Datasheet

ADP2108 Data Sheet
Rev. G | Page 12 of 20
SHORT-CIRCUIT PROTECTION
The ADP2108 includes frequency foldback to prevent output
current runaway on a hard short. When the voltage at the
feedback pin falls below half the target output voltage, indicat-
ing the possibility of a hard short at the output, the switching
frequency is reduced to half the internal oscillator frequency.
The reduction in the switching frequency allows more time for
the inductor to discharge, preventing a runaway of output current.
UNDERVOLTAGE LOCKOUT
To protect against battery discharge, undervoltage lockout
(UVLO) circuitry is integrated on the ADP2108. If the input
voltage drops below the 2.15 V UVLO threshold, the ADP2108
shuts down, and both the power switch and the synchronous
rectifier turn off. When the voltage rises above the UVLO thresh-
old, the soft start period is initiated, and the part is enabled.
THERMAL PROTECTION
In the event that the ADP2108 junction temperature rises above
150°C, the thermal shutdown circuit turns off the converter.
Extreme junction temperatures can be the result of high current
operation, poor circuit board design, or high ambient temperature.
A 20°C hysteresis is included so that when thermal shutdown
occurs, the ADP2108 does not return to operation until the
on-chip temperature drops below 130°C. When coming out
of thermal shutdown, soft start is initiated.
SOFT START
The ADP2108 has an internal soft start function that ramps the
output voltage in a controlled manner upon startup, thereby
limiting the inrush current. This prevents possible input voltage
drops when a battery or a high impedance power source is
connected to the input of the converter.
After the EN pin is driven high, internal circuits start to power up.
The time required to settle after the EN pin is driven high is called
the power-up time. After the internal circuits are powered up, the
soft start ramp is initiated and the output capacitor is charged
linearly until the output voltage is in regulation. The time required
for the output voltage to ramp is called the soft start time.
Start-up time in the ADP2108 is the measure of when the
output is in regulation after the EN pin is driven high. Start-up
time consists of the power-up time and the soft start time.
CURRENT LIMIT
The ADP2108 has protection circuitry to limit the amount of
positive current flowing through the PFET switch and the
synchronous rectifier. The positive current limit on the power
switch limits the amount of current that can flow from the input
to the output. The negative current limit prevents the inductor
current from reversing direction and flowing out of the load.
100% DUTY OPERATION
With a drop in V
IN
or with an increase in I
LOAD
, the ADP2108
reaches a limit where, even with the PFET switch on 100% of
the time, V
OUT
drops below the desired output voltage. At this
limit, the ADP2108 smoothly transitions to a mode where the
PFET switch stays on 100% of the time. When the input conditions
change again and the required duty cycle falls, the ADP2108
immediately restarts PWM regulation without allowing over-
shoot on V
OUT
.