Data Sheet

ADP1754/ADP1755

Rev. F | Page 11 of 20

THEORY OF OPERATION

The ADP1754/ADP1755 are low dropout linear regulators that

use an advanced, proprietary architecture to provide high power

supply rejection ratio (PSRR) and excellent line and load transient

response with only a small 4.7 µF ceramic output capacitor. Both

devices operate from a 1.6 V to 3.6 V input rail and provide up

to 1.2 A of output current. Supply current in shutdown mode is

typically 2 µA.

UVLO

VOUT

VIN

SENSE

SS

SHORT-CIRCUIT

AND THERMAL

PROTECTION

R1

0.5V

REF

R2

SHUTDOWN

EN

PG

GND

ADP1754

REVERSE POLARITY

PROTECTION

PG

DETECT

0.9µA

Figure 24. ADP1754 Internal Block Diagram

UVLO

VOUT

VIN

ADJ

SS

SHORT-CIRCUIT

AND THERMAL

PROTECTION

0.5V

REF

SHUTDOWN

EN

PG

GND

ADP1755

REVERSE POLARITY

PROTECTION

PG

DETECT

0.9µA

Figure 25. ADP1755 Internal Block Diagram

Internally, the ADP1754/ADP1755 consist of a reference, an

error amplifier, a feedback voltage divider, and a PMOS pass

transistor. Output current is delivered via the PMOS pass

transistor, 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 ADP1754 is available in seven fixed output voltage options

between 0.75 V and 2.5 V. The ADP1754 allows for connection

of an external soft start capacitor that controls the output

voltage ramp during startup. The ADP1755 is the adjustable

version with an output voltage that can be set to a value

between 0.75 V and 3.3 V by an external voltage divider. Both

devices are controlled by an enable pin (EN).

SOFT START FUNCTION (ADP1754/ADP1755)

For applications that require a controlled startup, the ADP1754/

ADP1755 provide a programmable soft start function. The

programmable soft start is useful for reducing inrush current

upon startup and for providing voltage sequencing. To implement

soft start, connect a small ceramic capacitor from SS to GND.

Upon startup, a 0.9 µA current source charges this capacitor.

The ADP1754/ADP1755 start-up output voltage is limited by

the voltage at SS, providing a smooth ramp-up to the nominal

output voltage. The soft start time is calculated as follows:

(1)

where:

When the ADP1754/ADP1755 is disabled (using the EN pin), the

soft start capacitor is discharged to GND through an internal 100 Ω

resistor.

2.50

0

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

0

2

4

6

8

10

TIME (ms)

EN

1nF

4.7nF

10nF