APW7083 datasheet(13/22 Pages) ANPEC

13 / 22 page

Rev. A.5 - Sep., 2011

APW7083

www.anpec.com.tw

Function Description

Main Control Loop

The APW 7083 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

OUT and ground allows the EAMP to receive an output

feedback voltage V

at FB pin. When the load current

increases, it causes a slight decrease in V

FB 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 APW7083 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. Connecting a RC network from EN to

GND is for setting 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 APW7083. The linear regulator designed to be stable

with a low-ESR ceramic output capacitor powers the in-

ternal control circuitry, then bypasses VCC to GND with a

ceramic capacitor of at least 0.22

µF. In order to provide

good noise decoupling, please place the capacitor physi-

cally close to the IC. The linear regulator is not intended

for powering up any external loads. Do not connect any

external load to VCC. The linear regulator is also equipped

with current-limit protection 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 0.22

µF. It is also

equipped with current-limit function to protect itself dur-

ing over-load or short-circuit conditions between VIN and

UGND.

The APW7083 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 APW7083 has a built-in digital soft-start to control

the output voltage rise and limit the input current surge

during start-up. During soft-start, an internal ramp, con-

nected 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 refer-

ence voltage.

The device is designed with a preceding delay about

10.8ms (typical) before soft-start process.

Enable/Shutdown

Driving EN to ground places the APW7083 in shutdown.

When in shutdown, the internal power MOSFET turns off,

all internal circuitry shuts down and the quiescent supply

current of VIN reduces to <1

µA (typical).

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-

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