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AN-6073

Highly Integrated Green-Mode PWM Controller

Fairchild Semiconductor 

AN-6073

Figure 8. UVLO Effect

FB Input

The FAN6751 is designed for peak-current-mode control. A

current-to-voltage conversion is done externally with a

current-sense resistor RS. Under normal operation, the FB

level controls the peak inductor current:

VSENSE

= Ipk

× RS

= VFB − 0.6

4

(2)

where VFB is the voltage on FB pin and 4 is an internal

divider ratio.

When VFB is less than 0.6V, the FAN6751 terminates the

output pulses.

APPLICATION NOTE

down at no load. The value of the biasing resistor Rb is

determined as:

Vo −VD −VZ • K ≥ 1.5mA

Rb

(3)

where:

VD is the drop voltage of photodiode, approximately 1.2V;

VZ is the minimum operating voltage, 2.5V of the shunt

regulator; and

K is the current transfer rate (CTR) of the opto-coupler.

For an output voltage VO=5V with CTR=100%, the

maximum value of Rb is 860Ω.

Green Mode Operation

Green mode includes off-time modulation and burst mode to

reduce the PWM frequency at light-load and in no-load

conditions. The feedback voltage of the FB pin is taken as a

reference. When the feedback voltage is lower than VFB-N,

the PWM frequency decreases. Because most losses in a

switching-mode power supply are proportional to the PWM

frequency, the off-time modulation reduces the power

consumption of the power supply at light-load and no-load

conditions. Figure 10 is the PWM frequency is 65KHz at

nominal load and decreases to 18KHz at light load.

Frequency

Fosc:65KHz

PWM Frequency

Fosc:18KHz

VFB-ZDC

VFB-G

VFB-N

Figure 10. PWM Frequency vs. FB Voltage

Figure 9. Feedback Circuit

Figure 9 is a typical feedback circuit consisting mainly of a

shunt regulator and an opto-coupler. R1 and R2 form a

voltage divider for the output voltage regulation. R3 and C1

are adjusted for control-loop compensation. A small-value

RC filter (e.g. RFB= 100Ω, CFB= 1nF) placed on the FB pin

to the GND can further increase the stability. The maximum

sourcing current of the FB pin is 1.5mA. The phototransistor

must be capable of sinking this current to pull FB level

The power supply enters “burst mode” in no-load

conditions. As shown in Figure 11 and Figure 12, when VFB

drops below VFB-ZDC, the PWM output is shuts off and the

output voltage drops at a rate dependent on load current.

This causes the feedback voltage to rise. Once VFB exceeds

VFB-ZDC, the internal circuit starts to provide switching pulse.

The feedback voltage then falls and the process repeats.

Burst mode operation alternately enables and disables

switching of the MOSFET, reducing the switching losses in

standby mode.

© 2008 Fairchild Semiconductor Corporation

Rev. 1.0.0 • 9/26/08

4

www.fairchildsemi.com

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