Data Sheet

ADP1764

Rev. A | Page 13 of 20

THEORY OF OPERATION

The ADP1764 is a low dropout (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 22 µF ceramic output

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

to provide up to 4 A of output current. The supply current in

shutdown mode is less than 4 µA.

SS BLOCK

REFCAP

SS

PG

SHORT-CIRCUIT,

THERMAL

PROTECTION

INTERNAL

BIAS SUPPLY

ADP1764

VIN

VREG

EN

GND

REFERENCE,

BIAS

VOUT

SENSE

Figure 38. Functional Block Diagram, Fixed Output

SS BLOCK

SHORT-CIRCUIT,

THERMAL

PROTECTION

INTERNAL

BIAS SUPPLY

REFCAP

SS

PG

VIN

VREG

VADJ

EN

GND

VOUT

SENSE

ADP1764

Figure 39. Functional Block Diagram, Adjustable Output

Internally, the ADP1764 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 ADP1764 is available in output voltages ranging from 0.55 V

to 1.5 V for a fixed output. Contact your 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 ADP1764 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 ADP1764

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 ADP1764 start-up output voltage, providing a smooth ramp up

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

use the following equations:

(1)

(2)

where:

1.6

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

–1.0

2.5

TIME (ms)

–0.5

0

0.5

1.0

1.5

2.0

EN

1.8

1.7

1.6

1.5

1.4

1.3

1.2

1.1

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.1

0.2

0

–1.0

–0.5

0

0.5

1.5

1.0

2.0

2.5

TIME (ms)

EN

VOUT = 0.5V; CSS = 0nF

VOUT = 0.5V; CSS = 10nF

VOUT = 0.5V; CSS = 22nF

VOUT = 1.5V; CSS = 0nF

VOUT = 1.5V; CSS = 10nF

VOUT = 1.5V; CSS = 22nF