ADP172

Rev. B | Page 11 of 20

THEORY OF OPERATION

The ADP172 is a low quiescent current, low-dropout linear

regulator that operates from 1.6 V to 3.6 V and can provide up to

300 mA of output current. Drawing a low 170 μA of quiescent

current (typical) at full load makes the ADP172 ideal for battery-

operated portable equipment. Shutdown current consumption is

typically 100 nA.

Optimized for use with small 1 μF ceramic capacitors, the

ADP172 provides excellent transient performance.

SHORT CIRCUIT,

UVLO AND

THERMAL

PROTECT

0.5V REFERENCE

SHUTDOWN

VIN

VOUT

R1

R2

GND

EN

ADP172

Figure 23. ADP172 Internal Block Diagram

Internally, the ADP172 consists of a reference, an error

amplifier, a feedback voltage divider, and a PMOS pass transistor.

Output current is delivered via the PMOS pass device, which is

controlled by the error amplifier. The error amplifier compares

the reference voltage with the feedback voltage from the output

and amplifies the difference. If the feedback voltage is lower than

the reference voltage, the gate of the PMOS device is pulled lower,

allowing more current to pass and increasing the output voltage.

If the feedback voltage is higher than the reference voltage, the

gate of the PMOS device is pulled higher, allowing less current

to pass and decreasing the output voltage.

The ADP172 uses the EN pin to enable and disable the VOUT

pin under normal operating conditions. When EN is high, VOUT

turns on; when EN is low, VOUT turns off. For automatic

startup, EN can be tied to VIN.