APW8802 datasheet(14/25 Pages) ANPEC

14 / 25 page

Rev. A.3 - Jan., 2013

APW8802

www.anpec.com.tw

Function Description

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

ward

The constant-on-time control architecture is a pseudofixed

frequency with input voltage feed-forward. This architec-

ture 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 oneshot

whose pulse width is inversely proportional to input volt-

age 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

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

line transients.

Another one-shot sets a minimum off-time (450ns,

typical). 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 oneshot

has timed out.

Pulse-Frequency Modulation (PFM)

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

PWM)

LOAD(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 UGATE

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.

Power-On-Reset

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

wrong logic controls when the PVCC or VCC voltage is

low. The POR function continually monitors the bias sup-

ply voltage on the PVCC and VCC pins if at least one of

the enable pins is set high. When the rising PVCC volt-

age reaches the rising PVCC POR voltage threshold

(4.35V, typical) and the rising VCC voltage reaches the

rising VCC POR Threshold (4.35V, typical), the POR sig-

nal goes high and the chip initiates soft-start operations.

There is almost no hysteresis to POR voltage threshold

(about 100mV typical). When PVCC voltage drops lower

than 4.25V (typical) or VCC voltage drops lower than

4.25V (typical), the POR disables the chip.

EN Pin Control

When V

EN is above the EN high threshold (2.65V, typical),

the converter is enabled in automatic PFM/PWM opera-

tion mode. When EN pin is floating, APW8802 internal

circuit will pull V

EN up to 1.95V (typical). Furthermore,

APW8802 is in Forced-PWM operation mode. When V

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