APW8821 datasheet(13/24 Pages) ANPEC

13 / 24 page

Rev. A.1 - Feb., 2011

APW8821

www.anpec.com.tw

Function Description

Constant-On-Time PWM Controller with Input Feed-For-

ward

Pulse-Frequency Modulation (PFM)

The constant-on-time control architecture is a pseudo-

fixed frequency with input voltage feed-forward. This ar-

chitecture relies on the output filter capacitor’s effective

series resistance (ESR) to act as a current-sense resis-

tor so the output ripple voltage provides the PWM ramp

signal. In PFM operation, the high-side switch on-time is

controlled by the on-time generator is determined solely

by a one-shot whose pulse width is inversely propor-

tional to the input voltage and directly proportional to the

output voltage. In PWM operation, the high-side switch

on-time is determined by a switching frequency control

circuit in the on-time generator block.

The switching frequency control circuit senses the switch-

ing frequency of the high-side switch and keeps regulat-

ing it at a constant frequency in PWM mode. The design

improves the frequency variation and is more outstand-

ing than a conventional constant-on-time controller, which

has large switching frequency variation over input voltage,

output current, and temperature. Both in PFM and PWM,

the on-time generator, which senses input voltage on

PHASE pin, provides very fast on-time response to input

line transients.

Another one-shot sets a minimum off-time (typical:

450ns). The on-time one-shot is triggered if the error com-

parator is high, the low-side switch current is below the

current-limit threshold, and the minimum off-time one-

shot has timed out.

In PFM mode, an automatic switchover to pulse-frequency

modulation (PFM) takes place at light loads. This

switchover is affected by a comparator that truncates the

low-side switch on-time at the inductor current zero

crossing. This mechanism causes the threshold between

PFM and PWM operation to coincide with the boundary

between continuous and discontinuous inductor-current

operation (also known as the critical conduction point).

The on-time of PFM is given by:

Where F

SW is the nominal switching frequency of the con-

verter in PWM mode.

The load current at handoff from PFM to PWM mode is

given by:

OUT

PFM

−

OUT

PFM

OUT

)

PFMtoPWM

(

LOAD

−

Power-On-Reset (POR)

A Power-On-Reset (POR) function is designed to prevent

wrong logic controls when the VCC voltage is low. The

POR function continually monitors the bias supply volt-

age on the VCC pin if at least one of the enable pins is set

high. When the rising VCC voltage reaches the rising

POR voltage threshold (4.35V, typical), the POR signal

goes high and the chip initiates soft-start operations.

When this voltage drops lower than 4.25V (typical), the

POR disables the chip.

REFIN Pin Control

The voltage (V

REFIN) applied to REFIN pin selects either

enable-shutdown or adjustable external reference. When

REFIN is above the EN high threshold (2.8V, typical), the

PWM is enabled. When V

REFIN is from 0.5V to 2.5V, the

output voltage can be programmed as same as V

REFIN

voltage. When V

REFIN is below the EN low threshold, the

chip is in the shutdown and only low leakage current is

taken from VCC. Once APW8821 has been operating at

internal mode, it is unable to transform into external mode.

On the other hand, it is able to transform into internal

mode. The slew rate of V

REFIN must be faster than 0.5V/µs

to avoid wrong output voltage.

APW8821 数据表(PDF) 13 Page - Anpec Electronics Coropration

APW8821 数据表(HTML) 13 Page - Anpec Electronics Coropration

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

数据表下载