19-2142; Rev 1; 9/02
3A, 1MHz, DDR Memory Termination Supply
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 VIN, 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
Typical Operating Circuit
VIN
IN
LX
MAX1809 PGND
VCC GND
SHDN
VSET
EXTREF
FB
REF
TOFF SS
VOUT
Features
o Source/Sink 3A
o ±1% Output Accuracy
o Up to 1MHz Switching Frequency
o 93% Efficiency
o Internal PMOS/NMOS Switches
90mΩ/70mΩ On-Resistance at VIN = 4.5V
110mΩ/80mΩ On-Resistance at VIN = 3V
o 1.1V to VIN Adjustable Output Voltage
o 3V to 5.5V Input Voltage Range
o <1µA Shutdown Supply Current
o Programmable Constant-Off-Time Operation
o Thermal Shutdown
o Adjustable Soft-Start Inrush Current Limiting
o Output Short-Circuit Protection
Ordering Information
PART
TEMP RANGE
MAX1809EGI*
-40°C to +85°C
MAX1809EEE
-40°C to +85°C
MAX1809ETI
-40°C to +85°C
*Contact factory for availability.
PIN-PACKAGE
28 QFN
16 QSOP
28 Thin QFN
Pin Configurations
TOP VIEW
N.C. 1
IN 2
LX 3
IN 4
N.C. 5
SS 6
EXTREF 7
MAX1809
21 PGND
20 PGND
19 LX
18 LX
17 PGND
16 VCC
15 GND
THIN QFN
Pin Configurations continued at end of data sheet.
________________________________________________________________ Maxim Integrated Products 1
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.
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