operating voltage range units min typ max symbol parameter v 2.0 2.6 v lo undervoltage lockout threshold (note 2) ? 37 50 i in in supply current ? 30 40 i ps ps supply current ? 0.4 2 i shdn shutdown supply current ctlin = gnd, i shdn = i in + i ps i ref = 0 v 1.215 1.228 1.241 v ref reference output voltage t a = +25? 1.200 1.256 t a = -40? to +85? fb = gnd, ctlin = 0.1% duty cycle, i out = 0 to 0.5ma v v out (min) minimum output voltage 2.35 2.45 2.5 t a = -40? to +85? i out = 0 to 0.5ma v 16 16.40 v out (max) maximum output voltage fb = gnd, ctlin = 0.1% to 100% duty cycle, i out = 0 v/100% ctlin to v out gain 13.90 13.95 14.00 t a = 0? to +85? v ps = +18v (note 3) ppm/? ?1 30 tc out output voltage temperature coefficient t a = 0? to +85? ?8 65 t a = -40? to +85? ma 0.5 2.5 i out maximum output current v 1.178 1.228 1.278 v tc tc output voltage t a = +25? mv/? 15.5 16.5 17.5 tc tc tc output temperature coefficient (note 3) t a = 0? to +85? 14.5 16.5 18.5 t a = -40? to +85? ? ?0 i tc tc output current t a = 0? to +85? 2.35 2.52 t a = -40? to +85? 13.60 14.20 v 1.215 1.228 1.241 v fb feedback set voltage (fb) t a = +25? 1.200 1.256 t a = -40? to +85? mv 90 122 150 v mode fb mode threshold na 550 i fb fb bias current v fb = +1.25v v 2 v ih ctlin high voltage v in = +5.5v 1.3 v in = +2.7v
MAX1729 ecb and lcd display bias supply with accurate output voltage and temperature compensation _______________________________________________________________________________________ 3 note 1: specifications to -40? are guaranteed by design, not production tested. note 2: when v in is below this level, the boost and ldo outputs are disabled. note 3: guaranteed by design. note 4: minimum time to hold ctlin low to invoke shutdown. if ctlin is held low for less than t off , device does not enter shutdown. note 5: switching regulator regulates this voltage to keep ldo from dropping out. electrical characteristics (v in = +3v, ctlin = in, fb = gnd, t a = -40? to +85?, unless otherwise noted. typical values are at t a = +25?.) (note 1) v fb = 0, internal feedback mode v 0.4 0.6 1.0 v in = +5.5v, ctlin = gnd or in v in = +2.7v to +5.5v ps to out voltage (note 5) t a = +25? i diode = 30ma (note 4) i lx = 30ma ns 25 conditions 511 v-? 6810 k pfm on-time constant t ctlin ctlin minimum pulse width for v out control v fb = +1.25v ns w 60 150 r comp comp impedance ? ? i ihl v 0.4 v il ctlin low voltage ctlin bias current 20 t r /t f comp rise/fall time v in = +5.5v 1.5 mv 700 970 v lx-ps lx to ps diode forward voltage ? 700 1250 2400 t off ctlin minimum pulse width for shutdown w 2.5 5.0 r on switch on-resistance units min typ max symbol parameter v in = +2.7v v lx = 18v ? 0.1 1 i lx(off) switch off-leakage current v fb = +1.25v, external feedback mode 33 k w t a = -40? to +85?
MAX1729 ecb and lcd display bias supply with accurate output voltage and temperature compensation 4 _______________________________________________________________________________________ typical operating characteristics (circuit of figure 2, t a = +25?, unless otherwise noted.) 0 0.01 10 1 0.1 efficiency vs. output current v out = 9.4 (ctlin at 50% duty cycle) 20 10 30 60 50 40 MAX1729 toc 01 output current (ma) efficiency (%) v in = 5.5v v in = 2.7v 0 0.01 10 1 0.1 efficiency vs. output current v out = 16.4 (ctlin = in) 20 10 30 60 50 40 MAX1729 toc 02 output current (ma) efficiency (%) v in = 5.5v v in = 2.7v 0.05 0.15 0.10 0.25 0.20 0.35 0.30 0.40 2.5 3.5 4.0 3.0 4.5 5.0 5.5 6.0 no-load supply current vs. supply voltage MAX1729 toc 03 supply voltage (v) supply current (ma) v out = 16.4v v out = 9.4v v out = 2.5v 0 2 1 4 3 6 5 7 2.5 3.5 4.0 3.0 4.5 5.0 5.5 6.0 maximum output current vs. supply voltage MAX1729 toc 04 supply voltage (v) maximum output current (ma) v out = +2.5v v out = 9.4v v out = 16.4v 0 4 2 8 6 10 12 16 14 18 0 203040 10 50 60 70 80 90 100 output voltage vs. duty cycle MAX1729 toc 07 duty cycle (%) output voltage (v) 0 -100 1 10 100 1k 10k 100k ps to out (ldo) power-supply rejection ratio -80 MAX1729 toc 05 frequency (hz) psrr (db) -60 -40 -20 0 1.0 0.5 2.0 1.5 2.5 3.0 2.5 4.0 4.5 3.0 3.5 5.0 5.5 6.0 shutdown supply current MAX1729 toc 06 input voltage (v) shutdown current ( m a) 20ms/div start-up delay from shutdown v ctlin 5v/div v ref 1v/div v tc 1v/div v out 10v/div MAX1729 toc 08 v in = 5v i o = 0.5ma 500ms/div delay to shutdown v ctlin 5v/div v ref 1v/div v tc 1v/div v out 10v/div MAX1729 toc 09a v in = 5v i o = 0.5ma
MAX1729 ecb and lcd display bias supply with accurate output voltage and temperature compensation _______________________________________________________________________________________ 5 5ms/div delay to shutdown v ctlin 5v/div v ref 1v/div v tc 1v/div v out 10v/div MAX1729 toc 09b v in = 5v i o = 0.5ma 2 m s/div switching waveforms heavy load i l 20ma/div output ripple 10mv/div v lx 5v/div MAX1729 toc 10 2 m s/div switching waveforms medium load i l 20ma/div output ripple 10mv/div v lx 5v/div MAX1729 toc 11 typical operating characteristics (continued) (circuit of figure 2, t a = +25?, unless otherwise noted.) 1ms/div load-transient response v out 20mv/div i out 250 m a/div 20 m a 500 m a 16.400 16.404 MAX1729 toc 13 2ms/div line-transient response v out 20mv/div v in 1v/div 4v 5v MAX1729 toc 14 ctlin = in i out = 0.5ma
MAX1729 detailed description the MAX1729 is designed to provide bias voltage for ecb or lcd displays. it is composed of a step-up dc-dc converter followed by a linear regulator (figure 1), a combination that provides step-up/step- down voltage conversion while minimizing output rip- ple. the device allows you to adjust a display? color or contrast by dynamically adjusting the MAX1729? out- put voltage using a pwm control signal. in internal feedback mode, the output voltage is adjustable between +2.5v and +16v. in external feedback mode, the output voltage is adjustable, and its range is set by a resistor network that is programmed to match the out- put voltage range of lcd/ecb displays needing a max- imum output up to +18v. boost converter the MAX1729? dc-dc boost converter is implemented with an on-chip n-channel mosfet, a diode, and an error comparator. the ic? unique pfm control system varies the on-time and off-time of the switch based on the boost converter? input and output voltage values, as fol- lows: where k is typically 8v-?. this timing maintains discon- tinuous conduction and sets the peak inductor current (i peak ) to: where l is the inductance of l1 (figures 2, 3, and 4). when the error comparator detects that the drop across the linear regulator (v ps - v out ) is less than approximate- ly 0.6v, the internal switch is turned on (t on initiates) and current through the inductor ramps to i peak . at the end of t on , the switch is turned off for at least t off , allowing the i = k l peak t = k v t k vv on in off ps in 3 - ecb and lcd display bias supply with accurate output voltage and temperature compensation 6 _______________________________________________________________________________________ pin description drain of the internal mosfet switch lx 9 ground gnd 10 feedback and mode control input. connect to gnd for internal feedback mode operation. fb 5 control input. drive low for more than 1.2ms to put the device into shutdown. ctlin 6 bypass to gnd with a 1.0? capacitor. out 7 output of boost converter and input to ldo. bypass to gnd with a 0.068? capacitor. ps 8 compensation pin. in internal feedback mode (figure 2), bypass with a 1? capacitor. in external feedback mode, comp is a buffered inverse version of ctlin (figure 3). comp 4 reference voltage output. bypass to gnd with a 0.1? capacitor. ref 3 pin temperature-sensor output. bypass to gnd with a 1000pf capacitor. tc 2 supply input. bypass with 0.1? capacitor to ground. connect to supply side of inductor (l1). in 1 function name
inductor current to ramp down and v ps to increase. if, at the end of t off , v ps - v out is still too low, then another t on is initiated immediately. otherwise, the boost con- verter remains idle in a low-quiescent-current state until v ps - v out drops again and the error comparator initi- ates another cycle. linear regulator the pnp low-dropout linear regulator of the MAX1729 regulates the boost-converter output to the desired out- put voltage. the boost converter? regulation circuitry holds the linear regulator? input voltage (v ps ) approxi- mately 0.6v above the output voltage to keep the regu- lator out of dropout, thereby enhancing ripple rejection. the linear regulator incorporates short-circuit protec- tion, which limits the output current to approximately 6ma. temperature sensor output the MAX1729 generates a temperature sensor voltage (v tc ) that varies at 16.5mv/? (typ) and is nominally equal to the reference voltage at room temperature. tc is capable of sinking or sourcing 50?. this output is used to compensate for ecb color or lcd contrast variations caused by changes in temperature. it may be read with an adc and used to modify an external pwm control signal or, in external feedback mode, summed directly into the feedback-resistor network. control signal an externally generated pwm control signal on ctlin controls v out in internal feedback mode and influ- ences v out in external feedback mode. in either mode, if ctlin is held low for longer than 1.24ms, the MAX1729 enters shutdown mode, decreasing the sup- ply current below 2?. shutdown mode limits the mini- mum duty cycle and frequency that may be used to keep the device active. ctlin frequencies between 2khz and 12khz are recommended. internal feedback mode in internal feedback mode, the signal at ctlin is inverse- ly buffered, level-shifted, and output at comp through a resistor. internal resistance (33k w typical) and c6 then fil- ter the signal before it is used by the internal feedback network to set v out . if temperature compensation is used, the temperature sensor output voltage is read by an adc and used to adjust the duty cycle of the pwm control signal. see the designing for internal feedback mode section for more information. external feedback mode in external feedback mode, the output voltage of the MAX1729 is controlled by the duty cycle of the pwm control signal and an external resistor network, as shown in figure 3. in this mode, the signal at ctlin is inverted, level-shifted, and presented directly to comp. r3, r4, and c6 filter the signal, before it is summed into the feedback node. design procedure designing for internal feedback mode for a 3khz pwm control signal use a 1? low-leakage ceramic capacitor for c6. for applications requiring a higher-frequency pwm control signal, reduce the value of c6 to between 1? and 0.22? for frequencies between 3khz and 12khz. higher c6 values reduce output ripple. in figure 2, v out is governed by the fol- lowing equation: where v out(min) is 2.45v and gain is nominally 13.95v/100%, as listed in the electrical characteristics. v = v duty cycle gain out out(min) + MAX1729 ecb and lcd display bias supply with accurate output voltage and temperature compensation _______________________________________________________________________________________ 7 122mv v ref gnd note: switch states shown for internal feedback mode. comp tc lx ps out MAX1729 in ref fb ctlin boost converter temperature sensor voltage reference shutdown control ldo linear regulator feedback control gnd figure 1. internal block diagram
MAX1729 to use a dc control signal to adjust the output voltage, use the circuit shown in figure 4. in this configuration, v out is governed by the following equation: the impedance looking into comp is nominally 33k w . a source output impedance of less than 500 is rec- ommended. also, ensure v out 18v by keeping v comp above 0.6v. designing for external feedback mode to solve for v out in external feedback mode, assume the current into the fb pin is zero and the voltage at fb is 1.228v. then take the sum of the currents into fb and solve for v out : using the following formulas, calculate the external component values required for MAX1729 operation in external feedback mode, as shown in figure 3. an example follows the formulas. external component value formulas 1) given the maximum output voltage needed (v max ), choose the maximum feedback current and solve for r1 (10? to 30? is recommended for maximum feed- back current) as follows: r1 = v - v i max fb fb v = r1 1 r1 1 r2 1 rr 1 r5 v r1 rr v r1 r5 v out fb comp tc ++ + + ? ? ? ? - + ? ? ? ? - ? ? ? ? 34 34 v 24.67v - 22.71v out fb comp ? ecb and lcd display bias supply with accurate output voltage and temperature compensation 8 _______________________________________________________________________________________ ps out fb comp lx l1 220 m h tc ctlin digital pwm controller gnd in v in c1 0.1 m f c2 0.068 m f c4 1 m f c6 1 m f c3 0.1 m f c5 1000pf v out MAX1729 ref adc figure 2. internal feedback mode ps out fb tc lx l1 220 m h comp ctlin pwm generator gnd in v in r4 r3 c1 0.1 m f c2 0.068 m f c4 1 m f r1 r5 r2 c6 1 m f c3 0.1 m f v out MAX1729 c5 1000pf ref figure 3. external feedback mode ps out fb comp lx l1 tc ctlin gnd in 220 m h v in c1 0.1 m f c2 0.068 m f c4 1 m f v comp dc control input c3 0.1 m f c5 1000pf v out MAX1729 ref figure 4. using a dc control signal
2) given the maximum output voltage (v max ) and mini- mum output voltage (v min ), calculate values for r3 and r4 as follows: 3) for first-order temperature compensation, calculate r5 as shown below. (if temperature compensation is not used, leave r5 open.) where tempco is the negative temperature coefficient needed to compensate the ecb or lcd display for changes in temperature. 4) solve for v comp . the duty cycle used here corre- sponds to the duty cycle that yields the maximum out- put voltage, not including first-order temperature compensation. where a 90% duty cycle corresponds to duty cycle = 0.9. 5) use the results from the above calculations to solve for r2. (for applications not utilizing temperature com- pensation, use 1 / r5 = 0.) external component value example the example application requires the output voltage to adjust between 5v and 10v, using the circuit shown in figure 3. the device in our example needs a tempera- ture coefficient of 33mv/?, which yields the following results. 1) v max = 10v and i fb = 29.24? is within the limits and yields a reasonable resistor value, therefore: 2) v max = 10v and v min = 5v, therefore: with r3 = 36.7k w , then v min = 5.019v. let r4 = r3 = 36.7k w . 3) tempco = 33mv/?, therefore: 4) if external circuitry limits the duty cycle to 90%, the following equation is true: 5) solving for r2: with r2 = 56k w , a duty cycle of 87.4% generates a v out of 10v. component selection inductors use a 220? inductor to maximize output current (2.5ma typical). use an inductor with dc resistance less than 10 w and a saturation current exceeding 35ma. for lower peak inductor current, use a 470? inductor with dc resistance less than 20 w and a satu- ration current over 18ma. this limits output current to typically less than 1ma. see table 1 for a list of recom- mended inductors. the inductor should be connected from the battery to the lx pin, as close to the ic as pos- sible. capacitors the equivalent series resistance (esr) of output capac- itor c2 directly affects output ripple. to minimize output ripple, use a low-esr capacitor. a physically smaller capacitor, such as a common ceramic capacitor, mini- mizes board space and cost while creating an output ripple that? acceptable in most applications. refer to table 2 for recommended capacitor values. 1 r2 v r1 v r3 v r5 1 v 1 r1 1 r3 1 r5 1 56560 out comp fb fb =++ ? ? ? ? -++ ? ? ? ? = v 1.228 1 0.9 2 0.6754v comp =- ? ? ? ? = r5 300k 33mv/ c = ? ? ? ? = w w / 16. mv k 5 150 c r3 1/2 300k 5v 1.228 36,840 = ? ? ? ? = w w r1 10v 1.228v 29.24 a = - m = 300k w 1 r2 1 v v r1 v r3 v r5 1 r1 1 r3 1 r5 fb out comp fb =++ ? ? ? ? -++ ? ? ? ? v =v 1 duty cycle r4 r3 r4 comp fb + ? ? ? ? ? ? r5 = r1 tempco 16.5mv/ c ? ? ? ? r3 = 1/2 r1 v v v r4 = r3 max min fb ? ? ? ? MAX1729 ecb and lcd display bias supply with accurate output voltage and temperature compensation _______________________________________________________________________________________ 9
MAX1729 ecb and lcd display bias supply with accurate output voltage and temperature compensation 10 ______________________________________________________________________________________ dc resistance ( w ) inductance (?) supplier 8.4 220 murata saturation current (ma) 70 part lqh3c221k04m00 max height (mm) 2.2 table 1. recommended inductors applications information pc board layout considerations proper pc board layout minimizes output ripple and increases efficiency. for best results, use a ground plane, minimize the space between c1, c2, and gnd of the MAX1729, and place the inductor as close to lx and in as possible. for an example of proper pc board layout, refer to the MAX1729 evaluation kit. chip information transistor count: 1154 capacitance (?) 1000pf 1 1 c4 0.1 c3 0.068 capacitor c2 0.1 c1 c6* c5 *use a low-leakage capacitor. table 2. recommended capacitor values 19 470 panasonic 40 elt3kn115b 1.6
MAX1729 ecb and lcd display bias supply with accurate output voltage and temperature compensation ______________________________________________________________________________________ 11 package information 10lumaxb.eps
MAX1729 ecb and lcd display bias supply with accurate output voltage and temperature compensation notes 12 ______________________________________________________________________________________

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