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ADP1754

1.2 A, Low VIN, Low Dropout Linear Regulator

Analog Devices 

Data Sheet

ADP1754/ADP1755

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.

ADP1754

VIN

UVLO

REVERSE POLARITY

PROTECTION

VOUT

GND

PG

EN

SHORT-CIRCUIT

AND THERMAL

PROTECTION

PG

DETECT

0.5V

REF

SHUTDOWN

SENSE

R1

R2

0.9µA

SS

VIN

GND

Figure 24. ADP1754 Internal Block Diagram

ADP1755

UVLO

REVERSE POLARITY

PROTECTION

VOUT

SHORT-CIRCUIT

AND THERMAL

PROTECTION

PG

0.5V

REF

PG

DETECT

EN

SHUTDOWN

ADJ

0.9µA

SS

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:

tSS = VREF × (CSS/ISS)

(1)

where:

tSS is the soft start period.

VREF is the 0.5 V reference voltage.

CSS is the soft start capacitance from SS to GND.

ISS is the current sourced from SS (0.9 µA).

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

2.25

EN

2.00

1.75

1nF

1.50

1.25

4.7nF

1.00

10nF

0.75

0.50

0.25

0

0

2

4

6

8

10

TIME (ms)

Figure 26. VOUT Ramp-Up with External Soft Start Capacitor

Rev. F | Page 11 of 20

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