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general description the MAX15061 consists of a constant-frequency pulse- width modulating (pwm) step-up dc-dc converter with an internal switch and a high-side current monitor with high-speed adjustable current limiting. this device can generate output voltages up to 76v and provides current monitoring up to 4ma (up to 300mw). the MAX15061 can be used for a wide variety of applications such as avalanche photodiode biasing, pin biasing, or varactor biasing, and lcd displays. the MAX15061 operates from 2.7v to 11v. the constant-frequency (400khz), current-mode pwm architecture provides low-noise output voltage that is easy to filter. a high-voltage, internal power switch allows this device to boost output voltages up to 76v. internal soft-start circuitry limits the input current when the boost converter starts. the MAX15061 features a shutdown mode to save power. the MAX15061 includes a current monitor with more than three decades of dynamic range and monitors cur- rent ranging from 500na to 2ma with high accuracy. resistor-adjustable current limiting protects the apd from optical power transients. a clamp diode protects the monitors output from overvoltage conditions. other protection features include cycle-by-cycle current limit- ing of the boost converter switch, undervoltage lockout, and thermal shutdown if the die temperature reaches +160c. the MAX15061 is available in a thermally enhanced 4mm x 4mm, 16-pin tqfn package and operates over the -40c to +125c automotive temperature range. applications avalanche photodiode biasing and monitoring pin diode bias supplies low-noise varactor diode bias supplies fbon modules gpon modules lcd displays features input voltage range +2.7v to +5.5v (using internal charge pump) or +5.5v to +11v wide output-voltage range from (v in + 1v) to 76v internal 1 (typ) 80v switch 300mw boost converter output power accurate ?0% (500na to 1ma) and ?.5% (1ma to 4ma) high-side current monitor resistor-adjustable ultra-fast apd current limit (1? response time) open-drain current-limit indicator flag 400khz fixed switching frequency constant pwm frequency provides easy filtering in low-noise applications internal soft-start 2? (max) shutdown current -40? to +125? temperature range small thermally enhanced, 4mm x 4mm, 16-pin tqfn package MAX15061 80v, 300mw boost converter and current monitor for apd bias applications ________________________________________________________________ maxim integrated products 1 15 16 14 13 6 5 7 cp in 8 pwr clamp rlim apd 12 shdn 4 12 11 9 pgnd lx ilim cntrl fb sgnd cn mout 3 10 bias thin qfn (4mm top view MAX15061 + *ep *connect exposed pad to sgnd. pin configuration ordering information 19-5034; rev 0; 10/09 for pricing, delivery, and ordering information, please contact maxim direct at 1-888-629-4642, or visit maxim? website at www.maxim-ic.com. part temp range pin-package MAX15061ate+ -40c to +125c 16 tqfn-ep* + denotes a lead(pb)-free/rohs-compliant package. * ep = exposed pad. typical operating circuits appear at end of data sheet.
MAX15061 80v, 300mw boost converter and current monitor for apd bias applications 2 _______________________________________________________________________________________ absolute maximum ratings electrical characteristics (v in = v pwr = 3.3v. v shdn = 3.3v. c in = c pwr = 10f. c cp = 10nf, v cntrl = v in . v rlim = 0v. v pgnd = v sgnd = 0v. v bias = 40v. apd = unconnected. clamp = unconnected. ilim = unconnected, mout = unconnected. t a = t j = -40c to +125c, unless other- wise noted. typical values are at t a = +25c.) (note 2) stresses beyond those listed under absolute 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. note 1: package thermal resistances were obtained using the method described in jedec specification jesd51-7, using a four- layer board. for detailed information on package thermal considerations, refer to www.maxim-ic.com/thermal-tutorial . pwr, in to sgnd ...................................................-0.3v to +12v lx to pgnd ............................................................-0.3v to +80v bias, apd to sgnd ...............................................-0.3v to +80v shdn to sgnd............................................-0.3v to (v in + 0.3v) clamp to sgnd ......................................-0.3v to (v bias + 0.3v) fb, ilim , rlim, cp, cn, cntrl to sgnd .............-0.3v to +12v pgnd to sgnd .....................................................-0.3v to +0.3v mout to sgnd ....................................-0.3v to (v clamp + 0.3v) continuous power dissipation 16-pin tqfn (derate 25mw/c above +70c)..........2000mw thermal resistance (note 1) ja ...............................................................................40c/w jc .................................................................................6c/w operating temperature range .........................-40c to +125c junction temperature ......................................................+150c storage temperature range .............................-65c to +150c lead temperature (soldering, 10s) .................................+300c parameter symbol conditions min typ max units 2.7 5.5 supply voltage range v in , v pwr cp connected to in, c cp = open 5.5 11 v v fb = 1.4v, no switching 1 2 supply current i supply v in = 11v, v fb = 1.4v (no switching), c cp = open, cp = in 1.2 3 ma undervoltage lockout threshold v uvlo v in rising 2.375 2.5 2.675 v undervoltage lockout hysteresis v uvlo_hys 100 mv shutdown current i in_shdn shdn pulled low 2 a bias current during shutdown i bias_shdn v bias = 3.3v, v shdn = 0v 30 a boost converter output-voltage adjustment range v in + 1v 76 v v in = v pwr = 5v 400 switching frequency f sw 2.9v v pwr 11v, v in = v pwr 400 khz maximum duty cycle d clk 2.9v v pwr 11v, v in = v pwr 90 % fb set-point voltage v fb 1.2201 1.245 1.2699 v fb input bias current i fb 100 na v pwr = v in = 2.9v, v cp = 5.5v 12 i lx = 100ma v pwr = v in = 5.5v, v cp = 10v 12 v pwr = v in = v cp = 5.5v 1 2 internal switch on-resistance r on i lx = 100ma, v cp = v in v pwr = v in = v cp = 11v 1 2 peak switch current limit i lim_lx 0.8 1.2 1.6 a lx leakage current v lx = 76v 1 a line regulation 2.9v v pwr 11v, v pwr = v in , i load = 4.5ma 0.2 % load regulation 0 i load 4.5ma 1 %
MAX15061 80v, 300mw boost converter and current monitor for apd bias applications _______________________________________________________________________________________ 3 note 2: all minimum/maximum parameters are tested at t a = +125c. limits over temperature are guaranteed by design. note 3: guaranteed by design and not production tested. parameter symbol conditions min typ max units soft-start duration 8ms soft-start steps (0.25 x i lim_lx ) to i lim_lx 32 steps control input (cntrl) maximum control input-voltage range fb set point is regulated to v cntrl 1.25 v current monitor bias voltage range v bias 10 76 v i apd = 500na 100 a bias quiescent current i bias i apd = 2ma 3.2 ma voltage drop v drop i apd = 2ma, v drop = v bias - v apd 1v i apd = 500na 1 g dynamic output resistance at mout r mout i apd = 2.5ma 890 m mout output leakage apd is unconnected 1 na output clamp voltage v mout - v clamp forward diode current = 1ma 0.5 0.73 0.95 v output clamp leakage current v bias = v clamp = 76v 1 na output-voltage range v mout 10v v bias 76v, 0 i apd 1ma, clamp is unconnected v bias - 1v v i apd = 500na 0.1 current gain i mout /i apd i apd = 2ma 0.0965 0.1 0.1035 i apd = 500na -1000 +300 +1500 power-supply rejection ratio psrr ( i mout /i mout )/ v bias , v bias = 10v to 76v (note 3) i apd = 5a to 1ma -250 +24 +250 ppm/v apd input current limit i lim_apd v apd = 35v, r lim = 3.3k 3.15 3.75 4.35 ma current-limit adjustment range 12.45k r lim 2.5k 15ma i apd = 500na 7.5 ms power-up settling time t s i mout settles to within 0.1%, 10nf connected from apd to ground i apd = 2.5ma 90 s logic inputs/outputs shdn input-voltage low v il 0.8 v shdn input-voltage high v ih 2.4 v ilim output-voltage low v ol i lim = 2ma 0.3 v ilim output leakage current i oh v ilim = 11v 1 a thermal protection thermal shutdown temperature rising +160 c thermal shutdown hysteresis 10 c electrical characteristics (continued) (v in = v pwr = 3.3v. v shdn = 3.3v. c in = c pwr = 10f. c cp = 10nf, v cntrl = v in . v rlim = 0v. v pgnd = v sgnd = 0v. v bias = 40v. apd = unconnected. clamp = unconnected. ilim = unconnected, mout = unconnected. t a = t j = -40c to +125c, unless other- wise noted. typical values are at t a = +25c.) (note 2)
MAX15061 80v, 300mw boost converter and current monitor for apd bias applications 4 _______________________________________________________________________________________ typical operating characteristics (v pwr = v in = 3.3v, v out = 70v, circuit of figure 3 (figure 4 for v in > 5.5v), unless otherwise noted.) efficiency vs. load current MAX15061 toc01 load current (ma) efficiency (%) 3 2 1 10 20 30 40 50 60 70 0 04 v out = 30v v out = 55v v out = 70v v in = 3.3v efficiency vs. load current MAX15061 toc02 load current (ma) efficiency (%) 3 2 1 10 20 30 40 50 60 70 0 04 v out = 30v v out = 55v v out = 70v v in = 5v efficiency vs. load current MAX15061 toc03 load current (ma) efficiency (%) 3 2 1 10 20 30 40 50 60 70 0 04 v in = 3.3v v in = 5v v in = 8v v out = 70v minimum startup voltage vs. load current MAX15061 toc04 load current (ma) minimum startup voltage (v) 3 2 1 2.49 2.50 2.51 2.52 2.53 2.54 2.55 2.48 04 supply current vs. supply voltage MAX15061 toc05 supply voltage (v) supply current (ma) 10 9 7 8 2 3 4 5 6 1 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0 011 v fb = 1.4v t a = +125 c t a = +85 c t a = -40 c t a = +25 c no-load supply current vs. supply voltage xMAX15061 toc06 supply voltage (v) no-load supply current (ma) 10 9 8 7 6 5 4 10 20 30 40 50 60 0 311 t a = +85 c t a = -40 c t a = +25 c exiting shutdown MAX15061 toc07 1ms/div v out 50v/div i l 500ma/div v shdn 2v/div 3v 0v 0ma i out = 1ma entering shutdown MAX15061 toc08 4ms/div output voltage 50v/div inductor current 500ma/div shutdown voltage 2v/div 3v 70v 0v 0ma i load = 1ma
light-load switching waveform with rc filter MAX15061 toc09 1 = = MAX15061 80v, 300mw boost converter and current monitor for apd bias applications _______________________________________________________________________________________ 5 heavy-load switching waveform with rc filter MAX15061 toc10 1 = = line-transient response MAX15061 toc12 100ms/div v in 2v/div 3.3v v out ac-coupled 100mv/div v out = 70v i out = 1ma t rise = 50 s load-transient response MAX15061 toc11 100ms/div v out ac-coupled 200mv/div i load 5ma/div 0ma v out = 70v v in = 3.3v lx leakage current vs. temperature MAX15061 toc13 temperature ( c) lx leakage current (na) 110 95 65 80 -10 5 20 35 50 -25 20 40 60 80 100 120 140 160 180 200 0 -40 125 current into lx pin typical operating characteristics (continued) (v pwr = v in = 3.3v, v out = 70v, circuit of figure 3 (figure 4 for v in > 5.5v), unless otherwise noted.) maximum load current vs. input voltage MAX15061 toc15 input voltage (v) maximum load current (ma) 10 9 7 8 5 6 4 10 20 30 40 50 60 70 80 90 100 110 0 311 a b c a: v out = 30v, b: v out = 35v, c: v out = 45v, d: v out = 55v, e: v out = 60v, f: v out = 72v d e f load regulation MAX15061 toc14 load current (ma) output voltage (v) 4 3 2 1 68.2 68.4 68.6 68.8 69.0 69.2 69.4 69.6 69.8 70.0 68.0 05 bias current vs. bias voltage MAX15061 toc16 bias voltage (v) bias current (ma) 70 60 50 40 30 20 10 0.1 1 10 0.01 080 i apd = 2ma i apd = 500na
MAX15061 80v, 300mw boost converter and current monitor for apd bias applications 6 _______________________________________________________________________________________ typical operating characteristics (continued) (v pwr = v in = 3.3v, v out = 70v, circuit of figure 3 (figure 4 for v in > 5.5v), unless otherwise noted.) bias current vs. apd current MAX15061 toc17 apd current (ma) bias current (ma) 1 0.1 0.01 0.001 0.1 1 10 0.01 0.0001 10 v bias = 70v bias current vs. temperature MAX15061 toc18 temperature ( c) bias current (ma) 110 95 80 65 50 35 20 5 -10 -25 0.1 1 10 0.01 -40 125 i apd = 2ma i apd = 500na gain error vs. apd current MAX15061 toc19 i apd ( a) gain error (%) 1000 100 10 1 -4 -3 -2 -1 0 1 2 3 4 5 -5 0.1 10,000 v bias = 70v gain error vs. temperature MAX15061 toc20 temperature ( c) gain error (%) 110 95 -25 -10 5 35 50 65 20 80 -2.5 -2.0 -1.5 -1.0 -0.5 0 0.5 1.0 -3.0 -40 125 v bias = 70v i apd = 500na i apd = 50 a i apd = 5 a i apd = 2ma i apd = 500 a gain error vs. bias voltage MAX15061 toc21 bias voltage (v) gain error (%) 70 60 20 30 40 50 -0.60 -0.40 -0.20 0 0.20 0.40 0.60 0.80 -0.80 10 80 i apd = 500na i apd = 50 a i apd = 5 a i apd = 2ma i apd = 500 a apd transient response MAX15061 toc22 20 s/div v apd ac-coupled 70v 2v/div i apd 2.5ma/div i mout 0.25ma/div 0ma 0ma startup delay MAX15061 toc23 200 s/div v bias 20v/div i mout 20na/div 3v 0na i apd = 500na
MAX15061 80v, 300mw boost converter and current monitor for apd bias applications _______________________________________________________________________________________ 7 startup delay MAX15061 toc24 100 s/div v apd 20v/div i mout 50 a/div 3v 0na i apd = 2ma startup delay MAX15061 toc25 100 s/div v apd 2v/div i mout 20na/div 0v 0na i apd = 500na v bias = 5v startup delay MAX15061 toc26 40 s/div v bias 2v/div i mout 50 a/div 0v 0na i apd = 2ma v bias = 5v short-circuit response MAX15061 toc27 40ms/div i lim 2v/div i bias 2ma/div 0v 0ma v bias = 70v t a = +85 c r lim = 2k typical operating characteristics (continued) (v pwr = v in = 3.3v, v out = 70v, circuit of figure 3 (figure 4 for v in > 5.5v), unless otherwise noted.) voltage drop vs. apd current MAX15061 toc28 i apd ( a) v bias - v apd (v) 1000 100 10 1 0.20 0.40 0.60 0.80 1.00 1.20 1.40 0 0.1 10,000 t a = +25 c t a = -40 c t a = +125 c t a = +85 c switching frequency vs. temperature MAX15061 toc29 temperature ( c) switching frequency (khz) 110 95 65 80 -10 5 20 35 50 -25 320 340 360 380 400 420 440 460 480 500 300 -40 125
MAX15061 80v, 300mw boost converter and current monitor for apd bias applications 8 _______________________________________________________________________________________ switching frequency vs. input voltage MAX15061 toc30 input voltage (v) switching frequency (khz) 10 8 6 4 320 340 360 380 400 420 440 460 480 500 300 212 fb set-point variation vs. temperature MAX15061 toc32 temperature ( c) fb set-point voltage variation (v) 110 95 80 65 50 35 20 5 -10 -25 1.217 1.227 1.237 1.247 1.257 1.277 1.267 1.207 -40 125 v in = 5.5v v in = 2.9v v in = 2.9v v in = 5.5v fb rising fb falling apd output ripple voltage MAX15061 toc34 2 s/div v apd ac-coupled, 55v 100 v/div 0.1 f capacitor connected from apd to gnd. switching frequency and duty cycle vs. load current MAX15061 toc31 load current (ma) switching frequency (khz) duty cycle 3 2 1 385 390 395 400 405 410 415 420 380 10 40 30 20 50 60 0 04 duty cycle switching frequency measured at cn apd output ripple voltage MAX15061 toc33 2 s/div v apd ac-coupled, 55v 200 v/div apd output ripple voltage MAX15061 toc35 2 s/div v apd ac-coupled, 70v 500 v/div 0.1 f capacitor connected from apd to gnd. typical operating characteristics (continued) (v pwr = v in = 3.3v, v out = 70v, circuit of figure 3 (figure 4 for v in > 5.5v), unless otherwise noted.)
MAX15061 80v, 300mw boost converter and current monitor for apd bias applications _______________________________________________________________________________________ 9 pin description pin name function 1 pwr boost converter input voltage. pwr powers the switch driver and charge pump. bypass pwr to pgnd with a ceramic capacitor of 1f minimum value. 2cp positive terminal of the charge-pump flying capacitor for 2.7v to 5.5v supply voltage operation. connect cp to in when the input voltage is in the 5.5v to 11v range. 3cn negative terminal of the charge-pump flying capacitor for 2.7v to 5.5v supply voltage operation. leave cn unconnected when the input voltage is in the 5.5v to 11v range. 4in input supply voltage. in powers all blocks of the MAX15061 except the switch driver and charge pump. bypass in to pgnd with a ceramic capacitor of 1f minimum value. 5 sgnd signal ground. connect directly to the local ground plane. connect sgnd to pgnd at a single point, typically near the return terminal of the output capacitor. 6fb feedback regulation input. connect fb to the center tap of a resistive voltage-divider from the output (v out ) to sgnd to set the output voltage. the fb voltage regulates to 1.245v (typ) when v cntrl is above 1.5v (typ) and to v cntrl voltage when v cntrl is below 1.245v (typ). 7 cntrl control input for boost converter output-voltage programmability. allows the feedback set-point voltage to be set externally by cntrl when cntrl is less than 1.245v. pull cntrl above 1.5v (typ) to use the internal 1.245v (typ) feedback set-point voltage. 8 ilim open-drain current-limit indicator. ilim asserts low when the apd current limit has been exceeded. 9 rlim current-limit resistor connection. connect a resistor from rlim to sgnd to program the apd current-limit threshold. 10 mout current-monitor output. mout sources a current 1/10 of i apd . 11 clamp clamp voltage input. clamp is the external potential used for voltage clamping of mout. 12 apd reference current output. apd provides the source current to the cathode of the photodiode. 13 bias bias voltage input. connect bias to the boost converter output (v out ) either directly or through a lowpass filter for ripple attenuation. bias provides the voltage bias for the current monitor and is the current source for apd. 14 shdn active-low shutdown control input. apply a logic-low voltage to shdn to shut down the device and reduce the supply current to 2a (max). connect shdn to in for normal operation. ensure that v shdn is not greater than the input voltage, v in . 15 pgnd power ground. connect the negative terminals of the input and output capacitors to pgnd. connect pgnd externally to sgnd at a single point, typically at the return terminal of the output capacitor. 16 lx drain of internal 80v n-channel dmos. connect inductor and diode to lx. minimize the trace area at lx to reduce switching noise emission. ep exposed pad. connect ep to a large contiguous copper plane at sgnd potential to improve thermal dissipation. do not use as the main sgnd connection.
MAX15061 80v, 300mw boost converter and current monitor for apd bias applications 10 ______________________________________________________________________________________ functional diagram 80v dmos +a -c -a +c clk v ref v ref shdn bias v ref lx ilim pwr pgnd rlim mout 1x apd fb sgnd cntrl cn cp in switch control logic soft- start output error and current comparator reference comparator current monitor oscillator 400khz charge pump (doubler) bias and reference thermal shutdown switch current sense current- limit adjustment uvlo current limit mux peak current-limit comparator clamp 10x MAX15061 detailed description the MAX15061 constant-frequency, current-mode, pwm boost converter is intended for low-voltage systems that require a locally generated high voltage. this device can generate a low-noise, high output voltage required for pin and varactor diode biasing and lcd displays. the MAX15061 operates either from +2.7v to +5.5v or from +5.5v to +11v. for 2.7v to 5.5v operation, an internal charge pump with an external 10nf ceramic capacitor is used. for 5.5v to 11v operation, connect cp to in and leave cn unconnected. the MAX15061 operates in discontinuous mode in order to reduce the switching noise caused by reverse- voltage recovery charge of the rectifier diode. other continuous mode boost converters generate large volt- age spikes at the output when the lx switch turns on because there is a conduction path between the out- put, diode, and switch to ground during the time need- ed for the diode to turn off and reverse its bias voltage. to reduce the output noise even further, the lx switch turns off by taking 10ns typically to transition from on to off. as a consequence, the positive slew rate of the lx node is reduced and the current from the inductor does not force the output voltage as hard as would be the case if the lx switch were to turn off faster. the constant-frequency (400khz) pwm architecture generates an output voltage ripple that is easy to filter. an 80v vertical dmos device used as the internal power switch is ideal for boost converters with output voltages up to 76v. the MAX15061 can also be used in other topologies where the pwm switch is grounded, like sepic and flyback converters.
MAX15061 the MAX15061 includes a versatile current monitor intended for monitoring the apd, pin, or varactor diode dc current in fiber and other applications. the MAX15061 features more than three decades of dynamic current ranging from 500na to 4ma and pro- vides an output current accurately proportional to the apd current at mout. the MAX15061 also features a shutdown logic input to disable the device and reduce its standby current to 2a (max). fixed-frequency pwm controller the heart of the MAX15061 current-mode pwm con- troller is a bicmos multiple-input comparator that simultaneously processes the output-error signal and switch current signal. the main pwm comparator uses direct summing, lacking a traditional error amplifier and its associated phase shift. the direct summing configu- ration approaches ideal cycle-by-cycle control over the output voltage since there is no conventional error amplifier in the feedback path. the device operates in pwm mode using a fixed-fre- quency, current-mode operation. the current-mode fre- quency loop regulates the peak inductor current as a function of the output error signal. the current-mode pwm controller is intended for dis- continuous conduction mode (dcm) operation. no internal slope compensation is added to the current signal. charge pump at low supply voltages (2.7v to 5.5v), internal charge- pump circuitry and an external 10nf ceramic capacitor connected between cp and cn double the available inter- nal supply voltage to drive the internal switch efficiently. in the 5.5v to 11v supply voltage range, the charge pump is not required. in this configuration, disable the charge pump by connecting cp to in and leaving cn unconnected. monitor current limit (rlim) the current limit of the current monitor is programmable from 1ma to 5ma. connect a resistor from rlim to ground to program the current-limit threshold up to 5ma. the current monitor mirrors the current out of apd with a 1:10 ratio, and the mout current can be converted to a voltage signal by connecting a resistor from mout to sgnd. the apd current-monitor range is from 500na to 4ma, and the mout current-mirror output accuracy is 10% from 500na to 1ma of apd current and 3.5% from 1ma to 4ma of apd current. clamping the monitor output voltage (clamp) clamp provides a means for diode clamping the volt- age at mout; thus, v mout is limited to (v clamp + 0.6v). clamp can be connected to either an external supply or bias. clamp can be left unconnected if volt- age clamping is not required. adjusting the boost converter output voltage (fb/cntrl) the boost converter output voltage can be set by con- necting fb to a resistor-divider from v out to ground. the set-point feedback reference is the 1.245 (typ) internal reference voltage when v cntrl > 1.5v and is equal to the cntrl voltage when v cntrl < 1.25v. to change the converter output on the fly, apply a volt- age lower than 1.25v (typ) to the cntrl input and adjust the cntrl voltage, which is the reference input of the error amplifier when v cntrl < 1.25v (see the functional diagram ). this feature can be used to adjust the apd voltage based on the apd mirror current, which compensates for the apd avalanche gain varia- tion with temperature and manufacturing process. as shown in figure 4, the voltage signal proportional to the mout current is connected to the analog-to-digital (adc) input of the apd module, which then controls the reference voltage of the boost converter error amplifier through a digital-to-analog (dac) block connected to the cntrl input. the bias voltage and, therefore, the apd current, are controlled based on the mout mirror current, forming a negative feedback loop. shutdown ( shdn ) the MAX15061 features an active-low shutdown input ( shdn ). pull shdn low to enter shutdown. during shut- down, the supply current drops to 2a (30a from bias) (max). however, the output remains connected to the input through the inductor and the output diode, holding the output voltage to one diode drop below pwr when the MAX15061 shuts down. connect shdn to in for always-on operation. 80v, 300mw boost converter and current monitor for apd bias applications ______________________________________________________________________________________ 11
MAX15061 80v, 300mw boost converter and current monitor for apd bias applications 12 ______________________________________________________________________________________ design procedure setting the output voltage set the MAX15061 output voltage by connecting a resis- tive divider from the output to fb to sgnd (figure 1). select r 1 (fb to sgnd resistor) between 200k and 400k . calculate r 2 (v out to fb resistor) using the fol- lowing equation: where v out can range from (v in + 1v) to 76v and v ref = 1.245v or v cntrl depending on the v cntrl value. for v cntrl > 1.5v, the internal 1.245v (typ) reference voltage is used as the feedback set point (v ref = 1.245v) and for v cntrl < 1.25v, v ref = v cntrl . determining peak inductor current if the boost converter remains in the discontinuous mode of operation, then the approximate peak inductor current, i lpeak (in amperes), is represented by the for- mula below: where t s is the switching period in microseconds, v out is the output voltage in volts, v in_min is the mini- mum input voltage in volts, i out_max is the maximum output current in amperes, l is the inductor value in microhenrys, and is the efficiency of the boost con- verter (see the typical operating characteristics ). determining the inductor value three key inductor parameters must be specified for operation with the MAX15061: inductance value (l), inductor saturation current (i sat ), and dc resistance (dcr). in general, the inductor should have a saturation current rating greater than the maximum switch peak current-limit value (i lim-lx = 1.6a). choose an inductor with a low-dcr resistance for reasonable efficiency. use the following formula to calculate the lower bound of the inductor value at different output voltages and output currents. this is the minimum inductance value for discontinuous mode operation for supplying full 300mw of output power. where v in_min , v out (both in volts), and i out (in amperes) are typical values (so that efficiency is opti- mum for typical conditions), t s (in microseconds) is the period, is the efficiency, and i lim_lx is the peak switch current in amperes (see the electrical characteristics table). calculate the optimum value of l (l optimum ) to ensure the full output power without reaching the boundary between continuous conduction mode (ccm) and dcm using the following formula: for a design in which v in = 3.3v, v out = 70v, i out = 3ma, = 45%, i lim-lx = 1.3a, and t s = 2.5s: l min = 1.3h and l max = 23h. for a worse-case scenario in which v in = 2.9v, v out = 70v, i out = 4ma, = 43%, i lim-lx = 1.3a, and t s = 2.5s: l min = 1.8h and l max = 15h. the choice of 4.7h is reasonable given the worst-case scenario above. in general, the higher the inductance, the lower the switching noise. load regulation is also better with higher inductance. where l [ h] v(vv)t 2 max in_min 2 out in_min s = ? iv out out 2 l[h] l optimum max = [] . h 225 l[h] 2t i (v v ) i min s out out in_min lim-lx 2 = ? i 2t (v v )i l lpeak s out in_min out_max = ? rr v v 1 21 out ref = ? ? ? ? ? ? ? ? ? ? ? ? ? MAX15061 fb v out r 2 r 1 figure 1. adjustable output voltage
MAX15061 diode selection the MAX15061s 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 drop. ensure that the diodes peak current rating is greater than the peak inductor current. also the diode reverse-breakdown voltage must be greater than v out , the output voltage of the boost converter. output filter capacitor selection for most applications, use a small output capacitor of 0.1f or greater. to achieve low output ripple, a capaci- tor with low esr, low esl, and high capacitance value should be selected. if tantalum or electrolytic capacitors are used to achieve high capacitance values, always add a smaller ceramic capacitor in parallel to bypass the high-frequency components of the diode current. the higher esr and esl of electrolytic capacitors increase the output ripple and peak-to-peak transient voltage. assuming the contribution from the esr and capacitor discharge equals 50% (proportions may vary), calculate the output capacitance and esr required for a specified ripple using the following equations: for very low output ripple applications, the output of the boost converter can be followed by an rc filter to further reduce the ripple. figure 2 shows a 100 (r f ), 0.1f (c f ) filter used to reduce the switching output ripple to 1mv p-p with a 0.1ma load or 2mv p-p with a 4ma load. the output-voltage regulation resistor-divider must remain connected to the diode and output capacitor node. use x7r ceramic capacitors for more stability over the full temperature range. use an x5r capacitor for -40c to +85c applications. input capacitor selection bypass pwr to pgnd with a 1f (min) ceramic capaci- tor and bypass in to pgnd with a 1f (min) ceramic capacitor. depending on the supply source imped- ance, higher values may be needed. make sure that the input capacitors are close enough to the ic to provide adequate decoupling at in and pwr as well. if the lay- out cannot achieve this, add another 0.1f ceramic capacitor between in and pgnd (or pwr and pgnd) in the immediate vicinity of the ic. bulk aluminum elec- trolytic capacitors may be needed to avoid chattering at low input voltage. in case of aluminum electrolytic capacitors, calculate the capacitor value and esr of the input capacitor using the following equations: c vxi xv x0.5x in out out in_min in [] f v = t t ixl xv v(vv s lpeak optimum out in_min out i ? ? n n_min in in_m ) vxv ? ? ? ? ? ? ? ? [] = esr m 0.5 x x i in out vxi out c i 0.5 x t ixl out out out s lpeak optim [] f v =? u um out in_min (v v ) ? ? ? ? ? ? ? ? ? [] = esr m 0.5 x i out v o out MAX15061 pwr cntrl shdn pgnd cp c cp c out1 c f 0.1 f c pwr cn lx fb d1 bias sgnd in v in = 2.7v to 5.5v v out l1 c in r 2 r 1 r f 100 figure 2. typical operating circuit with rc filter 80v, 300mw boost converter and current monitor for apd bias applications ______________________________________________________________________________________ 13
MAX15061 80v, 300mw boost converter and current monitor for apd bias applications 14 ______________________________________________________________________________________ determining monitor current limit calculate the value of the monitor current-limit resistor, r lim , for a given apd current limit, i limit , using the fol- lowing equation: the r lim resistor, r lim , ranges from 12.45k to 2.5 for apd currents from 1ma to 5ma. applications information using apd or pin photodiodes in fiber applications when using the MAX15061 to monitor apd or pin pho- todiode currents in fiber applications, several issues must be addressed. in applications where the photodi- ode must be fully depleted, keep track of voltages bud- geted for each component with respect to the available supply voltage(s). the current monitors require as much as 1.1v between bias and apd, which must be considered part of the overall voltage budget. additional voltage margin can be created if a negative supply is used in place of a ground connection, as long as the overall voltage drop experienced by the MAX15061 is less than or equal to 76v. for this type of application, the MAX15061 is suggested so the output can be referenced to true ground and not the negative supply. the MAX15061s output current can be refer- enced as desired with either a resistor to ground or a transimpedance amplifier. take care to ensure that out- put voltage excursions do not interfere with the required margin between bias and mout. in many fiber applica- tions, mout is connected directly to an adc that oper- ates from a supply voltage that is less than the voltage at bias. connecting the MAX15061s clamping diode output, clamp, to the adc power supply helps avoid damage to the adc. without this protection, voltages can develop at mout that might destroy the adc. this protection is less critical when mout is connected directly to subsequent transimpedance amplifiers (linear or logarithmic) that have low-impedance, near-ground- referenced inputs. if a transimpedance amplfier is used on the low side of the photodiode, its voltage drop must also be considered. leakage from the clamping diode is most often insignificant over nominal operating condi- tions, but grows with temperature. to maintain low levels of wideband noise, lowpass filter- ing the output signal is suggested in applications where only dc measurements are required. connect the filter capacitor at mout. determining the required filtering components is straightforward, as the MAX15061 exhibits a very high output impedance of 890m . in some applications where pilot tones are used to identi- fy specific fiber channels, higher bandwidths are desired at mout to detect these tones. consider the minimum and maximum currents to be detected, then consult the frequency response and noise typical operating curves. if the minimum current is too small, insufficient bandwidth could result, while too high a current could result in excessive noise across the desired bandwidth. layout considerations careful pcb layout is critical to achieve low switching losses and clean and stable operation. protect sensitive analog grounds by using a star ground configuration. connect sgnd and pgnd together close to the device at the return terminal of the output bypass capacitor. do not connect them together anywhere else. keep all pcb traces as short as possible to reduce stray capaci- tance, trace resistance, and radiated noise. ensure that the feedback connection to fb is short and direct. route high-speed switching nodes away from the sen- sitive analog areas. use an internal pcb layer for sgnd as an emi shield to keep radiated noise away from the device, feedback dividers, and analog bypass capaci- tors. refer to the MAX15061 evaluation kit data sheet for a layout example. r10 1.245v i (ma) lim limit =
MAX15061 MAX15061 cntrl cp cn in pgnd fb bias shdn mout apd sgnd rlim pwr lx gpio ilim gpio clamp v dd c v dd apd c in 1 f c cp 10nf r mout 10k c out 0.1 f c mout (optional) r 1 6.34k r lim 2.87k r 2 348k v in c pwr 1 f l1 4.7 h d1 v out (70v max) dac adc r f 100 r adj c f 0.1 f figure 3. typical operating circuit for v in = 2.7v to 5.5v 80v, 300mw boost converter and current monitor for apd bias applications ______________________________________________________________________________________ 15
MAX15061 80v, 300mw boost converter and current monitor for apd bias applications 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. 16 ____________________maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 ? 2009 maxim integrated products maxim is a registered trademark of maxim integrated products, inc. MAX15061 cntrl cp cn in pgnd fb bias shdn mout apd sgnd rlim pwr lx gpio ilim gpio clamp v dd c v dd apd c in 1 f r mout 10k c out 0.1 f c mout (optional) r 1 634k r lim 2.87k r 2 348k v in = 5.5v to 11v c pwr 1 f l1 4.7 h d1 v out (70v max) dac adc r f 100 c f 0.1 f package information for the latest package outline information and land patterns, go to www.maxim-ic.com/packages . note that a "+", "#", or "-" in the package code indicates rohs status only. package draw- ings may show a different suffix character, but the drawing per- tains to the package regardless of rohs status. package type package code document no. 16 tqfn t1644-4 21-0139 chip information process: bicmos figure 4. typical operating circuit for v in = 5.5v to 11v

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