APW7120
Functional Description
Pleasepayattention totheRCdelayeffect.Itcauses
the OCP trip level to be the function of the
operating duty.The parasitic capacitance (including
thecapacitance inside the OCSET, external PCB trace
capacitance and the COSS of the shutdown MOSFET)
must be minimized, especially selecting a shutdown
MOSFET with very small COSS. The OCP trip level
follows the duty to increase a little at low operating
duty, but very much at high operating duty, like the
RCdelaycurve. Dueto load regulation or current-limit,
heavyloadnormallyreducesconverter’sinput voltage
and increases the power loses. During heavy load, the
APW7120 regulates the output voltage by expending
the duty. This rises up the OCP trip level at the same
time.
Power-On-Reset (POR)
The APW7120 monitors the VCC voltage (VCC) for
Power-On-Reset function, preventing wrong logic
operation during powering on. When the VCC voltage
is ready, the APW7120 starts a start-up process and
then rampsthe output voltage up to the target voltage.
Soft-Start
TheAPW7120 has a built-in digital soft-start to control
the output voltage rise and limit the current surge at
thestart-up.Duringsoft-start,aninternalrampconnected
to the one of the positive inputs of the Gm amplifier
rises up from 0V to 2V to replace the reference voltage
(0.8V) until the ramp voltage reaches the reference
voltage. The soft-start interval is about 3.2ms typical,
independent of the converter’s input and output
voltages.
Under-Voltage Protection (UVP)
The under-voltage function monitors the FB voltage
(VFB) to protect the converter against short-circuit
conditions. When the VFB falls below the falling UVP
threshold (67% VREF), the APW7120 shuts off the
converter. After a preceding delay, which starts at
the beginning of the under-voltage shutdown, the
APW7120 initiates a new soft-start to resume
regulating. The under-voltage protection shuts off
and then re-starts the converter repeatedly without
latching. The function is disabled during soft-start
process.
Over-Current Protection(OCP)
The over-current function protects the switching
converter against over-current or short-circuit
conditions. The controller senses the inductor current
by detecting the drain-to-source voltage, product of
the inductor’s current and the on-resistance, of the
low-side MOSFET during it’s on-state. This method
enhances the converter’s efficiency and reduces cost
by eliminating a current sensing resistor.
A resistor (ROCSET), connected from the OCSET to the
low-side MOSFET’s drain, programs the over-current
trip level. An internal 40mA (typical) current source
Over-Voltage Protection (OVP)
Theover-voltageprotection monitorsthe FBvoltage to
prevent theoutputfromover-voltage.Whentheoutputvolt-
age rises to 118% of the nominal output voltage,
the APW7120 turns on the low-side MOSFET until
the output voltage falls below the OVP threshold,
regulating the output voltage around the OVP
thresholds.
flowing throughtheROCSET developsavoltage(VROCSET
)
across the ROCSET. When the VOCSET (VROCSET+ VDS of
the low-side MOSFET) is less than the internal over-
current reference voltage (0.4V, typical), the IC shuts
off the converter and then initiates a new soft-start
process. After 4 over-current events are counted, the
deviceturnsoff both high-sideandlow-side MOSFETs
and the converter’s output is latched to be floating.
Copyright ã ANPEC Electronics Corp.
12
www.anpec.com.tw
Rev. A.4 - Jan., 2006
ANPEC [ ANPEC ELECTRONICS COROPRATION ]
