FAN6862WLTY Datasheet, PDF (11/16 Pages) Fairchild Semiconductor – Highly Integrated Green-Mode PWM Controller

Operation Description
Startup Operation
Figure 24 shows a typical startup circuit and transformer
auxiliary winding for a typical application. Before
switching operation begins, FAN6862W consumes only
startup current (typically 8 µA) and the current supplied
through the startup resistor charges the V_DD capacitor
(C_DD). When V_DD reaches turn-on voltage of 16 V (V_DD-
ON^{), switching begins and the current consumed}
increases to 2 mA. Power is then supplied from the
transformer auxiliary winding. The large hysteresis of
V_DD (7 V) provides more holdup time, which allows using
a small capacitor for V_DD. The startup resistor is typically
connected to AC line for a fast reset of latch protection.
Figure 25. PWM Frequency
Figure 24. Startup Circuit
Green-Mode Operation
The FAN6862W uses feedback voltage (V_FB) as an
indicator of the output load and modulates the PWM
frequency, as shown in Figure 25, such that the
switching frequency decreases as load decreases. In
heavy-load conditions, the switching frequency is
65 kHz. Once V_FB decreases below V_FB-N (2.5 V), the
PWM frequency starts to linearly decrease from 65 kHz
to 22.5 kHz to reduce the switching losses. As V_FB
decreases below V_FB-G (2.2 V), the switching frequency
is fixed at 22.5 kHz and FAN6862W enters “deep”
Green Mode, where the operating current decreases to
600 µA (maximum), further reducing the standby power
consumption. As V_FB decreases below V_FB-ZDC (1.6V),
FAN6862W enters Burst-Mode operation. When V_FB
drops below V_FB-ZDC, switching stops and the output
voltage starts to drop, which causes the feedback
voltage to rise. Once V_FB rises above V_FB-ZDC, switching
resumes. Burst Mode alternately enables and disables
switching, thereby reducing switching loss in Standby
Mode, as shown in Figure 26.
Figure 26. Burst-Mode Operation
Frequency Hopping
EMI reduction is accomplished by frequency hopping,
which spreads the energy over a wider frequency range
than the bandwidth measured by the EMI test
equipment. An internal frequency-hopping circuit
changes the switching frequency between 61.0 kHz and
69.0 kHz with a period of 4.4 ms, as shown in Figure 27.
This covers the whole frequency range and shrinks the
period with operation frequency proportionally.
© 2012 Fairchild Semiconductor Corporation
FAN6862W • Rev. 1.0.0
Figure 27. Frequency Hopping
www.fairchildsemi.com
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