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MAX1638EAG+ |MAX1638EAGMAXN/a30avaiHigh-Speed Step-Down Controller with Synchronous Rectification for CPU Power
MAX1638EAG+T |MAX1638EAGTMAXIMN/a8000avaiHigh-Speed Step-Down Controller with Synchronous Rectification for CPU Power


MAX1638EAG+T ,High-Speed Step-Down Controller with Synchronous Rectification for CPU Powerapplications in which♦ Greater than 90% Efficiency Using N-Channeloutput voltage precision and good ..
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MAX1638EAG+-MAX1638EAG+T
High-Speed Step-Down Controller with Synchronous Rectification for CPU Power
General Description
The MAX1638 is an ultra-high-performance, step-down
DC-DC controller for CPU power in high-end computer
systems. Designed for demanding applications in which
output voltage precision and good transient response are
critical for proper operation, it delivers over 35A from 1.3V
to 3.5V with ±1% total accuracy from a +5V ±10% supply.
Excellent dynamic response corrects output transients
caused by the latest dynamically clocked CPUs. This
controller achieves over 90% efficiency by using synchro-
nous rectification. Flying-capacitor bootstrap circuitry
drives inexpensive, external N-channel MOSFETs.
The switching frequency is pin-selectable for 300kHz,
600kHz, or 1MHz. High switching frequencies allow the
use of a small surface-mount inductor and decrease out-
put filter capacitor requirements, reducing board area
and system cost.
The MAX1638 is available in 24-pin SSOP and QSOP
(future package) packages, and offers additional fea-
tures such as a digitally programmable output;
adjustable transient response; and selectable 0.5%, 1%,
or 2% AC load regulation. Fast recovery from load tran-
sients is ensured by a GlitchCatcher™ current-boost cir-
cuit that eliminates delays caused by the buck inductor.
Output overvoltage protection is enforced by a crowbar
circuit that turns on the low-side MOSFET with 100%
duty factor when the output is 200mV above the normal
regulation point. Other features include internal digital
soft-start, a power-good output, and a 3.5V ±1% refer-
ence output.
________________________Applications

Pentium Pro™, Pentium II™, PowerPC™, Alpha™,
and K6™ Systems
Workstations
Desktop Computers
LAN Servers
GTL Bus Termination
____________________________Features
Better than ±1% Output Accuracy Over
Line and Load
Greater than 90% Efficiency Using N-Channel
MOSFETs
Pin-Selected High Switching Frequency (300kHz,
600kHz, or 1MHz)
Over 35A Output CurrentDigitally Programmable Output from 1.3V to 3.5VCurrent-Mode Control for Fast Transient
Response and Cycle-by-Cycle Current-Limit
Protection
Short-Circuit Protection with Foldback Current
Limiting
Glitch-Catcher Circuit for Fast Load-Transient
Response
Crowbar Overvoltage ProtectionPower-Good (PWROK) OutputDigital Soft-StartHigh-Current (2A) Drive OutputsComplies with Intel VRM 8.2 Specification
MAX1638
High-Speed Step-Down Controller with
Synchronous Rectification for CPU Power

19-1313; Rev 1; 8/05
PART

MAX1638EAG
MAX1638EAG+-40°C to +85°C
-40°C to +85°C
TEMP RANGEPIN-PACKAGE

24 SSOP
24 QSOP
Ordering Information
Typical Operating Circuit
Pin Configuration appears at end of data sheet.

Pentium Pro and Pentium II are trademarks of Intel Corp.
PowerPC is a trademark of IBM Corp.
Alpha is a trademark of Digital Equipment Corp.
K6 is a trademark of Advanced Micro Devices.
GlitchCatcher is a trademark of Maxim Integrated Products.
VCC
AGND
REF
FREQ
CC1
CC2
PWROK
BST
PGND
TO VDD
CSH
OUTPUT
1.3V TO 4.5V
INPUT
+5V
VDD
CSL
MAX1638
EVALUATION KIT
AVAILABLE

+Denotes lead-free package.
MAX1638
High-Speed Step-Down Controller with
Synchronous Rectification for CPU Power
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS

(VDD= VCC = D4 = +5V, PGND = AGND = D0–D3 = 0V, FREQ = REF, TA= 0°C to +85°C, unless otherwise noted.)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
VDD, VCC, PWROK to AGND......................................-0.3V to 6V
PGND to AGND..................................................................±0.3V
CSH, CSL to AGND....................................-0.3V to (VCC+ 0.3V)
NDRV, PDRV, DL to PGND.........................-0.3V to (VDD+ 0.3V)
REF, CC1, CC2, LG, D0–D4, FREQ,
FB to AGND.............................................-0.3V to (VCC + 0.3V)
BST to PGND............................................................-0.3V to 12V
BST to LX....................................................................-0.3V to 6V
DH to LX.............................................(LX - 0.3V) to (BST + 0.3V)
Continuous Power Dissipation (TA= +70°C)
QSOP (derate 8.70mW/°C above +70°C).....................696mW
QSOP θJC..................................................................40°C/W
SSOP (derate 8.00mW/°C above +70°C).....................640mW
SSOP θJC..................................................................45°C/W
Operating Temperature Range...........................-40°C to +85°C
Storage Temperature Range.............................-65°C to +160°C
Lead Temperature (soldering, 10s).................................+300°C
FREQ = AGND
FREQ = REF
VCC= VDD
FREQ = VCC
PWROK = 5.5V
Over line and load
(Note 1)
ISINK= 2mA, VCC= 4.5V
Falling FB, 1% hysteresis with respect to VREF
Rising FB, 1% hysteresis with respect to VREF
CSH - CSL =
0mV to 80mV
0µA < IREF< 100µA
No load
VCC= VDD= 5.5V, FB forced 200mV above
regulation point, operating or standby mode
VCCrising edge, 1% hysteresis
VREF= 0V
Rising edge, 1% hysteresis
CONDITIONS

kHz
Switching Frequency540600660
850100011501PWROK Output Current High0.4PWROK Output Voltage Low6.589.5PWROK Trip Level-7.5-6-4.5
AC Load Regulation
(Note 2)1
0.50.54.0Reference Short-Circuit Current2.73.0Reference Undervoltage Lockout4.55.5Input Voltage Range±1.5Output Voltage (FB) Accuracy10Reference Load Regulation3.4653.53.535Reference Voltage0.1VDDSupply Current4.04.2Input Undervoltage Lockout
UNITSMINTYPMAXPARAMETER

CSH - CSL =
0mV to 80mV%
DC Load Regulation
(Note 2)0.1
0.05= +25°C to +85°C= 0°C to +85°C
LG = GND
LG = REF
LG = VCC
LG = GND
LG = REF
LG = VCC
2.5FB overdrive = 200mVFB overdrive = 0V
Operating
modeVCC= VDD
= 5.5VmA
VCCSupply Current
VREF= 0V
Shutdown
mode
0.3DAC code = 11111
ELECTRICAL CHARACTERISTICS (continued)
(VDD= VCC = D4 = +5V, PGND = AGND = D0–D3 = 0V, FREQ = REF, TA= 0°C to +85°C, unless otherwise noted.)
MAX1638

DH = DL = 2.5V
VDD= 4.5V
BST - LX = 4.5V
GND (low)
100mV overdrive
FREQ = VCC
With respect to VREF,
FB going low
Minimum
D0–D4 = 0V
D0–D4, VCC= 5.5V
REF (mid)
CSH = CSL = 1.3V, D0–D3 = 5V, D4 = 0V
D0–D4, VCC= 4.5V
CONDITIONS

VCC (high)030DH, DL Dead Time2DH, DL Source/Sink Current
Maximum
DH On-Resistance0.72-3-1100CC2 Source/Sink Current
4VCCV2.43.0
mmho110CC1 Output Resistance±0.1
0.28590Maximum Duty Cycle
LG, FREQ Input Voltage50CSH, CSL Input Current4LG, FREQ Input Current2510D0–D4 Source Current2.0Logic Input Voltage High
VCC- 0.1
0.8Logic Input Voltage Low
UNITSMINTYPMAXPARAMETER

FB Input Current
CC2 Clamp Voltage
CC2 Transconductance
PDRV Trip Level
PDRV, NDRV Response TimeFB overdrive = 5%ns75
PDRV, NDRV On-ResistanceVDD= 4.5VΩ25
PDRV, NDRV Source/Sink CurrentPDRV = NDRV = 2.5VA0.5
PDRV, NDRV Minimum On-Timens100
FB = 3.5V85100115
Soft-Start TimeTo full current limit1 / fOSC1536
BST Leakage CurrentBST = 12V, LX = 7V, REF = GNDµA50
High-Speed Step-Down Controller with
Synchronous Rectification for CPU Power
DL On-Resistance
NDRV Trip LevelWith respect to VREF,
FB going high
1.2522.75%13= +25°C= 0°C to +85°C= +25°C= 0°C to +85°C
Current-Limit Trip VoltageFB = 0V (Foldback)mV153870
MAX1638
High-Speed Step-Down Controller with
Synchronous Rectification for CPU Power

FREQ = REF
FREQ = VCC
Falling FB, 1% hysteresis with respect to VREF
Rising FB, 1% hysteresis with respect to VREF
FREQ = AGND
Over line and load (Note 1)
VCC= VDD
CONDITIONS

Switching Frequency10
kHz
240300360-8-6-4PWROK Trip Level±2.5Output Voltage (FB) Accuracy4.55.5Input Voltage Range
UNITSMINTYPMAXPARAMETER

BST - LX = 4.5V
FREQ = VCC
VDD= 4.5V0.7284900.7 2
Maximum Duty Cycle
DL On-Resistance
DH On-Resistance
FB = 3.5VCurrent-Limit Trip VoltagemV70100130
ELECTRICAL CHARACTERISTICS

(VDD= VCC= D4 = +5V, PGND = AGND = D0–D3= 0V, FREQ = REF, TA= -40°C to +85°C, unless otherwise noted.) (Note 3)
Note 1:
FB accuracy is 100% tested at FB = 3.5V (code 10000) with VCC= VDD= 4.5V to 5.5V and CSH - CSL = 0mV to 80mV. The
other DAC codes are tested with VCC= VDD= 5V and CSH - CSL = 0.
Note 2:
AC load regulation sets the AC loop gain, to make tradeoffs between output filter capacitor size and transient response,
and has only a slight effect on DC accuracy or DC load-regulation error.
Note 3:
Specifications from 0°C to -40°C are guaranteed by design, not production tested.
No loadV3.4483.53.553Reference Voltage
VCCrising edge, 1% hysteresisV3.94.3Input Undervoltage Lockout
Operating mode3
VCCSupply Current
VCC= VDD= 5.5V, FB forced 200mV above
regulation point, operating or shutdown modemA0.2
VCC= VDD= 5.5V,
FB overdrive =
200mV
VDDSupply Current
Shutdown
modeVREF= 0V
DAC code = 11111
MAX1638
High-Speed Step-Down Controller with
Synchronous Rectification for CPU Power
FOLDBACK CURRENT LIMIT

MAX1638-04
VO = 2.0V NOMINAL
A: VOUT = 0.5V/div
B: INDUCTOR CURRENT, 5A/div
10μs/div
STARTUP WAVEFORMS

MAX1638-05
A: VOUT = 0.5V/div
B: INDUCTOR CURRENT, 5A/div
400μs/div1μs/div
SWITCHING WAVEFORMS

MAX1638-06
VIN = 5V, VOUT = 2.5V, LOAD = 5A
A:LX, 5V/divC:INDUCTOR CURRENT,
B:VOUT, 20mV/div, AC COUPLED5A/div
LOAD-TRANSIENT RESPONSE
WITHOUT GLITCHCATCHER (COUT = 880μF)

MAX1638-01
VIN = 5V, VOUT = 2.0V, LOAD = 14A, 3A/μs
A: VOUT, 50mV/div, AC COUPLED
B: INDUCTOR CURRENT, 10A/div
10μs/div
LOAD-TRANSIENT RESPONSE
WITHOUT GLITCHCATCHER (COUT = 440μF)

MAX1638-02
VIN = 5V, VOUT = 2.0V, LOAD = 14A, 3A/μs
A: VOUT, 100mV/div, AC COUPLED
B: INDUCTOR CURRENT, 10A/div
10μs/div
LOAD-TRANSIENT RESPONSE
WITH GLITCHCATCHER

MAX1638-03
COUT = 440μF, VIN = 5V, VOUT = 2.0V, LOAD = 14A, 30A/μs
A:VOUT, 100mV/div, C:NDRV, 5V/div
AC COUPLEDD:INDUCTOR CURRENT,
B:PDRV, 5V/div10A/div
10μs/div
__________________________________________Typical Operating Characteristics

(TA = +25°C, using the MAX1638 evaluation kit, unless otherwise noted.)
EFFICIENCY vs. OUTPUT CURRENT
MAX1638-07
EFFICIENCY (%)
VOUT = 2.0V
VOUT = 1.3V
VOUT = 3.5V
REFERENCE VOLTAGE
vs. OUTPUT CURRENT
MAX1638-08
OUTPUT CURRENT (mA)
REFERENCE VOLTAGE (V)
SOURCING
CURRENT
SINKING
CURRENT
MAXIMUM DUTY CYCLE
vs. SWITCHING FREQUENCY

MAX1638-09
SWITCHING FREQUENCY (kHz)
MAXIMUM DUTY CYCLE (%)
600400
MAX1638
High-Speed Step-Down Controller with
Synchronous Rectification for CPU Power
PIN

High-Side Main MOSFET Switch Gate-Drive Output. DH is a floating driver output that swings from LX
to BST, riding on the LX switching-node voltage. See the section BST High-Side Gate-Driver Supply
and MOSFET Drivers.24
Switching Node. Connect LX to the high-side MOSFET source and inductor.LX23
Power GroundPGND22Low-Side Synchronous Rectifier Gate-Drive Output. DL swings between PGND and VDD. See the
section BST High-Side Gate-Driver Supply and MOSFET Drivers.21
VDD5V Power Input for MOSFET Drivers. Bypass VDDto PGND within 0.2 in. (5mm) of the VDDpin using a
0.1µF capacitor and 4.7µF capacitor connected in parallel. 20
PDRVGlitchCatcher P-Channel MOSFET Driver Output. PDRV swings between VDDand PGND. 19
NDRVGlitchCatcher N-Channel MOSFET Driver Output. NDRV swings between VDDand PGND. 18
D4, D3Digital Inputs for Programming the Output Voltage 16, 17
FREQ
Frequency-Select Input.FREQ = VCC: 1MHz
FREQ = REF: 600kHz
FREQ = AGND: 300kHz
CC2Slow-Loop Compensation Capacitor Input. Connect a ceramic capacitor from CC2 to AGND. See the
section Compensating the Feedback Loop.14
BSTBoost-Capacitor Bypass for High-Side MOSFET Gate Drive. Connect a 0.1µF capacitor and low-leak-
age Schottky diode as a bootstrapped charge-pump circuit to derive a 5V gate drive from VDDfor DH.1
NAMEFUNCTION
______________________________________________________________Pin Description

CC1Fast-Loop Compensation Capacitor Input. Connect a ceramic capacitor and resistor in series from
CC1 to AGND. See the section Compensating the Feedback Loop.13Voltage-Feedback Input. Connect FB to the CPU’s remote voltage-sense point. The voltage at this input
is regulated to a value determined by D0–D4.12
PWROKOpen-Drain Logic Output. PWROK is high when the voltage on FB is within +8% and -6% of its set-
point. 2
CSLCurrent-Sense Amplifier’s Inverting Input. Place the current-sense resistor very close to the controller IC,
and use a Kelvin connection.3
CSHCurrent-Sense Amplifier’s Noninverting Input4
D2, D1,
Digital Inputs for Programming the Output Voltage. D0–D4 are logic inputs that set the output to a volt-
age between 1.3V and 3.5V (Table 2). D0–D4 are internally pulled up to VCCwith 5µA current sources.5, 6, 7
Loop Gain-Control Input. LG is a three-level input that is used to trade off loop gain vs. AC load-regula-
tion and load-transient response. Connect LG to VCC, REF, or AGND for 2%, 1%, or 0.5% AC load-reg-
ulation errors, respectively.
VCCAnalog Supply Input, 5V. Use an RC filter network, as shown in Figure 1. 9
REFReference Output, 3.5V. Bypass REF to AGND with 0.1µF (min). Sources up to 100µA for external
loads. Force REF below 2V to turn off the controller. 10
AGNDAnalog Ground11
MAX1638
High-Speed Step-Down Controller with
Synchronous Rectification for CPU Power

Figure 1. Standard Application Circuit
_____Standard Application Circuits

The predesigned MAX1638 circuit shown in Figure 1
meets a wide range of applications with output currents
up to 19A and higher. Use Table 1 to select compo-
nents appropriate for the desired output current range,
and adapt the evaluation kit PC board layout as neces-
sary. This circuit represent a good set of trade-offs
between cost, size, and efficiency while staying within
the worst-case specification limits for stress-related
parameters, such as capacitor ripple current.
The MAX1638 circuit was designed for the specified
frequency. Do not change the switching frequency
without first recalculating component values—particu-
larly the inductance, output filter capacitance, and RC1
resistance values. Table 2 lists the voltage adjustment
DAC codes.
_______________Detailed Description

The MAX1638 is a BiCMOS power-supply controller
designed for use in switch-mode, step-down (buck)
topology DC-DC converters. Synchronous rectification
provides high efficiency. It is intended to provide the
high precision, low noise, excellent transient response,
and high efficiency required in today’s most demand-
ing applications.
D1
(OPTIONAL)
(OPTIONAL)
(OPTIONAL)
VCCVDD
PWROKBST
PGND
CSH
PDRV
NDRV
AGND
CC1
CC2
CC1
1000pF
CC2
0.056μF
REF
C4, 1.0μF
CERAMIC
RC1TO
AGND
100k
0.1μF
10μF
10Ω
TO VDD
FREQ
0.1μF
CMPSH-3
0.1μF
VIN = 5V
LOCAL
BYPASSING
MAX1638
1μF
VOUT = 1.3V
TO 3.5VLOAD
CSL
MAX1638
High-Speed Step-Down Controller with
Synchronous Rectification for CPU Power

REF
REF4REF3REF2
REFD0–D4PWROKPDRVNDRV
CC2
CC1
REF1
10k
40k
WINDOW
CONTROL AND
DRIVE LOGIC
OSCILLATOR
SLOPE
COMPENSATION
AGND
VCC
FREQ
REF4REF1
REF
REF3REF2CSL
CSH
BST
VDD
RESETQ
SET
PGND
MAX1638
Figure 2. Simplified Block Diagram
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