ADP1715 Analog Devices, ADP1715 Datasheet - Page 10

• Manufacturer Part Number: ADP1715
• Description: 500 mA Low-Dropout CMOS Linear Regulator with Soft Start
• Manufacturer: Analog Devices

ADP1715/ADP1716
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.
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,
EN
IN
THERMAL PROTECT
SHUTDOWN
CURRENT LIMIT
REFERENCE
Figure 26. Internal Block Diagram
GND
START
SOFT
OUT
SS/
ADJ/
TRK
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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
where:
T
V
C
I
When the ADP1715 is disabled (using EN), the soft-start capacitor
is discharged to GND through an internal 100 Ω resistor.
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.
SS
SS
SS
REF
is the current sourced from SS (1.2 μA).
is the soft-start period.
is the soft-start capacitance from SS to GND.
T
is the 0.8 V reference voltage.
SS
Figure 27. OUT Ramp-Up with External Soft-Start Capacitor
1
1
2
2
= V
REF
Figure 28. OUT Ramp-Up with Internal Soft-Start
× (C
OUT
EN
OUT
EN
SS
/I
SS
)
TIME (20µs/DIV)
TIME (4ms/DIV)
V
V
C
C
I
LOAD
V
V
C
I
LOAD
IN
OUT
OUT
SS
IN
OUT
OUT
= 5V
= 22nF
= 5V
= 3.3V
= 2.2µF
= 500mA
= 1.6V
= 2.2µF
= 10mA
(1)