ADP121

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THEORY OF OPERATION

The ADP121 is a low quiescent current, low dropout linear

regulators that operate from 2.3 V to 5.5 V and provide up

to 150 mA of output current. Drawing a low 30 μA quiescent

current (typical) at full load makes the ADP121 ideal for battery-

operated portable equipment. Shutdown current consumption

is typically 100 nA.

Optimized for use with small 1 μF ceramic capacitors,

the ADP121 provides excellent transient performance.

0.8V REFERENCE

SHORT CIRCUIT,

UVLO, AND

THERMAL

PROTECT

SHUTDOWN

R1

R2

VOUT

VIN

GND

EN

Figure 27. Internal Block Diagram

Internally, the ADP121 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 con-

trolled 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 flow 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 flow and decreasing the output voltage.

The ADP121 is available in 16 output voltage options ranging

from 1.2 V to 3.3 V. The ADP121 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.