APW8811 datasheet(11/23 Pages) ANPEC

11 / 23 page

Rev. A.3 - Sep., 2012

APW8811

www.anpec.com.tw

Function Description

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

ward

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 switching frequency of the high-side switch

and keeps regulating it at a constant frequency in PWM

mode. The design improves the frequency variation and

is more outstanding than a conventional constant-on-

time controller, which has large switching frequency varia-

tion 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 (typ.: 300ns).

The on-time one-shot is triggered if the error comparator

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

limit threshold, and the minimum off-time one-shot has

timed out.

Forced-PWM Mode

Connect SKIP# to REF for normal Forced-PWM operation.

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 the most useful for

reducing audio frequency noise, improving load-transient

response, and providing sink-current capability for dy-

namic output voltage adjustment.

Pulse-Frequency Modulation (PFM) Mode

Connect SKIP# to GND for normal PFM operation. In PFM

mode, an automatic switchover to pulse-frequency modu-

lation (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:

Ultrasonic Mode

Connecting SKIP# to LDO3 or LDO5 for ultrasonic mode.

The ultrasonic mode activates a unique PFM mode with

a minimum switching frequency of 37kHz. The minimum

frequency 37KHz of ultrasonic mode eliminates audio-

frequency interference in light load condition. It will transit

to unique PFM mode when output loading makes the

frequency bigger than ultrasonic frequency. In ultrasonic

mode, the controller automatically transits to fixed-fre-

quency PWM operation when the load reaches the same

critical conduction point (I

LOAD(PFM to PWM)).

When the controller detects that no switching has oc-

curred within about 27

µs (typ.), an ultrasonic pulse will

occurre. The ultrasonic controller turns on the low-side

MOSFET first to reduce the output voltage. After feedback

voltage drops below the internal reference voltage, the

OUT

PFM

Where F

SW is the nominal switching frequency of the con-

verter in PWM mode. Similarly, the on-time of ultrasonic

mode is the same with PFM mode. The description of

ultrasonic mode will be illustrated later.

The load current at handoff from PFM to PWM mode is

given by:

PFM

OUT

PWM)

LOAD(PFM

−

OUT

−

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