AN-6300

APPLICATION NOTE

© 2009 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.2 • 5/21/10

7

1

DET

(8µs/38µs)

5V

Valley

Detector

1st

Valley

+9µs

To SR F/F

0.3V

0.3V

To VDD

+

-

(3µs/13µs)

for H version

for H version

Figure 12. Valley Detection Block

Figure 13. First Valley Switching

The proprietary green-mode function provides off-time

modulation to linearly decrease the switching frequency

the voltage feedback loop, is taken as the reference. In

finishes. Therefore, the valley detect circuit is activated

switching frequency and provides extended valley voltage

switching. In very light load conditions, it might fail to

cycle start after a 9μs(5µs for H version) delay. Figure 15

Figure 15. QR Operation in Extended Valley Voltage

Detection Mode

Failure to Detect Valley Voltage (with 5µs Delay for

FAN6300H)

is the minimum switching frequency at the minimum input

is the minimum frequency when a 9μs(5µs for H version)

timer is enabled. When output load is gradually lighter

extended linearly. The flyback converter is forced to enter

discontinuous conduction mode (DCM); therefore, the

MOSFET drain voltage is switched at further extended

than 20kHz to prevent audio noise. Once the converter