ADP2108

Rev. A | Page 12 of 16

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

reaches a limit where, even with the PFET switch on 100% of

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-