Datasheet

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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 Current

♦ Digitally Programmable Output from 1.3V to 3.5V

♦ Current-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 Protection

♦ Power-Good (PWROK) Output

♦ Digital Soft-Start

♦ High-Current (2A) Drive Outputs

♦ Complies with Intel VRM 8.2 Specification

MAX1638

High-Speed Step-Down Controller with

Synchronous Rectification for CPU Power

________________________________________________________________ Maxim Integrated Products 1

19-1313; Rev 1; 8/05

PART

MAX1638EAG

MAX1638EAG+ -40°C to +85°C

-40°C to +85°C

TEMP RANGE PIN-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.

AGND

REF

FREQ

CC1

CC2

PWROK

BST

PGND

TO V

CSH

OUTPUT

1.3V TO 4.5V

INPUT

+5V

CSL

MAX1638

For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at

1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.

EVALUATION KIT

AVAILABLE

+Denotes lead-free package.

Summary of content (16 pages)

PAGE 1
9-1313; Rev 1; 8/05 KIT ATION EVALU E L B AVAILA High-Speed Step-Down Controller with Synchronous Rectification for CPU Power 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.
PAGE 2
MAX1638 High-Speed Step-Down Controller with Synchronous Rectification for CPU Power ABSOLUTE MAXIMUM RATINGS 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 ...........................
PAGE 3
High-Speed Step-Down Controller with Synchronous Rectification for CPU Power MAX1638 ELECTRICAL CHARACTERISTICS (continued) (VDD = VCC = D4 = +5V, PGND = AGND = D0–D3 = 0V, FREQ = REF, TA = 0°C to +85°C, unless otherwise noted.) PARAMETER Maximum Duty Cycle CONDITIONS FREQ = VCC MIN TYP 85 90 GND (low) LG, FREQ Input Voltage 3.3 VCC (high) VCC - 0.1 D0–D4, VCC = 4.5V Logic Input Voltage High D0–D4, VCC = 5.5V D0–D4 Source Current D0–D4 = 0V % 3.7 V 0.8 V 10 µA 4 µA 50 µA 2.
PAGE 4
MAX1638 High-Speed Step-Down Controller with Synchronous Rectification for CPU Power ELECTRICAL CHARACTERISTICS (VDD = VCC = D4 = +5V, PGND = AGND = D0–D3= 0V, FREQ = REF, TA = -40°C to +85°C, unless otherwise noted.) (Note 3) MAX UNITS Input Voltage Range PARAMETER VCC = VDD CONDITIONS MIN 4.5 TYP 5.5 V Input Undervoltage Lockout VCC rising edge, 1% hysteresis 3.9 4.3 V VCC Supply Current VCC = VDD = 5.5V, FB overdrive = 200mV Operating mode 3 DAC code = 11111 0.
PAGE 5
High-Speed Step-Down Controller with Synchronous Rectification for CPU Power (TA = +25°C, using the MAX1638 evaluation kit, unless otherwise noted.) LOAD-TRANSIENT RESPONSE WITHOUT GLITCHCATCHER (COUT = 880μF) LOAD-TRANSIENT RESPONSE WITH GLITCHCATCHER MAX1638-03 MAX1638-02 MAX1638-01 LOAD-TRANSIENT RESPONSE WITHOUT GLITCHCATCHER (COUT = 440μF) A A A B C B D B 10μs/div 10μs/div 10μs/div VIN = 5V, VOUT = 2.
PAGE 6
MAX1638 High-Speed Step-Down Controller with Synchronous Rectification for CPU Power ______________________________________________________________Pin Description 6 PIN NAME FUNCTION 1 BST Boost-Capacitor Bypass for High-Side MOSFET Gate Drive. Connect a 0.1µF capacitor and low-leakage Schottky diode as a bootstrapped charge-pump circuit to derive a 5V gate drive from VDD for DH. 2 PWROK 3 CSL Current-Sense Amplifier’s Inverting Input.
PAGE 7
High-Speed Step-Down Controller with Synchronous Rectification for CPU Power C5 0.1μF VIN = 5V C6 10μF C7 0.1μF C1 D2 CMPSH-3 VDD VCC TO VDD MAX1638 R5 10Ω R6 100k BST PWROK D0 R4 (OPTIONAL) D2 P1 LX D3 D4 N1 DH D1 C8 1μF C3 0.1μF VOUT = 1.3V TO 3.5V R1 MAX1638 R3 (OPTIONAL) LG DL N2 D1 (OPTIONAL) C2 FREQ N3 REF C4, 1.0μF CERAMIC LOCAL BYPASSING LOAD PGND CSH CC2 TO AGND R2 L1 CC2 RC1 0.056μF 1k CSL NDRV PDRV CC1 CC1 1000pF FB AGND Figure 1.
PAGE 8
MAX1638 High-Speed Step-Down Controller with Synchronous Rectification for CPU Power REF REF1 REF4 REF3 REF2 + - AGND MAX1638 VCC CSL CSH LG SLOPE COMPENSATION BST DH RESET OSCILLATOR FREQ Q LX CONTROL AND DRIVE LOGIC REF Q SET VDD DL CC1 40k PGND CC2 10k + - gm WINDOW REF3 REF2 REF4 REF REF1 5 N FB D0–D4 PWROK PDRV NDRV Figure 2.
PAGE 9
High-Speed Step-Down Controller with Synchronous Rectification for CPU Power MAX1638 Table 1. Component List for Standard Applications LOAD REQUIREMENT COMPONENT 2.0V, 14A 2.0V, 19A 1.
PAGE 10
MAX1638 High-Speed Step-Down Controller with Synchronous Rectification for CPU Power Synchronous-Rectifier Driver Synchronous rectification reduces conduction losses in the rectifier by shunting the normal Schottky diode or MOSFET body diode with a low-on-resistance MOSFET switch. The synchronous rectifier also ensures proper start-up by precharging the boost-charge pump used for the high-side switch gate-drive circuit.
PAGE 11
High-Speed Step-Down Controller with Synchronous Rectification for CPU Power 100 90 80 ILIM (%) 70 60 50 Internal Soft-Start 40 Soft-start allows a gradual increase of the internal current limit at start-up to reduce input surge currents. An internal DAC raises the current-limit threshold from 0V to 100mV in four steps (25mV, 50mV, 75mV, and 100mV) over the span of 1536 oscillator cycles. 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 VFB (%) Figure 4.
PAGE 12
MAX1638 High-Speed Step-Down Controller with Synchronous Rectification for CPU Power R6 10Ω C9 0.1μF VIN = 5V C5 0.1μF C7 10μF C8 4.7μF C1 C11 3.3nF TO VDD D2 CMPSH-3 VDD VCC CSH R7 C12 39Ω 3.3nF R5 100k PWROK R1 CSL R8 39Ω D0 BST D1 D2 REF MAX1638 P1 R11 L1 VOUT = 1.3V TO 3.5V LX LG R9 (OPTIONAL) DL NO CONNECTION C4 0.1μF R10 (OPTIONAL) D3 D4 N1 DH N2 D1 (OPTIONAL) LOCAL BYPASSING C2 N3 FREQ PGND CC2 CC2 NDRV RC1 CC1 PDRV CC1 FB TO AGND REF C6, 1.
PAGE 13
High-Speed Step-Down Controller with Synchronous Rectification for CPU Power D4 D3 D2 0 0 0 D1 D0 0 0 OUTPUT VOLTAGE (V) COMPATIBILITY Table 3. LG Pin Adjustment Settings AC LOADLG REGULATION CONNECTED ERROR TO: (%) DC LOADREGULATION ERROR (%) TYPICAL AE (VGAIN/ IGAIN) 2.050 VCC 2 0.2 2 REF 1 0.1 4 GND 0.5 0.05 8 0 0 0 0 1 2.000 0 0 0 1 0 1.950 0 0 0 1 1 1.900 0 0 1 0 0 1.850 0 0 1 0 1 1.800 0 0 1 1 0 1.750 0 0 1 1 1 1.
PAGE 14
MAX1638 High-Speed Step-Down Controller with Synchronous Rectification for CPU Power Use IPEAK from the equation in the section Specifying the Inductor. RSENSE = 85mV IPEAK The high inductance of standard wire-wound resistors can degrade performance. Low-inductance resistors, such as surface-mount power metal-strip resistors, are preferred.
PAGE 15
High-Speed Step-Down Controller with Synchronous Rectification for CPU Power I2R losses are the greatest heat contributor to MOSFET power dissipation and are distributed between the high- and low-side MOSFETs according to duty factor, as follows: 2 PD (high side) = ILOAD 2 PD (low side) = ILOAD V x RDS(ON) x OUT VIN ⎛ ⎞ V x RDS(ON) x ⎜1 − OUT ⎟ V ⎝ IN ⎠ Gate-charge losses are dissipated in the IC, and do not heat the MOSFETs.
PAGE 16
MAX1638 High-Speed Step-Down Controller with Synchronous Rectification for CPU Power __________Applications Information INPUT 5V Efficiency Considerations Refer to the MAX796–MAX799 data sheet for information on calculating losses and improving efficiency. C8 (OPTIONAL) P1 PC Board Layout Considerations Good PC board layout and routing are required in highcurrent, high-frequency switching power supplies to achieve good regulation, high efficiency, and stability.