ADP1755 Analog Devices, ADP1755 Datasheet - Page 11

• Manufacturer Part Number: ADP1755
• Description: 1.2A Low-Vin, Adjustable-Vout LDO Linear Regulator
• Manufacturer: Analog Devices

GND
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.
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.
VIN
PG
EN
GND
VIN
PG
EN
ADP1755
ADP1754
SHORT-CIRCUIT
SHUTDOWN
AND THERMAL
PROTECTION
SHORT-CIRCUIT
SHUTDOWN
AND THERMAL
Figure 24. ADP1754 Internal Block Diagram
Figure 25. ADP1755 Internal Block Diagram
UVLO
PROTECTION
UVLO
DETECT
DETECT
PG
PG
0.5V
REF
REVERSE POLARITY
0.5V
REF
REVERSE POLARITY
PROTECTION
PROTECTION
0.9µA
0.9µA
R1
R2
VOUT
ADJ
SS
VOUT
SENSE
SS
Rev. B | Page 11 of 20
The ADP1754 are 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 3.0 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:
where:
t
V
C
I
When the ADP1754/ADP1755 is disabled (using the EN pin), the
soft start capacitor is discharged to GND through an internal 100 Ω
resistor.
SS
SS
SS
REF
is the soft start period.
is the current sourced from SS (0.9 μA).
is the soft start capacitance from SS to GND.
is the 0.5 V reference voltage.
t
SS
2.50
2.25
2.00
1.75
1.50
1.25
1.00
0.75
0.50
0.25
= V
Figure 26. V
0
0
REF
× (C
OUT
SS
2
/I
Ramp-Up with External Soft Start Capacitor
EN
1nF
4.7nF
SS
)
4
TIME (ms)
10nF
ADP1754/ADP1755
6
8
10
(1)