APW7085K View Datasheet(PDF) - Anpec Electronics
Part Name
Description
Manufacturer
APW7085K Datasheet PDF : 24 Pages
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APW7085
Function Description
Main Control Loop
The APW7085 is a constant frequency current mode
switching regulator. During normal operation, the inter-
nal P-channel power MOSFET is turned on each cycle
when the oscillator sets an internal RS latch and would
be turned off when an internal current comparator (ICMP)
resets the latch. The peak inductor current at which ICMP
resets the RS latch is controlled by the voltage on the
COMP pin, which is the output of the error amplifier
(EAMP). An external resistive divider connected between
V and ground allows the EAMP to receive an output
OUT
feedback voltage VFB at FB pin. When the load current
increases, it causes a slight decrease in VFB relative to
the 0.8V reference, which in turn causes the COMP volt-
age to increase until the average inductor current matches
the new load current.
VCC Power-On-Reset (POR) and EN Undervoltage
Lockout
The APW7085 keeps monitoring the voltage on VCC pin
to prevent wrong logic operations which may occur when
VCC voltage is not high enough for the internal control
circuitry to operate. The VCC POR has a rising threshold
of 3.9V (typical) with 0.15V of hysteresis.
An external undervoltage lockout (UVLO) is sensed and
programmed at the EN pin. The EN UVLO has a rising
threshold of 2.5V with 0.2V of hysteresis. The EN UVLO
should be programmed by connecting a resistive divider
from VIN to EN to GND.
After the VCC, EN and VIN-to-UGND voltages exceed their
respective voltage thresholds, the IC starts a start-up pro-
cess and then ramps up the output voltage to the setting
of output voltage. Connect a RC network from EN to GND
to set a turn-on delay that can be used to sequence the
output voltages of multiple devices.
VCC 4.2V Linear Regulator
VCC is the output terminal of the internal 4.2V linear regu-
lator which is powered from VIN and provides power to
the APW7085. The linear regulator designed to be stable
with a low-ESR ceramic output capacitor powers the in-
ternal control circuitry. Bypass VCC to GND with a ceramic
capacitor of at least 0.22µF. Place the capacitor physically
close to the IC to provide good noise decoupling. The
linear regulator is not intended for powering up any exter-
nal loads. Do not connect any external loads to VCC. The
linear regulator is also equipped with current-limit pro-
tection to protect itself during over-load or short-circuit
conditions on VCC pin.
VIN-to-UGND 5.5V Linear Regulator
The built-in 5.5V linear regulator regulates a 5.5V voltage
between VIN and UGND pins to supply bias and gate
charge for the P-channel Power MOSFET gate driver. The
linear regulator is designed to be stable with a low-ESR
ceramic output capacitor of at least 0.22µF. It is also
equipped with current-limit function to protect itself dur-
ing over-load or short-circuit conditions between VIN and
UGND.
The APW7085 shuts off the output of the converters when
the output voltage of the linear regulator is below 3.5V
(typical). The IC resumes working by initiating a new soft-
start process when the linear regulator’s output voltage
is above the undervoltage lockout voltage threshold.
Digital Soft-Start
The APW7085 has a built-in digital soft-start to control the
output voltage rise and limit the input current surge dur-
ing start-up. During soft-start, an internal ramp, connected
to the one of the positive inputs of the error amplifier,
rises up from 0V to 1V to replace the reference voltage
(0.8V) until the ramp voltage reaches the reference voltage.
The device is designed with a preceding delay about
10.8ms (typical) before soft-start process.
Output Under-Voltage Protection
In the process of operation, if a short-circuit occurs, the
output voltage will drop quickly. Before the current-limit
circuit responds, the output voltage will fall out of the re-
quired regulation range. The undervoltage continually
monitors the FB voltage after soft-start is completed. If a
load step is strong enough to pull the output voltage lower
than the under-voltage threshold, the IC shuts down
converter’s output.
The under-voltage threshold is 70% of the nominal out-
put voltage. The undervoltage comparator has a built-in
Copyright © ANPEC Electronics Corp.
15
Rev. A.7 - Sep., 2010
www.anpec.com.tw
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