APW7137
Function Description
Main Control Loop
Over-Temperature Protection (OTP)
The APW7137 is a constant frequency and current-mode
switching regulator. In normal operation, the internal N-
channel power MOSFET is turned on each cycle when the
oscillator sets an internal RS latch, and then turned off
when an internal comparator (ICMP) resets the latch. The
peak inductor current at which ICMP resets the RS latch
is controlled by the voltage on the COMP node which is
the output of the error amplifier (EAMP). An external resis-
tive divider connected between VOUT and ground allows
the EAMP to receive an output feedback voltage VFB at FB
pin. When the load current increases, it causes a slightly
to decrease in VFB associated with the 1.23V reference,
which in turn, it causes the COMP voltage to increase
until the average inductor current matches the new load
current.
The over-temperature circuit limits the junction tempera-
ture of the APW7137. When the junction temperature ex-
ceeds 150oC, a thermal sensor turns off the power
MOSFET allowing the devices to cool. The thermal sen-
sor allows the converters to start a soft-start process and
regulates the output voltage again after the junction tem-
perature cools by 40oC. The OTP is designed with a 40oC
hysteresis to lower the average Junction Temperature
(TJ) during continuous thermal overload conditions in-
creasing the lifetime of the device.
Enable/Shutdown
Driving EN to the ground places the APW7137 in shut-
down mode. When in shutdown, the internal power
MOSFET turns off, all internal circuitry shuts down, and
the quiescent supply current reduces to 1mA maximum.
VIN Under-Voltage Lockout (UVLO)
The Under-Voltage Lockout (UVLO) circuit compares the
input voltage at VIN with the UVLO threshold to ensure
the input voltage is high enough for reliable operation.
The 100mV (typ) hysteresis prevents supply transients
from causing a restart. Once the input voltage exceeds
the UVLO rising threshold, startup begins. When the in-
put voltage falls below the UVLO falling threshold, the
controller turns off the converter.
Soft-Start
The APW7137 has a built-in soft-start to control the output
voltage rise during start-up. During soft-start, an internal
ramp voltage, connected to the one of the positive inputs
of the error amplifier, raises up to replace the reference
voltage (1.23V typical) until the ramp voltage reaches the
reference voltage.
Current-Limit Protection
TheAPW7137 monitors the inductor current, flows through
the N-channel MOSFET, and limits the current peak at
current-limit level to prevent loads and the APW7137 from
damaging during overload or short-circuit conditions.
Copyright ã ANPEC Electronics Corp.
12
www.anpec.com.tw
Rev. A.4 - Oct., 2008
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
