3A, 1MHz, DDR Memory Termination Supply
MAX1809
Pin Description
PIN (QFN)
1, 5, 10, 11, 12,
22, 24, 26, 28
2, 4
PIN (QSOP)
—
2, 4
NAME
N.C.
IN
FUNCTION
No Connection. Not internally connected.
Supply Voltage Input for the Internal PMOS Power Switch. Not internally
connected. Externally connect all pins for proper operation.
Inductor Connection. Connection for the drains of the PMOS power switch
and NMOS synchronous-rectifier switch. Connect the inductor from this
node to the output filter capacitor and load. Not internally connected.
Externally connect all pins for proper operation.
Soft-Start. Connect a capacitor from SS to GND to limit inrush current
during startup.
External Reference Input. Feedback input regulates to V
EXTREF
. The PWM
controller remains off until EXTREF is greater than REF.
Off-Time Select Input. Sets the PMOS power switch constant-off-time.
Connect a resistor from TOFF to GND to adjust the PMOS switch off-time.
Feedback Input. Connect directly to output for fixed-voltage operation or to
a resistive-divider for adjustable operating modes.
Analog Ground. Connect exposed backside pad and corner tabs to analog
GND.
Reference Output. Bypass REF to GND with a 1µF capacitor.
Tie to GND (pin 13 QFN; pin 9 QSOP)
Analog Supply Voltage Input. Supplies internal analog circuitry. Bypass
V
CC
with a 10Ω and 2.2µF low-pass filter (see Figure 1).
Power Ground. Internally connected to the internal NMOS synchronous-
rectifier switch.
Shutdown Control Input. Drive
SHDN
low to disable the reference, control
circuitry, and internal MOSFETs. Drive high or connect to V
CC
for normal
operation.
3, 18, 19, 23, 25
3, 14, 16
LX
6
7
8
9
13, backside pad,
corner tabs
14
15
16
17, 20, 21
5
6
7
8
9
10
11
12
13, 15
SS
EXTREF
TOFF
FB
GND
REF
GND
V
CC
PGND
27
1
SHDN
Detailed Description
The MAX1809 synchronous, current-mode, constant-
off-time, PWM DC-DC converter steps down input volt-
ages of 3V to 5.5V to an adjustable output voltage from
1.1V to V
IN
, as set by the voltage applied at EXTREF. It
sources and sinks up to 3A of output current. Internal
switches composed of a 90mΩ PMOS power switch
and a 70mΩ NMOS synchronous-rectifier switch
improve efficiency, reduce component count, and elim-
inate the need for an external Schottky diode across
the synchronous switch.
The MAX1809 operates in a constant-off-time mode
under all loads. A single resistor-programmable con-
stant-off-time control sets switching frequencies up to
1MHz, allowing the user to optimize performance trade-
6
offs in efficiency, switching noise, component size, and
cost.
When power is drawn from a regulated supply, con-
stant-off-time PWM architecture essentially provides
constant-frequency operation. This architecture has the
inherent advantage of quick response to line and load
transients. The MAX1809’s current-mode, constant-off-
time PWM architecture regulates the output voltage by
changing the PMOS switch on-time relative to the con-
stant off-time.
Constant-Off-Time Operation
In the constant-off-time architecture, the FB voltage
comparator turns the PMOS switch on at the end of
each off-time, keeping the device in continuous-con-
duction mode. The PMOS switch remains on until the
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