ANP015

Application Note

AP2011 High Efficiency Synchronous PWM Controller

Preliminary

Anachip Corp.

www.anachip.com.tw

Rev. A.0 Oct, 29, 2004

6/15

Voltage Clamping for High Input Voltage

Normally the MOSFETs AP2011 used can be bought easily, but always the easy-buying parts have 25V

function for protecting the MOSFETs. AP2011 is built-in a 8V voltage drop circuit to clamp the voltage

(Figure. 4), AP2011 will make a current to charge the outside capacitor by the resistor. And if that charging

value is not enough, the P channel MOSFET may conduct a large short-through current during supply

transition.

VCC

outside inside

I

CAP

R

VCC

I

CE

Mirror

PGATE

PDRV

Cboost

0.7V

+

-

CAP

Ccap

Ccap > 20*Cboost

Cboost = 47nF (recommendation)

Figure 4. Voltage Clamping Function Circuit

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 5 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