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AP2011

Synchronous PWM with VFC Controller

Anachip Corporation 

Synchronous PWM Controller

AP2011

„ 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.

VIN

ERROR

COMP

Vref

+

-

VFC Pulse

VIRTUAL FREQ

OSCILLATOR

GATE

CONTROL

LOGIC

FB Pulse

Lout

Cout

Vout

Rfb1

Rfb2

DON

>

0.1,

240-50 × (1-

VOUT

VIN

)

-

VIN

DON < 0.1,

10000× VOUT/ VIN

5.9 - 0.01VIN

Figure 1: Virtual Frequency Control Loop-

Synchronous single supply application.

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.

Anachip Corp

www.anachip.com.tw

Rev. 1.1 Apr 1, 2005

6/9

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