ADP2138/ADP2139

Rev. 0 | Page 12 of 16

SHORT-CIRCUIT PROTECTION

The ADP2138/ADP2139 include frequency fold back to prevent

output current runaway on a hard short. When the voltage at

the feedback pin falls below half the target output voltage, indi-

cating 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 ADP2138/ADP2139. If

the input voltage drops below the 2.15 V UVLO threshold, the

ADP2138/ADP2139 shut down, and both the power switch and

the synchronous rectifier turn off. When the voltage rises above

the UVLO threshold, the soft start period is initiated, and the

part is enabled.

THERMAL PROTECTION

In the event that the ADP2138/ADP2139 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 ADP2138/ADP2139 do 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 ADP2138/ADP2139 have 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 begin to power

up. Start-up time in the ADP2138/ADP2139 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

Each ADP2138/ADP2139 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

ADP2139 reach a limit where, even with the PFET switch on

At this limit, the ADP2138/ADP2139 smoothly transition 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 ADP2138/ADP2139 immediately restart PWM regulation

DISCHARGE SWITCH

The ADP2139 has an integrated switched resistor (of typically

100 Ω) to discharge the output capacitor when the EN pin goes

low or when the device enters undervoltage lockout or thermal

shutdown. The time to discharge is typically 200 μs.

PWM/

PSM

CONTROL

LOW

CURRENT

PSM

COMP

SOFT START

UNDER-VOLTAGE

LOCK OUT

THERMAL

SHUTDOWN

DRIVER

AND

ANTISHOOT

THROUGH

OSCILLATOR

PWM

COMP

GM ERROR

AMP

ADP2139

VOUT

MODE

GND

EN

SW

VIN

Figure 33. ADP2139 Functional Block Diagram