Low-Output-Voltage, 800mA, PWM Step-Down

DC-DC Converters

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7

Pin Description

PIN

NAME

FUNCTION

1

PWM

Forced-PWM Input. Drive to GND to use PWM at medium to heavy loads and pulse-skipping at light loads.

Drive to BATT to force PWM operation at all loads.

2

GND

Ground

3

REF

Internal 1.25V Reference. Bypass to GND with a 0.1µF capacitor.

4FB

Output Feedback Sense Input. To set the output voltage to the preset voltage (MAX1928), connect FB directly

to the output. To adjust the output voltage (MAX1927R), connect FB to the center of an external resistor-

divider between the output and GND. FB regulation voltage is 0.75V.

5

COMP

Compensation Input. See the Compensation, Stability, and Output Capacitor section for compensation

component selection.

6

SHDN

Shutdown Control Input. Drive low to shut down the converter. Drive high for normal operation.

7

PGND

Power Ground

8

LX

Inductor Connection to the drains of the internal power MOSFETs.

9

BATT

Supply Voltage Input. Connect to a 2.6V to 5.5V source. Bypass to GND with a low-ESR 10µF capacitor.

10

POK

Power-OK Open-Drain Output. Once the soft-start routine has completed, POK goes high impedance 20ms

after FB exceeds 90% of its expected final value.

N

N

P

P

SLOPE

COMPENSATION

POWER-OK

CONTROL

1MHz

OSC

BIAS

COMP

BATT

PWM

CONTROL

1.25V

REFERENCE

PFM CURRENT

COMPARATOR

PWM

COMPARATOR

PWM

ILIM

COMPARATOR

TO

COMP

N-CHANNEL

CURRENT COMPARATOR

PGND

POK

FB

REF

MAX1927R

ONLY

GND

MAX1928

ONLY

LX

SHDN

MAX1927

MAX1928

Figure 1. Simplified Functional Diagram