Datasheet
Data Sheet ADP1740/ADP1741
Rev. F | Page 11 of 20
THEORY OF OPERATION
The ADP1740/ADP1741 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 capac-
itor. Both devices operate from a 1.6 V to 3.6 V input rail and
provide up to 2 A of output current. Supply current in shutdown
mode is typically 2 µA.
UVLO
VOUTVIN
SENSE
SS
SHORT-CIRCUIT
AND THERMAL
PROTECTION
R1
0.5V
REF
R2
SHUTDOWN
EN
PG
GND
ADP1740
REVERSE POLARITY
PROTECTION
PG
DETECT
0.9µA
07081-023
Figure 24. ADP1740 Internal Block Diagram
UVLO
VOUTVIN
ADJ
SS
SHORT-CIRCUIT
AND THERMAL
PROTECTION
0.5V
REF
SHUTDOWN
EN
PG
GND
ADP1741
REVERSE POLARITY
PROTECTION
PG
DETECT
0.9µA
07081-024
Figure 25. ADP1741 Internal Block Diagram
Internally, the ADP1740/ADP1741 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 feed-
back 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 ADP1740 is available in seven fixed output voltage options
from 0.75 V to 2.5 V. The ADP1740 allows for connection of an
external soft start capacitor, which controls the output voltage
ramp during startup. The ADP1741 is an adjustable version with
an output voltage that can be set to a value from 0.75 V to 3.3 V
by an external voltage divider. Both devices are controlled by an
enable pin (EN).
SOFT START FUNCTION
For applications that require a controlled startup, the ADP1740/
ADP1741 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 ADP1740/ADP1741 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:
t
SS
= V
REF
×
(C
SS
/I
SS
) (1)
where:
t
SS
is the soft start period.
V
REF
is the 0.5 V reference voltage.
C
SS
is the soft start capacitance from SS to GND.
I
SS
is the current sourced from SS (0.9 µA).
When the ADP1740/ADP1741 are disabled (using the EN pin),
the soft start capacitor is discharged to GND through an
internal 100 Ω resistor.
2.50
0
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
0
2 4 6 8 10
VOLTAGE (V)
TIME (ms)
EN
1nF
4.7nF
10nF
07081-025
Figure 26. V
OUT
Ramp-Up with External Soft Start Capacitor