Datasheet

ADP2138/ADP2139 Data Sheet
Rev. C | Page 12 of 20
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
With a drop in V
IN
or with an increase in I
LOAD
, the ADP2138/
ADP2139 reach 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 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
without allowing overshoot on V
OUT
.
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.
09496-028
PWM/
PSM
CONTROL
I
LIMIT
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 34. ADP2139 Functional Block Diagram