Data Sheet

ADP1763

Rev. 0 | Page 11 of 18

THEORY OF OPERATION

The ADP1763 is an LDO, low noise linear regulator that uses an

advanced proprietary architecture to achieve high efficiency

regulation. It also provides high PSRR and excellent line and load

transient response using a small 10 F ceramic output capacitor.

The device operates from a 1.10 V to 1.98 V input rail to

provide up to 3 A of output current. Supply current in

shutdown mode is typically 2 μA.

SS BLOCK

REFCAP

SS

PG

SHORT-CIRCUIT,

THERMAL PROJECT

INTERNAL

BIAS SUPPLY

ADP1763

VIN

VREG

EN

GND

REFERENCE,

BIAS

VOUT

SENSE

Figure 23. Functional Block Diagram, Fixed Output

SS BLOCK

SHORT-CIRCUIT,

THERMAL PROJECT

INTERNAL

BIAS SUPPLY

REFCAP

SS

PG

VIN

VREG

VADJ

EN

GND

VOUT

SENSE

ADP1763

Figure 24. Functional Block Diagram, Adjustable Output

Internally, the ADP1763 consists of a reference, an error amplifier,

and a pass device. The output current is delivered via the pass

device, which is controlled by the error amplifier, forming a

negative feedback system that ideally drives the feedback voltage

to equal the reference voltage. If the feedback voltage is lower

than the reference voltage, the negative feedback drives more

current, increasing the output voltage. If the feedback voltage is

higher than the reference voltage, the negative feedback drives

less current, decreasing the output voltage.

The ADP1763 is available in output voltages ranging from 0.9 V to

1.5 V for a fixed output. Contact a local Analog Devices, Inc.,

sales representative for other fixed voltage options. The adjustable

output option can be set from 0.5 V to 1.5 V. The ADP1763 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, tie EN to VIN.

SOFT START FUNCTION

For applications that require a controlled startup, the ADP1763

provides 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. At startup, a 10 μA

current source charges this capacitor. The voltage at SS limits

the ADP1763 start-up output voltage, providing a smooth ramp-up

to the nominal output voltage. To calculate the start-up time for

the fixed output and adjustable output, use the following equations:

(1)

(2)

where:

TIME (ms)

–0.1

0.1

0.3

0.5

0.7

0.9

1.1

1.3

1.5

1.7

–0.2

0.3

0.8

1.3

1.8

EN

–0.5

0

0.5

1.0

1.5

2.0

–0.2

0.3

0.8

1.3

1.8

TIME (ms)

EN

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