AP2011

Synchronous PWM Controller

Anachip Corp

www.anachip.com.tw

Rev. 1.1 Apr 1, 2005

6/9

Virtual Frequency Control

Virtual

Frequency

Control



combines

the

advantages of constant frequency and constant

off-time control in a single mode of operation. This

allows fix frequency, precision switching voltage

regulator control with fast transient response and

the smallest solution size. Switch duty cycle can be

adjusted from 0% to 100% on a pulse by pulse basis

when responding to transient conditions. Both 0%

and 100% duty cycle operation can be maintained

for extended periods of time in response to load or

line transients. Figure 1 depicts a simplified

operation

of

the

Virtual

Frequency

Control

technique: The VFC oscillator generates a pulse of

a known duration (VFC_Pulse). The regulator loop

responds by returning a complementary feedback

pulse (FB_Pulse). The FB_Pulse duration is a result

of external conditions such as inductor size, the

voltage across the inductor and the duration of the

VFC_Pulse. A VFC control loop is then formed

whereby the duration of the VFC_Pulse is modified

as a result of the FB_Pulse duration. The VFC loop

arrives at a state of equilibrium, where the operating

frequency remains inherently constant.

GATE

CONTROL

LOGIC

VIRTUAL FREQ

OSCILLATOR

+

-

FB Pulse

VFC Pulse

Vref

ERROR

COMP

V

IN

Lout

Cout

Vout

Rfb1

Rfb2

Figure 1: Virtual Frequency Control Loop-

Synchronous single supply application.

D

(1-

) - V

IN

V

OUT

V

IN

×

D

V

×

10000

/

5.9 - 0.01V

IN

Virtual frequency control is a technique that

provides stable, constant frequency of operation for

pulse controlled architectures such as constant

off-time/on-time. This is all done internal to the IC

with minimal number of components and without the

need for connections to external terminals such as

input and/or output. No external compensation is

required,

thus

providing

a

low

cost,

high

performance fix frequency solution for switching

voltage regulators.

Virtual Frequency Control is a trademark of

PWRTEK, LLC.