APW7199 datasheet(14/24 Pages) ANPEC

14 / 24 page

Rev. A.4 - Oct., 2010

APW7199

www.anpec.com.tw

Function Description

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

ward

Pulse-Frequency Modulation (PFM)

Ultrasonic Mode

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:

300ns). 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:

In this case, APW7199 operates in ultrasonic mode with

PFM when the load is zero. The ultrasonic mode is illus-

trated as below description.

The ultrasonic mode activates an unique PFM mode with

a minimum switching frequency of 20kHz. The minimum

frequency 20kHz 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-frequency 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 40

µs (Typical), an ultrasonic pulse

will be occurred. The ultrasonic controller turns on the

low-side MOSFET firstly to reduce the output voltage. Af-

ter feedback voltage drops below the internal reference

voltage, the controller turns off the low-side MOSFET and

triggers a constant-on-time. When the constant-on-time

has expired, the controller turns on the low-side MOSFET

again until the inductor current is below the zero-cross-

ing threshold. The behavior is the same as PFM mode.

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 (4.2V, typical), the POR signal goes

high and the chip initiates soft-start operations. When

this voltage drops lower than 4.0V (typical), the POR dis-

ables the chip.

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