Datasheet
Data Sheet ADP1754/ADP1755
Rev. F | Page 11 of 20
THEORY OF OPERATION
The ADP1754/ADP1755 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 capacitor. Both
devices operate from a 1.6 V to 3.6 V input rail and provide up
to 1.2 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
ADP1754
REVERSE POLARITY
PROTECTION
PG
DETECT
0.9µA
07722-021
Figure 24. ADP1754 Internal Block Diagram
UVLO
VOUTVIN
ADJ
SS
SHORT-CIRCUIT
AND THERMAL
PROTECTION
0.5V
REF
SHUTDOWN
EN
PG
GND
ADP1755
REVERSE POLARITY
PROTECTION
PG
DETECT
0.9µA
07722-022
Figure 25. ADP1755 Internal Block Diagram
Internally, the ADP1754/ADP1755 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
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 ADP1754 is available in seven fixed output voltage options
between 0.75 V and 2.5 V. The ADP1754 allows for connection
of an external soft start capacitor that controls the output
voltage ramp during startup. The ADP1755 is the adjustable
version with an output voltage that can be set to a value
between 0.75 V and 3.3 V by an external voltage divider. Both
devices are controlled by an enable pin (EN).
SOFT START FUNCTION (ADP1754/ADP1755)
For applications that require a controlled startup, the ADP1754/
ADP1755 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 ADP1754/ADP1755 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 ADP1754/ADP1755 is 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
07722-023
Figure 26. V
OUT
Ramp-Up with External Soft Start Capacitor