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19-1758; Rev 0; 8/00
MAX1855 Evaluation Kit Evaluates: MAX1716/MAX1854/MAX1855
General Description
The MAX1855 evaluation kit (EV kit) demonstrates a highpower, dynamically adjustable notebook CPU power-supply application circuit. The MAX1855 DC-DC converter steps down high-voltage batteries and/or AC adapters, generating a precision, low-voltage CPU core VCC rail. The MAX1855 EV kit is designed for CPU core applications requiring a voltage-positioned supply. Voltage positioning and a high-DC-accuracy control loop decrease full-load power dissipation and reduce the required number 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 output current. The EV kit operates at 300kHz switching frequency 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). o o o o o o o
Features
High Speed, Accuracy, and Efficiency Voltage-Positioned Output Low Output Capacitor Count (5) Reduces CPU Power Consumption Fast-response Quick-PWMTM 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
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Ordering Information
PART TEMP. RANGE 0C to +70C IC PACKAGE 24 QSOP MAX1855EVKIT
Quick-PWM is a trademark of Maxim Integrated Products.
Note: To evaluate the MAX1716/MAX1854, request a MAX1716EEG/MAX1854EE free sample with the MAX1855EVKIT.
Component List
DESIGNATION QTY DESCRIPTION 10F, 25V ceramic capacitors (1812) Taiyo Yuden TMK432BJ106KM or TDK C4532X5R1E106M 220F, 2.5V, 15m low-ESR specialty polymer capacitors Panasonic EEFUE0E221R 0.1F ceramic capacitor (0805) 10F, 6.3V X5R ceramic capacitor (1210) Taiyo Yuden JMK325BJ106MN or equivalent 0.22F, 16V X5R ceramic capacitors (0805) Taiyo Yuden EMK212BJ224KG or equivalent 1000pF ceramic capacitor (0805) DESIGNATION QTY C14 C1-C4, C18 5 1 DESCRIPTION 47pF ceramic capacitor (0805) 1F, 10V X5R ceramic capacitor (0805) Taiyo Yuden LMK212BJ105MG or equivalent Not installed 20 5% resistor (1206) 0.006 1% 1W resistors (2512) Dale WSL-2512-R006F 1M 5% resistors (0805) 100k 1% resistor (0805) 100 5% resistor (0805) 1k 1% resistors (0805) Not installed (0805) 100k 5% resistor (0805) 200k 1% resistor (0805)
C15
1
C5-C8, C16 C9
5 1
C17 R1 R2, R14 R3, R4 R6 R8 R5, R9, R13 R10 R11 R12
0 1 2 2 1 1 3 0 1 1
C10
1
C11, C12
2
C13
1
________________________________________________________________ Maxim Integrated Products
1
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.
MAX1855 Evaluation Kit Evaluates: MAX1716/MAX1854/MAX1855
Component List (continued)
DESIGNATION QTY DESCRIPTION 2A Schottky diode Central Semiconductor CMSH2-40 STM-Microelectronics STPS2L25U or International Rectifier 10MQ040 100mA Schottky diode Central Semiconductor CMPSH-3 1A Schottky diode Motorola MBRS130LT3 or International Rectifier 10BQ040 or Nihon EC10QS03 0.68H power inductor Sumida CEP125 #4712-T007 or Sumida CDEP134H-0R6 or Panasonic ETQP6F0R6BFA N-channel MOSFETs (8-pin SO) International Rectifier IRF7811 or International Rectifier IRF7811A N-channel MOSFETs (8-pin SO) International Rectifier IRF7811 or International Rectifier IRF7811A or Fairchild FDS7764A DESIGNATION QTY N5 U1 JU1 None SW1 SW2 D3 1 J1 0 1 2 2 1 1 1 DESCRIPTION Not installed MAX1855EEG (24-pin QSOP) 2-pin headers Shunts (JU1, JU2) DIP-5 dip switch Momentary switch, normally open Digi-Key P8006/7S Scope-probe connector Berg Electronics 33JR135-1 Rubber bumpers 3M SJ-5007 or Mouser 517-SJ-5007BK or equivalent MAX1716/MAX1854/MAX1855 PC board MAX1855 EV kit data sheet MAX1716/MAX1854/MAX1855 data sheet
D1
1
D2
1
None
4
L1
1
None N1, N4 2 None None N2, N3 2
1 1 1
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 = VCC). 3) Turn on battery power prior to +5V bias power; otherwise, 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 oscilloscope. 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 output voltage.
* * *
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 VOUT = 1.6V, inductor ripple is approximately 30%, with a resulting pulse-skipping threshold at roughly ILOAD = 3A with VIN = 12V.
Setting the Output Voltage
The MAX1855 uses an internal 5-bit DAC as a feedback resistor voltage divider. The output voltage can be digitally set from 0.6V to 1.75V, using the D0-D4 inputs (Table 1).
Recommended Equipment
* * +7V to +24V, >30W power supply, battery, or notebook AC adapter DC bias power supply, 5V at 100mA
Load-Transient Experiment
One interesting experiment is to subject the output to large, fast load transients and observe the output with
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MAX1855 Evaluation Kit Evaluates: MAX1716/MAX1854/MAX1855
Table 1. MAX1855 Output Voltage Adjustment Settings
D4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 D3 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 D2 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 D1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 D0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 OUTPUT VOLTAGE (V) MAX1716 NO CPU* NO CPU* NO CPU* NO CPU* NO CPU* NO CPU* NO CPU* NO CPU* 1.600 1.550 1.500 1.450 1.400 1.350 1.300 NO CPU* 1.275 1.250 1.225 1.200 1.175 1.150 1.125 1.100 1.075 1.050 1.025 1.000 0.975 0.950 0.925 NO CPU* MAX1854 2.000 1.950 1.900 1.850 1.800 1.750 1.700 1.650 1.600 1.550 1.500 1.450 1.400 1.350 1.300 NO CPU* 1.275 1.250 1.225 1.200 1.175 1.150 1.125 1.100 1.075 1.050 1.025 1.000 0.975 0.950 0.925 NO CPU* MAX1855 1.750 1.700 1.650 1.600 1.550 1.500 1.450 1.400 1.350 1.300 1.250 1.200 1.150 1.100 1.050 1.000 0.975 0.950 0.925 0.900 0.875 0.850 0.825 0.800 0.775 0.750 0.725 0.700 0.675 0.650 0.625 0.600
* In the NO-CPU state, DH and DL are held low.
an oscilloscope. This necessitates careful instrumentation of the output, using the supplied scope-probe jack. Accurate measurement of output ripple and load-transient 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 powersupply 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
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3
MAX1855 Evaluation Kit Evaluates: MAX1716/MAX1854/MAX1855
least 10A/s load transients. One easy method for generating 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 prohibited here by the need for low resistance and inductance in the path of the dummy-load MOSFET. There are two easy alternative methods of determining how much load current a particular pulse-generator amplitude 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 inductor 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 MOSFET load must be well heatsinked for this to work without causing smoke and flames).
Jumper Settings
Table 2. Jumper JU1 Functions (Shutdown Mode)
SHUNT LOCATION ON OFF SHDN PIN Connected to VCC Connected to GND MAX1855 OUTPUT MAX1855 enabled Shutdown mode, VOUT = 0V
Table 3. Jumper JU2 Functions (Low-Noise Mode)
SHUNT LOCATION SKIP PIN Connected to VCC MAX1855 OUTPUT Low-noise mode, forced fixed-frequency PWM operation Normal operation, allows automatic PWM/PFM switchover for pulse-skipping at light load, resulting in highest efficiency
ON
Component Suppliers
SUPPLIER Central Semiconductor Dale-Vishay Fairchild International Rectifier Kemet Nihon ON Semiconductor (Motorola) Panasonic Sanyo STMMicroelectronics Sumida Taiyo Yuden TDK PHONE 516-435-1110 402-564-3131 408-721-2181 310-322-3331 408-986-0424 847-843-7500 602-303-5454 714-373-7939 619-661-6835 617-259-0300 708-956-0666 408-573-4150 847-390-4373 FAX 516-435-1824 402-563-6418 408-721-1635 310-322-3332 408-986-1442 847-843-2798 602-994-6430 714-373-7183 619-661-1055 617-259-9442 708-956-0702 408-573-4159 847-390-4428
OFF
Connected to GND
Table 4. Jumpers JU3/JU4/JU5 Functions (Switching-Frequency Selection)
JUMPER JU3 JU4, JU5 JU4 JU3, JU5 JU5 JU3, JU4 JU3, JU4, JU5 SHUNT LOCATION ON OFF ON OFF ON OFF OFF TON PIN Connected to REF Connected to VCC Connected to GND Floating FREQUENCY (kHz) 400 200 550 300
Note: Don't change the operating frequency without first recalculating component values because the frequency has a significant 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 ________________________________________________________________________________________
MAX1855 Evaluation Kit
Table 5. Jumper JU6 Functions (Fixed/Adjustable Current-Limit Selection)
SHUNT LOCATION ON ILIM PIN Connected to VCC Connected to resistor divider R6/R12. Refer to the Setting the Current Limit section in the MAX1855 data sheet for more information. CURRENT-LIMIT THRESHOLD 120mV
Evaluates: MAX1716/MAX1854/MAX1855
OFF
Adjustable between 50mV and 200mV.
Table 6. Troubleshooting Guide
SYMPTOM Circuit won't start when power is applied. POSSIBLE PROBLEM Power-supply sequencing: +5V bias supply was applied first. Output overvoltage due to shorted high-side MOSFET. Circuit won't start when RESET is pressed, +5V bias supply cycled. Output overvoltage due to load recovery overshoot. Transient overload condition. Broken connection, bad MOSFET, or other catastrophic problem. On-time pulses are erratic or have unexpected changes in period. Load-transient waveform shows excess ringing. OR LX switching waveform exhibits double-pulsing (pulses separated only by a 400ns min off-time). POSSIBLE PROBLEM Cycle SHDN Press the RESET button. Replace the MOSFET. Reduce the inductor value, raise the switching frequency, or add more output capacitance. Add more low-ESR output capacitors. Troubleshoot the power stage. Are the DH and DL gate-drive signals present? Is the 2V VREF present? Add a bulk electrolytic bypass capacitor across the bench-top power supply or substitute a real battery. 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). 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). Use a smaller high-side MOSFET or add more heatsinking.
VBATT power source has poor impedance characteristic.
Instability due to low-ESR ceramic or polymer capacitors placed across fast feedback path (FB-GND).
Excessive EMI, poor efficiency at high input voltages.
Gate-drain capacitance of N2/N3 is causing shoot-through cross-conduction.
Poor efficiency at high input voltages, N1/N4 get hot.
N1/N4 have excessive gate capacitance.
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5
Evaluates: MAX1716/MAX1854/MAX1855
7V TO 24V VDD R1 20 VDD +5V 3 SHDN VCC VDD V+ R7 SHORT (PC TRACE) 22 D2 CMPSH-3 7 8 DH C9 0.1F 20 D0 LX 7 8 19 D1 9 18 D2 24 R9 1k 1% R2 0.006 1% R14 0.006 1% R10 OPEN REF CC FB C13 1000pF 12 VCC R11 100k PGND 14 10 AGND REF 4 R8 100 8 17 D3 11 VPS 16 D4 6 5 9 C14 47pF C12 0.22F R13 1k 1% 7 CS 6 5 4 3 3 D1 7 8 6 5 7 8 6 5 L1 0.6H C5 220F 2.5V C6 220F 2.5V 23 C7 220F 2.5V D3 C10 10F 6.3V 3 3 VOUT 1 4 4 N1 1 2 N4 1 2 6 5 6 5 7 8 J1 SCOPE JACK C15 1F 2 R3 1M BST 21 SKIP R4 1M VBIAS C1 10F 25V C2 10F 25V C3 10F 25V C18 10F 25V C4 10F 25V
VBATT R5 1k VCC C11 0.22F
SW2
RESET
JU1
MAX1855 Evaluation Kit
Figure 1. MAX1855 EV Kit Schematic
U1
3 DL 13 4 N5 1 2 C8 220F 2.5V N2 4 1 2 N3 1 2
SHDN
VCC
JU2
SKIP
D0
SW1-A 1 10
D1
SW1-B 2
MAX1855
GND
D2
SW1-C 3
C16 220F 2.5V
C17 OPEN
D3
SW1-D 4
D4
SW1-E 5
2V
REF VGATE 8 TDN VGATE
JU3 400kHz
FLOAT = 300kHz
VCC 6 ILIM REF R12 200k 1%
6 ________________________________________________________________________________________
JU4 200kHz
JU5 550kHz
R6 100k 1%
MAX1855 Evaluation Kit Evaluates: MAX1716/MAX1854/MAX1855
1.0"
1.0"
Figure 2. MAX1855 EV Kit Component Placement Guide--Top Silkscreen
Figure 3. MAX1855 EV Kit PC Board Layout--Component Side
1.0"
1.0"
Figure 4. MAX1855 EV Kit PC Board Layout--Layers 2 and 3
Figure 5. MAX1855 EV Kit PC Board Layout--Solder Side 7
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MAX1855 Evaluation Kit Evaluates: MAX1716/MAX1854/MAX1855
1.0"
Figure 6. MAX1855 EV Kit Component Placement Guide-- Solder Side
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit 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 (c) 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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