APW8713 datasheet(14/24 Pages) ANPEC

14 / 24 page

Rev. A.3 - Sep., 2013

APW8713

www.anpec.com.tw

Function Description

Constant-On-Time PWM Controller with Input Feed-

Forward

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 resistor,

so the output ripple voltage provides the PWM ramp signal.

In PFM operation, the high-side switch on-time controlled

by the on-time generator is determined solely by a one-

shot whose pulse width is inversely proportional to input

voltage and directly proportional to output voltage. In PWM

operation, the high-side switch on-time is determined by

a switching frequency control circuit in the on-time gen-

erator 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

VIN pin, provides very fast on-time response to input line

transients.

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

250ns). 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.

Over-Current Protection of the PWM Converter

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 FSW is the nominal switching frequency of the

converter in PWM mode.

The load current at handoff from PFM to PWM mode is

given by:

OUT

PFM

OUT

PWM)

(PFM

LOAD

OUT

PFM

Forced-PWM Mode

The Forced-PWM mode disables the zero-crossing

comparator, which truncates the low-side switch on-time

at the inductor current zero crossing. This causes the

low-side gate-drive waveform to become the complement

of the high-side gate-drive waveform. This in turn causes

the inductor current to reverse at light loads while UG

maintains a duty factor of V

OUT/VIN. The benefit of Forced-

PWM mode is to keep the switching frequency fairly

constant. The Forced-PWM mode is most useful for re-

ducing audio frequency noise, improving load-transient

response, and providing sink-current capability for dy-

namic output voltage adjustment.

When V

PFM is above the PFM high threshold (2.5V,

minimum), the converter is in forced-PWM mode. When

PFM is below the PFM low threshold (0.5V, maximum),

the chip is in automatic PFM/PWM Mode.

Power-On-Reset

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.

Should this voltage drop lower than 4.25V (typical), the

POR disables the chip.

En Pin Control

When V

EN is

above the EN high threshold (2.5V,

minimum), the converter is enabled. When V

EN is below

the EN low threshold (0.5V, maximum), the chip is in the

shutdown and only low leakage current is taken from

VCC.

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