general description the max1855 evaluation kit (ev kit) demonstrates a high- power, dynamically adjustable notebook cpu power-sup- ply application circuit. the max1855 dc-dc converter steps down high-voltage batteries and/or ac adapters, generating a precision, low-voltage cpu core v cc rail. the max1855 ev kit is designed for cpu core applica- tions requiring a voltage-positioned supply. voltage posi- tioning and a high-dc-accuracy control loop decrease full-load power dissipation and reduce the required num- ber of output capacitors. this fully assembled and tested circuit board provides a digitally adjustable 0.6v to 1.75v output voltage from a +7v to +24v battery input range. it delivers up to 18a out- put current. the ev kit operates at 300khz switching fre- quency and has superior line-and load-transient response. this ev kit can also be used to evaluate the max1716 (0.925v to 1.6v output) and the max1854 (0.925v to 2.0v output). features high speed, accuracy, and efficiency voltage-positioned output low output capacitor count (5) reduces cpu power consumption fast-response quick-pwm tm architecture +7v to +24v input voltage range adjustable output range (5-bit dac) max1716: 0.925v to 1.6v max1854: 0.925v to 2.0v max1855: 0.6v to 1.75v 18a load-current capability 300khz switching frequency vgate transition-complete indicator 24-pin qsop package low-profile components fully assembled and tested evaluates: max1716/max1854/max1855 max1855 evaluation kit ________________________________________________________________ maxim integrated products 1 19-1758; rev 0; 8/00 component list for free samples and the latest literature, visit www.maxim-ic.com or phone 1-800-998-8800. for small orders, phone 1-800-835-8769. ordering information part temp. range ic package max1855evkit 0 c to +70 c 24 qsop note: to evaluate the max1716/max1854, request a max1716eeg/max1854ee free sample with the max1855evkit. quick-pwm is a trademark of maxim integrated products. designation qty description c1?4, c18 5 10 f, 25v ceramic capacitors (1812) taiyo yuden tmk432bj106km or tdk c4532x5r1e106m c5?8, c16 5 220 f, 2.5v, 15m ? low-esr specialty polymer capacitors panasonic eefue0e221r c9 1 0.1 f ceramic capacitor (0805) c10 1 10 f, 6.3v x5r ceramic capacitor (1210) taiyo yuden jmk325bj106mn or equivalent c11, c12 2 0.22 f, 16v x5r ceramic capacitors (0805) taiyo yuden emk212bj224kg or equivalent c13 1 1000pf ceramic capacitor (0805) designation qty description c14 1 47pf ceramic capacitor (0805) c15 1 1 f, 10v x5r ceramic capacitor (0805) taiyo yuden lmk212bj105mg or equivalent c17 0 not installed r1 1 20 ? 5% resistor (1206) r2, r14 2 0.006 ? 1% 1w resistors (2512) dale wsl-2512-r006f r3, r4 2 1m ? 5% resistors (0805) r6 1 100k ? 1% resistor (0805) r8 1 100 ? 5% resistor (0805) r5, r9, r13 3 1k ? 1% resistors (0805) r10 0 not installed (0805) r11 1 100k ? 5% resistor (0805) r12 1 200k ? 1% resistor (0805)
quick start 1) ensure that the circuit is connected correctly to the supplies and dummy load prior to applying any power. 2) ensure that the shunt is connected at ju1 ( shdn = v cc ). 3) turn on battery power prior to +5v bias power; oth- erwise, the output uvlo timer will time out and the fault latch will be set, disabling the regulator until +5v power is cycled or shutdown is toggled. 4) observe the output with the dmm and/or oscillo- scope. look at the lx switching-node and mosfet gate-drive signals while varying the load current. 5) set switch sw1 per table 1 to get the desired out- put voltage. recommended equipment +7v to +24v, >30w power supply, battery, or note- book ac adapter dc bias power supply, 5v at 100ma dummy load capable of sinking 18a digital multimeter (dmm) 100mhz dual-trace oscilloscope detailed description this 18a buck-regulator design is optimized for a 300khz frequency and output voltage settings around 1.35v to 1.6v. at lower output voltages, transient response degrades slightly and efficiency worsens. at v out = 1.6v, inductor ripple is approximately 30%, with a resulting pulse-skipping threshold at roughly i load = 3a with v in = 12v. setting the output voltage the max1855 uses an internal 5-bit dac as a feed- back resistor voltage divider. the output voltage can be digitally set from 0.6v to 1.75v, using the d0 d4 inputs (table 1). load-transient experiment one interesting experiment is to subject the output to large, fast load transients and observe the output with evaluates: max1716/max1854/max1855 max1855 evaluation kit 2 ________________________________________________________________________________________ component list (continued) designation qty description d1 1 2a schottky diode central semiconductor cmsh2-40 stm-microelectronics stps2l25u or international rectifier 10mq040 d2 1 100ma schottky diode central semiconductor cmpsh-3 d3 1 1a schottky diode motorola mbrs130lt3 or international rectifier 10bq040 or nihon ec10qs03 l1 1 0.68 h power inductor sumida cep125 #4712-t007 or sumida cdep134h-0r6 or panasonic etqp6f0r6bfa n1, n4 2 n-channel mosfets (8-pin so) international rectifier irf7811 or international rectifier irf7811a n2, n3 2 n-channel mosfets (8-pin so) international rectifier irf7811 or international rectifier irf7811a or fairchild fds7764a designation qty description n5 0 not installed u1 1 max1855eeg (24-pin qsop) ju1 2 2-pin headers none 2 shunts (ju1, ju2) sw1 1 dip-5 dip switch sw2 1 momentary switch, normally open digi-key p8006/7s j1 1 scope-probe connector berg electronics 33jr135-1 none 4 rubber bumpers 3m sj-5007 or mouser 517-sj-5007bk or equivalent none 1 max1716/max1854/max1855 pc board none 1 max1855 ev kit data sheet none 1 max1716/max1854/max1855 data sheet
output voltage (v) d4 d3 d2 d1 d0 max1716 max1854 max1855 00000 no cpu* 2.000 1.750 00001 no cpu* 1.950 1.700 00010 no cpu* 1.900 1.650 00011 no cpu* 1.850 1.600 00100 no cpu* 1.800 1.550 00101 no cpu* 1.750 1.500 00110 no cpu* 1.700 1.450 00111 no cpu* 1.650 1.400 01000 1.600 1.600 1.350 01001 1.550 1.550 1.300 01010 1.500 1.500 1.250 01011 1.450 1.450 1.200 01100 1.400 1.400 1.150 01101 1.350 1.350 1.100 01110 1.300 1.300 1.050 01111 no cpu* no cpu* 1.000 10000 1.275 1.275 0.975 10001 1.250 1.250 0.950 10010 1.225 1.225 0.925 10011 1.200 1.200 0.900 10100 1.175 1.175 0.875 10101 1.150 1.150 0.850 10110 1.125 1.125 0.825 10111 1.100 1.100 0.800 11000 1.075 1.075 0.775 11001 1.050 1.050 0.750 11010 1.025 1.025 0.725 11011 1.000 1.000 0.700 11100 0.975 0.975 0.675 11101 0.950 0.950 0.650 11110 0.925 0.925 0.625 11111 no cpu* no cpu* 0.600 evaluates: max1716/max1854/max1855 max1855 evaluation kit _______________________________________________________________________________________ 3 an oscilloscope. this necessitates careful instrumenta- tion of the output, using the supplied scope-probe jack. accurate measurement of output ripple and load-tran- sient response invariably requires that ground clip leads be completely avoided and that the probe hat be removed to expose the gnd shield, so the probe can be plugged directly into the jack. otherwise, emi and noise pickup will corrupt the waveforms. most bench-top electronic loads intended for power- supply testing lack the ability to subject the dc-dc converter to ultra-fast load transients. emulating the supply current di/dt at the cpu vcore pins requires at table 1. max1855 output voltage adjustment settings * in the no-cpu state, dh and dl are held low.
evaluates: max1716/max1854/max1855 max1855 evaluation kit least 10a/s load transients. one easy method for gen- erating such an abusive load transient is to solder a mosfet, such as an mtp3055 or 12n05, directly across the scope-probe jack. then drive its gate with a strong pulse generator at a low duty cycle (10%) to minimize heat stress in the mosfet. vary the high-level output voltage of the pulse generator to vary the load current. to determine the load current, you might expect to insert a meter in the load path, but this method is pro- hibited here by the need for low resistance and induc- tance in the path of the dummy-load mosfet. there are two easy alternative methods of determining how much load current a particular pulse-generator ampli- tude is causing. the first and best is to observe the inductor current with a calibrated ac current probe, such as a tektronix am503. in the buck topology, the load current is equal to the average value of the induc- tor current. the second method is to first put on a static dummy load and measure the battery current. then, connect the mosfet dummy load at 100% duty momentarily, and adjust the gate-drive signal until the battery current rises to the appropriate level (the mos- fet load must be well heatsinked for this to work with- out causing smoke and flames). supplier phone fax central semiconductor 516-435-1110 516-435-1824 dale-vishay 402-564-3131 402-563-6418 fairchild 408-721-2181 408-721-1635 international rectifier 310-322-3331 310-322-3332 kemet 408-986-0424 408-986-1442 nihon 847-843-7500 847-843-2798 on semiconductor (motorola) 602-303-5454 602-994-6430 panasonic 714-373-7939 714-373-7183 sanyo 619-661-6835 619-661-1055 stm- microelectronics 617-259-0300 617-259-9442 sumida 708-956-0666 708-956-0702 taiyo yuden 408-573-4150 408-573-4159 tdk 847-390-4373 847-390-4428 component suppliers jumper settings shunt location shdn pin max1855 output on connected to v cc max1855 enabled off connected to gnd shutdown mode, v out = 0v table 2. jumper ju1 functions (shutdown mode) shunt location skip pin max1855 output on connected to v cc low-noise mode, forced fixed-frequency pwm operation off connected to gnd normal operation, allows automatic pwm/pfm switchover for pulse-skipping at light load, resulting in highest efficiency table 3. jumper ju2 functions (low-noise mode) jumper shunt location ton pin frequency (khz) ju3 on ju4, ju5 off connected to ref 400 ju4 on ju3, ju5 off connected to v cc 200 ju5 on ju3, ju4 off connected to gnd 550 ju3, ju4, ju5 off floating 300 table 4. jumpers ju3/ju4/ju5 functions (switching-frequency selection) note: don? change the operating frequency without first recal- culating component values because the frequency has a sig- nificant effect on the peak current-limit level, mosfet heating, preferred inductor value, pfm/pwm switchover point, output noise, efficiency, and other critical parameters. note: please indicate that you are using the max1855, max1716, or max1854 when contacting these component suppliers. 4 ________________________________________________________________________________________
evaluates: max1716/max1854/max1855 max1855 evaluation kit _______________________________________________________________________________________ 5 symptom possible problem possible problem circuit won t start when power is applied. power-supply sequencing: +5v bias supply was applied first. cycle shdn press the reset button. output overvoltage due to shorted high-side mosfet. replace the mosfet. output overvoltage due to load recovery overshoot. reduce the inductor value, raise the switching frequency, or add more output capacitance. transient overload condition. add more low-esr output capacitors. circuit won t start when reset is pressed, +5v bias supply cycled. broken connection, bad mosfet, or other catastrophic problem. troubleshoot the power stage. are the dh and dl gate-drive signals present? is the 2v v ref present? on-time pulses are erratic or have unexpected changes in period. vbatt power source has poor impedance characteristic. add a bulk electrolytic bypass capacitor across the bench-top power supply or substitute a real battery. load-transient waveform shows excess ringing. or lx switching waveform exhibits double-pulsing (pulses separated only by a 400ns min off-time). instability due to low-esr ceramic or polymer capacitors placed across fast feedback path (fb-gnd). add parasitic pc board trace resistance between the lx-fb connection and the ceramic capacitor. or substitute a different capacitor type (os-con, tantalum, aluminum electrolytic, and polymer types work well). excessive emi, poor efficiency at high input voltages. gate-drain capacitance of n2/n3 is causing shoot-through cross-conduction. observe the gate-source voltage of n2/n3 during the low-to-high lx node transition (this requires careful instrumentation). is the gate voltage being pulled above 1.5v, causing n2/n3 to turn on? use a smaller low-side mosfet or add a bst resistor (r7). poor efficiency at high input voltages, n1/n4 get hot. n1/n4 have excessive gate capacitance. use a smaller high-side mosfet or add more heatsinking. table 6. troubleshooting guide shunt location ilim pin current-limit threshold on connected to v cc 120mv off connected to resistor divider r6/r12. refer to the setting the current limit section in the max1855 data sheet for more information. adjustable between 50mv and 200mv. table 5. jumper ju6 functions (fixed/adjustable current-limit selection)
evaluates: max1716/max1854/max1855 max1855 evaluation kit 6 ________________________________________________________________________________________ figure 1. max1855 ev kit schematic 21 d0 d1 d2 d3 d4 d0 d1 d2 d3 d4 cc 20 19 18 sw1-a 110 17 tdn 2v float = 300khz ju3 400khz ju4 200khz ju5 550khz r12 200k 1% r6 100k 1% 16 ref ref v cc ref 6 ilim 2 r7 short (pc trace) d2 cmpsh? v+ 22 bst 1 23 dh 13 4 4 4 lx dl 11 cs 24 vps 4 fb vgate vgate 12 5 9 3 8 u1 max1855 agnd pgnd 14 10 shdn shdn skip skip sw1-c 38 sw1-b 29 sw1-d 47 sw1-e 56 r4 1m ? r3 1m ? ju2 ju1 sw2 reset v cc c12 0.22 f c14 47pf r5 1k r1 20 ? v cc v dd v cc v dd v dd c11 0.22 f c1 10 f 25v c5 220 f 2.5v c6 220 f 2.5v c10 10 f 6.3v c7 220 f 2.5v c8 220 f 2.5v c16 220 f 2.5v c17 open c2 10 f 25v c15 1 f c9 0.1 f c3 10 f 25v +5v vbias c4 10 f 25v c18 10 f 25v n1 n4 4 4 66 5 7 8 7 8 5 11 2 3 n3 d1 6 5 7 8 1 2 3 n2 6 5 7 8 1 2 3 n5 6 5 7 8 1 2 3 3 2 l1 0.6 h v out j1 scope jack d3 r9 1k 1% r13 1k 1% r10 open r2 0.006 ? 1% r14 0.006 ? 1% r8 100 ? r11 100k c13 1000pf v cc gnd v batt 7v to 24v ref
evaluates: max1716/max1854/max1855 max1855 evaluation kit _______________________________________________________________________________________ 7 figure 4. max1855 ev kit pc board layout layers 2 and 3 figure 5. max1855 ev kit pc board layout solder side figure 2. max1855 ev kit component placement guide top silkscreen 1.0" figure 3. max1855 ev kit pc board layout component side 1.0" 1.0" 1.0"
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. 8 _____________________maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 ? 2000 maxim integrated products printed usa is a registered trademark of maxim integrated products. evaluates: max1716/max1854/max1855 max1855 evaluation kit figure 6. max1855 ev kit component placement guide solder side 1.0"
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