Datasheet

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General Description

The MAX1809 is a reversible energy flow, constant-off-

time, pulse-width modulated (PWM), step-down DC-DC

converter. It is ideal for use in notebook and subnote-

book computers that require 1.1V to 5V active

termination power supplies. This device features an

internal PMOS power switch and internal synchronous

rectifier for high efficiency and reduced component

count. The internal 90mΩ PMOS power switch and

70mΩ NMOS synchronous-rectifier switch easily deliver

continuous load currents up to 3A. The MAX1809 accu-

rately tracks an external reference voltage, produces

an adjustable output from 1.1V to V

IN,

and achieves

efficiencies as high as 93%.

The MAX1809 uses a unique current-mode, constant-

off-time, PWM control scheme that allows the output to

source or sink current. This feature allows energy to

return to the input power supply that otherwise would

be wasted. The programmable constant-off-time archi-

tecture sets switching frequencies up to 1MHz, allowing

the user to optimize performance trade-offs between

efficiency, output switching noise, component size, and

cost. The MAX1809 features an adjustable soft-start to

limit surge currents during startup, a 100% duty-cycle

mode for low-dropout operation, and a low-power shut-

down mode that disables the power switches and

reduces supply current below 1µA. The MAX1809 is

available in a 28-pin QFN with an exposed backside

pad, a 28-pin thin QFN, or a 16-pin QSOP.

Applications

DDR Memory Termination

Active Termination Buses

Features

♦ Source/Sink 3A

♦ ±1% Output Accuracy

♦ Up to 1MHz Switching Frequency

♦ 93% Efficiency

♦ Internal PMOS/NMOS Switches

90mΩ/70mΩ On-Resistance at V

= 4.5V

110mΩ/80mΩ On-Resistance at V

= 3V

♦ 1.1V to V

Adjustable Output Voltage

♦ 3V to 5.5V Input Voltage Range

♦ <1µA Shutdown Supply Current

♦ Programmable Constant-Off-Time Operation

♦ Thermal Shutdown

♦ Adjustable Soft-Start Inrush Current Limiting

♦ Output Short-Circuit Protection

MAX1809

3A, 1MHz, DDR Memory Termination Supply

________________________________________________________________ Maxim Integrated Products 1

N.C.

EXTREF

PGND

GND

N.C.

REF

GND

N.C.

TOFF

TOP VIEW

THIN QFN

SHDN

MAX1809

Pin Configurations

Ordering Information

MAX1809

OUT

SET

SHDN

EXTREF

TOFF

PGND

GND

REF

Typical Operating Circuit

19-2142; Rev 1; 9/02

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.

PART TEMP RANGE PIN-PACKAGE

MAX1809EGI* -40°C to +85°C 28 QFN

MAX1809EEE -40°C to +85°C 16 QSOP

MAX1809ETI -40°C to +85°C 28 Thin QFN

Pin Configurations continued at end of data sheet.

*Contact factory for availability.

Summary of content (17 pages)

PAGE 1
9-2142; Rev 1; 9/02 3A, 1MHz, DDR Memory Termination Supply The MAX1809 uses a unique current-mode, constantoff-time, PWM control scheme that allows the output to source or sink current. This feature allows energy to return to the input power supply that otherwise would be wasted. The programmable constant-off-time architecture sets switching frequencies up to 1MHz, allowing the user to optimize performance trade-offs between efficiency, output switching noise, component size, and cost.
PAGE 2
MAX1809 3A, 1MHz, DDR Memory Termination Supply ABSOLUTE MAXIMUM RATINGS VCC, IN to GND ........................................................-0.3V to +6V IN to VCC .............................................................................±0.3V GND to PGND.....................................................................±0.3V SHDN, SS, FB, TOFF, RREF, EXTREF to GND.......................................-0.3V to (VCC + 0.3V) LX Current (Note 1)........................................................
PAGE 3
A, 1MHz, DDR Memory Termination Supply (VIN = VCC = 3.3V, VEXTREF = 1.1V, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL SS Source Current ISS SS Sink Current ISS SHDN Input Current CONDITIONS MIN TYP MAX UNITS 4 5 6 µA VSS = 1V 1 50 V SHDN = 0, VCC -1 VIL SHDN Logic Levels 0.8 VIH Maximum Output RMS Current mA +1 2 IOUT(RMS) µA V 3.1 ARMS MAX UNITS 3.0 5.5 V -24 +24 mV VREF 0.01V VIN 1.9V V 1.067 1.
PAGE 4
Typical Operating Characteristics (Circuit of Figure 1, VOUT = 1.25V, for VIN = 5V: L = 1µH, RTOFF = 130kΩ; for VIN = 3.3V: L = 0.68µH, RTOFF = 73.2kΩ.) RDROOP = 0Ω 85 VIN = 5V, VOUT = 2.5V 75 70 VIN = 3.3V, VOUT = 1.25V 65 55 75 VIN = 5V, VOUT = 1.25V 70 65 VIN = 3.3V, VOUT = 1.25V 60 VIN = 5V, VOUT = 1.25V 60 80 -0.2 1 2 OUTPUT CURRENT (A) 3 0 1 2 OUTPUT CURRENT (A) NO-LOAD SUPPLY CURRENT vs. SUPPLY VOLTAGE 5.0 4.5 4.0 3.0 2.5 1.5 1.0 4 -1 0 2 3 1200 VIN = 3.
PAGE 5
3A, 1MHz, DDR Memory Termination Supply LINE-TRANSIENT RESPONSE MAX1809 toc10 RDROOP 12mΩ MAX1809 toc09 LOAD-TRANSIENT RESPONSE VOUT (AC-COUPLED) 50mV/div VOUT (AC-COUPLED) 50mV/div VIN 2V/div V(LX) 5V/div 0V IOUT 5A/div 0A 0V 20µs/div 10µs/div VEXTREF = 1.25V, VIN = 3.3V, IOUT = -2A to +2A to -2A IOUT = 2A, VIN = 5V to 3.
PAGE 6
3A, 1MHz, DDR Memory Termination Supply MAX1809 Pin Description PIN (QFN) PIN (QSOP) NAME FUNCTION 1, 5, 10, 11, 12, 22, 24, 26, 28 — N.C. 2, 4 2, 4 IN Supply Voltage Input for the Internal PMOS Power Switch. Not internally connected. Externally connect all pins for proper operation. No Connection. Not internally connected. 3, 18, 19, 23, 25 3, 14, 16 LX Inductor Connection. Connection for the drains of the PMOS power switch and NMOS synchronous-rectifier switch.
PAGE 7
3A, 1MHz, DDR Memory Termination Supply IN 33µF LX MAX1809 10Ω VCC 2.2µF SHDN VSET VDDQ (2.5V) EXTREF VOUT = MAX1809 RDROOP L VIN (VDDQ 2 ) 270µF 2V 15mΩ PGND GND FB REF 1µF 10kΩ 1000pF TOFF SS 0.01µF RTOFF 10kΩ 1000pF FOR VIN = 5V: L = 1µH, RTOFF = 130kΩ FOR VIN = 3.3V: L = 0.68µH, RTOFF = 73.2kΩ VSSQ Figure 1. Typical Application Circuit feedback voltage exceeds the external reference voltage (VEXTREF) or the positive current limit is reached.
PAGE 8
MAX1809 3A, 1MHz, DDR Memory Termination Supply Setting the Output Voltage where C = 2.5nF and fSW is the switching frequency. Resistive losses in the two power switches are approximated by: The output voltage of the MAX1809 is set by an external voltage applied to the EXTREF pin. This can come directly from another voltage source or external reference. PD(RES) = IOUT2 ✕ RPMOS where RPMOS is the on-resistance of the PMOS switch.
PAGE 9
3A, 1MHz, DDR Memory Termination Supply MAX1809 VIN 1400 ISOURCE VOUT OPERATING FREQUENCY (kHz) 1200 1000 VOUT = 2.5V 800 600 VOUT = 1.25V 400 200 SYNCHRONOUS BUCK MODE (SOURCING CURRENT) 0 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 VIN (V) VIN Figure 4. Maximum Recommended Operating Frequency vs.
PAGE 10
MAX1809 3A, 1MHz, DDR Memory Termination Supply to maximum DC load current. A higher value of LIR allows smaller inductance but results in higher losses and ripple. A good compromise between size and losses is found at approximately a 25% ripple current to load current ratio (LIR = 0.25). L = (VOUT × t OFF ) (ISOURCE − ISINK ) × LIR The peak inductor current at full load is calculated by: IPEAK = IOUT + (VOUT × tOFF ) 2×L where IOUT is the maximum source or sink current.
PAGE 11
3A, 1MHz, DDR Memory Termination Supply MAX1809 VOUT = VEXTREF LX VOUT LX VDDQ MAX1809 MAX1809 EXTREF VEXTREF (1.1V ≤ VEXTREF ≤ VIN - 1.7V) EXTREF FB R2 FB R1 R2 = R1[(VOUT / VEXTREF) - 1] Figure 5. Adjusting the Output Voltage Using EXTREF Figure 6. Adjusting the Output Voltage at FB The output current limit during soft-start varies with the voltage on the soft-start pin, SS, according to the equation: careful component placement, and correct routing of traces using appropriate trace widths.
PAGE 12
MAX1809 3A, 1MHz, DDR Memory Termination Supply output regulates at a slightly lower voltage under a given load, allowing more voltage headroom as the load changes suddenly to zero or to the opposite polarity (sinking mode). By utilizing the full-voltage tolerance limits, the total output capacitance can be reduced and the capacitor’s ESR can be increased. Choose RDROOP such that the output voltage at the maximum load current, including ripple, is just above the lower limit of the output tolerance.
PAGE 13
3A, 1MHz, DDR Memory Termination Supply L IN MAX1809 INPUT VOLTAGE (3V TO 5.5V) VTT = VDDQ/2 LX CIN FB COUT MAX1809 10Ω SHDN VCC (a) PGND 2.2µF 2N7002 GND SHDN VDDQ (2.5V) SHDN (b) 10kΩ 0.1µF 10kΩ 0.1µF EXTREF VSSQ Figure 9. Discharging the Output of the MAX1809 in Shutdown Pin Configurations (continued) INPUT VOLTAGE (3V TO 5.5V) L IN LX CIN VTT =VDDQ/2 FB 10Ω 100Ω VCC MAX1809 COUT TOP VIEW IN 2 15 PGND LX 3 14 LX PGND IN 4 2.
PAGE 14
before startup, a 100Ω resistor should be added in the feedback path, and a diode from FB to SS as shown in Figure 10. SS will keep FB low during the startup sequence, ensuring that the MAX1809 enters into PWM mode and begins sinking current. See the Soft-Start Sink Current specification in the Electrical Characteristics for resistor selection. Chip Information TRANSISTOR COUNT: 3662 Package Information (The package drawing(s) in this data sheet may not reflect the most current specifications.
PAGE 15
3A, 1MHz, DDR Memory Termination Supply ______________________________________________________________________________________ 15 MAX1809 Package Information (continued) (The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.
PAGE 16
Package Information (continued) (The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.) QSOP.
PAGE 17
3A, 1MHz, DDR Memory Termination Supply b CL 0.10 M C A B D2/2 D/2 PIN # 1 I.D. QFN THIN 5x5x0.8 .EPS D2 0.15 C A D k 0.15 C B PIN # 1 I.D. 0.35x45 E/2 E2/2 CL (NE-1) X e E E2 k L DETAIL A e (ND-1) X e CL CL L L e e 0.10 C A C 0.08 C A1 A3 PROPRIETARY INFORMATION TITLE: PACKAGE OUTLINE 16, 20, 28, 32L, QFN THIN, 5x5x0.8 mm APPROVAL COMMON DIMENSIONS DOCUMENT CONTROL NO. REV. 21-0140 C 1 2 EXPOSED PAD VARIATIONS NOTES: 1. DIMENSIONING & TOLERANCING CONFORM TO ASME Y14.