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FAN6863RTY 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, FAN6863 consumes only
startup current (typically 8µA) and the current supplied
through the startup resistor charges the VDD capacitor
(CDD). When VDD reaches turn-on voltage of 16V (VDD-
ON), switching begins and the current consumed
increases to 2mA. Then power is supplied from the
transformer auxiliary winding. The large hysteresis of
VDD (7V) provides more holdup time, which allows using
a small capacitor for VDD. 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 FAN6863 uses feedback voltage (VFB) 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
65kHz. Once VFB decreases below VFB-N (2.5V), the
PWM frequency starts to linearly decrease from 65kHz
to 22.5kHz to reduce the switching losses. As VFB
decreases below VFB-G (2.2V), the switching frequency
is fixed at 22.5kHz and FAN6863 enters “deep” Green
Mode, where the operating current decreases to 600µA
(maximum), further reducing the standby power
consumption. As VFB decreases below VFB-ZDC (1.6V),
FAN6863 enters Burst-Mode operation. When VFB drops
below VFB-ZDC, switching stops and the output voltage
starts to drop, which causes the feedback voltage to
rise. Once VFB rises above VFB-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 60.8kHz and
69.2kHz with a period of 4.4ms, as shown in Figure 27.
Figure 27. Frequency Hopping
© 2011 Fairchild Semiconductor Corporation
FAN6863 • Rev. 1.0.1
11
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