FAN6862 Fairchild Semiconductor, FAN6862 Datasheet - Page 10

• Manufacturer Part Number: FAN6862
• Description: A highly integrated PWM controller, FAN6862 provides several features to enhance the performance of flyback converters
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FAN6862 • Rev. 1.0.2
Operation Description
Startup Operation
Figure
transformer auxiliary winding for a FAN6862 application.
Before
consumes only startup current (typically 8μA) and the
current supplied through the startup resistor charges
the V
voltage of 16V (V
the current consumed increases to 3mA. Then, the
power required is supplied from the transformer
auxiliary winding. The large hysteresis of V
provides more holdup time, which allows using a small
capacitor for V
connected to AC line for a fast reset of latch protection.
Green-Mode Operation
The FAN6862 uses feedback voltage (V
indicator of the output load and modulates the PWM
frequency, as shown in Figure 23, such that the
switching frequency decreases as load decreases. In
heavy load conditions, the switching frequency is
65KHz. Once V
PWM frequency starts to linearly decrease from 65KHz
to 22.5kHz to reduce the switching losses. As V
decreases below V
is fixed at 22.5kHz and FAN6862 enters “deep” green
mode, where the operating current decreases to 2.5mA
(maximum),
consumption. As V
FAN6862 enters burst-mode operation. When V
below V
voltage starts to drop, which causes the feedback
voltage to rise. Once V
resumes. Burst mode alternately enables and disables
switching, thereby reducing switching loss in standby
mode, as shown in Figure 24.
DD
FB-ZDC
22
FAN6862
capacitor (C
shows
, FAN6862 stops switching and the output
Figure 22. Startup Circuit
further
DD
FB
DD-ON
. The startup resistor is typically
FB-G
decreases below V
FB
begins
a
DD
), FAN6862 begins switching and
decreases below V
reducing
FB
(2.1V), the switching frequency
). When V
rises above V
typical
switching
the
startup
DD
reaches turn-on
standby power
FB-ZDC
FB-N
operation,
FB-ZDC
circuit
(2.2V), the
FB
, switching
DD
) as an
FB
(1.7V),
(8.5V)
drops
and
FB
it
10
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 25.
V
(1.7V)
V
FB.ZDC
I
Vo
FB
ds
Figure 24. Burst Mode Operation
Figure 25. Frequency Hopping
Figure 23. PWM Frequency
Switching
Disabled
Switching
Disabled
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