ADP1706/ADP1707/ADP1708
THEORY OF OPERATION
providing a smooth ramp-up to the nominal output voltage.
The soft start time is calculated by
The ADP1706/ADP1707/ADP1708 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 a small 4.7 μF ceramic
output capacitor. All devices operate from a 2.5 V to 5.5 V input
rail and provide up to 1 A of output current. Supply current in
shutdown mode is typically 100 nA.
T
SS = VREF ×(CSS/ISS)
where:
SS is the soft start period.
REF is the 0.8 V reference voltage.
SS is the soft start capacitance from SS to GND.
SS is the current sourced from SS (1.2 μA).
(1)
T
V
C
I
When the ADP1706 is disabled (using EN), the soft start capacitor
is discharged to GND through an internal 100 Ω resistor.
IN
OUT
SENSE
CURRENT LIMIT
THERMAL PROTECT
EN
SHUTDOWN
ADJ/
TRK/
SS
2
EN
SOFT
START
REFERENCE
OUT
GND
V
V
= 5V
= 3.3V
IN
OUT
Figure 27. Internal Block Diagram
C
C
I
= 4.7μF
OUT
1
= 10nF
= 1A
SS
Internally, the ADP1706/ADP1707/ADP1708 consist of a
LOAD
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.
TIME (2ms/DIV)
Figure 28. OUT Ramp-Up with External Soft Start Capacitor
The ADP1707 and ADP1708 have no pins for soft start;
therefore, the function is switched to an internal soft start
capacitor, which sets the soft start ramp-up period to approxi-
mately 48 μs. Note that the ramp-up period is the time it takes
OUT to go from 0% to 90% of the nominal value and is
different from the start-up time in Table 1, which is the time
between the rising edge of EN to OUT being at 90% of the
nominal value. For the worst-case output voltage of 5 V, using
the suggested 4.7 μF output capacitor, the resulting input
inrush current is approximately 490 mA, which is less than
the maximum 1 A load current.
The ADP1706/ADP1707 are available in 16 fixed output voltage
options between 0.75 V and 3.3 V. The ADP1706 allows for
connection of an external soft start capacitor, which controls
the output voltage ramp during startup. The ADP1707 features
a TRK pin that allows the output voltage to follow the voltage at
this pin. The ADP1708 is available in an adjustable version with
an output voltage that can be set to between 0.8 V and 5.0 V by
an external voltage divider. All devices are controlled by an
enable pin (EN).
EN
2
SOFT START FUNCTION (ADP1706)
For applications that require a controlled startup, the ADP1706
provides a programmable soft start function. The programma-
ble soft start is useful for reducing inrush current upon startup
and for providing voltage sequencing. To implement a soft start,
connect a small ceramic capacitor from SS to GND. Upon
startup, a 1.2 μA current source charges this capacitor. The
ADP1706 start-up output voltage is limited by the voltage at SS,
V
V
= 5V
= 1.6V
IN
OUT
OUT
1
C
I
= 4.7μF
= 10mA
OUT
LOAD
TIME (20µs/DIV)
Figure 29. OUT Ramp-Up with Internal Soft Start
Rev. 0 | Page 10 of 20
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