ADP1715/ADP1716

Rev. 0 | Page 10 of 20

THEORY OF OPERATION

The ADP1715/ADP1716 are low dropout, CMOS linear

regulators that use an advanced, proprietary architecture to

provide high power supply rejection ratio (PSRR) and excellent

line and load transient response with just a small 2.2 μF ceramic

output capacitor. Both devices operate from a 2.5 V to 5.5 V

input rail and provide up to 500 mA of output current. Supply

current in shutdown mode is typically 100 nA.

SOFT

START

REFERENCE

CURRENT LIMIT

THERMAL PROTECT

SHUTDOWN

GND

OUT

SS/

ADJ/

TRK

IN

EN

Figure 26. Internal Block Diagram

Internally, the ADP1715/ADP1716 consist 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 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 ADP1715 is available in two versions, one with fixed output

voltage options and one with an adjustable output voltage. The

fixed output voltage options are set internally to one of sixteen

values between 0.75 V and 3.3 V, using an internal feedback

network. The adjustable output voltage can be set to between

0.8 V and 5.0 V by an external voltage divider connected from

OUT to ADJ. The fixed output version of ADP1715 allows for

connection of an external soft-start capacitor, which controls

the output voltage ramp during startup. The ADP1716 features

a track pin and is available with fixed output voltage options. All

devices are controlled by an enable pin (EN).

SOFT-START FUNCTION (ADP1715)

For applications that require a controlled startup, the ADP1715

provides a programmable soft-start function. 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 1.2 μA current source charges this capacitor. The

ADP1715 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 by

(1)

where:

When the ADP1715 is disabled (using EN), the soft-start capacitor

is discharged to GND through an internal 100 Ω resistor.

2

1

EN

OUT

TIME (4ms/DIV)

Figure 27. OUT Ramp-Up with External Soft-Start Capacitor

The ADP1715 adjustable version and the ADP1716 have no

pins for soft start, so the function is switched to an internal soft-

start capacitor. This sets the soft-start ramp-up period to

approximately 24 μs. For the worst-case output voltage of 5 V,

using the suggested 2.2 μF output capacitor, the resulting input

inrush current is approximately 460 mA, which is less than the

maximum 500 mA load current.

2

1

OUT

EN

TIME (20µs/DIV)

Figure 28. OUT Ramp-Up with Internal Soft-Start