3A, 1MHz, DDR Memory Termination Supply

6

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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

sources and sinks up to 3A of output current. Internal

switches composed of a 90m

Ω PMOS power switch

and a 70m

Ω 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-

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

Pin Description

PIN (QFN)

PIN (QSOP)

NAME

FUNCTION

1, 5, 10, 11, 12,

22, 24, 26, 28

—

N.C.

No Connection. Not internally connected.

2, 4

2, 4

IN

Supply Voltage Input for the Internal PMOS Power Switch. Not internally

connected. Externally connect all pins for proper operation.

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. Connect the inductor from this

node to the output filter capacitor and load. Not internally connected.

Externally connect all pins for proper operation.

65

SS

Soft-Start. Connect a capacitor from SS to GND to limit inrush current

during startup.

7

6

EXTREF

controller remains off until EXTREF is greater than REF.

8

7

TOFF

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.

98

FB

Feedback Input. Connect directly to output for fixed-voltage operation or to

a resistive-divider for adjustable operating modes.

13, backside pad,

corner tabs

9

GND

Analog Ground. Connect exposed backside pad and corner tabs to analog

GND.

14

10

REF

Reference Output. Bypass REF to GND with a 1µF capacitor.

15

11

GND

Tie to GND (pin 13 QFN; pin 9 QSOP)

16

12

Analog Supply Voltage Input. Supplies internal analog circuitry. Bypass

Ω and 2.2µF low-pass filter (see Figure 1).

17, 20, 21

13, 15

PGND

Power Ground. Internally connected to the internal NMOS synchronous-

rectifier switch.

27

1

SHDN

Shutdown Control Input. Drive SHDN low to disable the reference, control

operation.