FAN6862HR Fairchild Semiconductor, FAN6862HR Datasheet - Page 10

• Manufacturer Part Number: FAN6862HR
• Description: A highly integrated PWM controller, FAN6862H(HR) provides several features to enhance the performance of flyback converters
• Manufacturer: Fairchild Semiconductor

© 2010 Fairchild Semiconductor Corporation
FAN6862H(HR) • Rev. 1.0.0
Operation Description
Startup Operation
Figure
transformer auxiliary winding for a FAN6862H(HR)
application. Before FAN6862H(HR) begins switching, it
consumes only startup current (typically 8µA) and the
current supplied through the startup resistor charges
the V
voltage of 16V (V
current consumed increases to 2mA. Then, the power
required is supplied from the transformer auxiliary
winding. The large hysteresis of V
more holdup time, which allows using a small capacitor
for V
AC line for a fast reset of latch protection.
Green-Mode Operation
The FAN6862H(HR) 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
100KHz to 25kHz to reduce the switching losses. As
V
frequency is fixed at 25kHz and FAN6862H(HR) enters
“deep” Green Mode, where the operating current
decreases to 2.5mA (maximum), further reducing the
standby power consumption. As V
V
When V
switching and the output voltage starts to drop, which
causes the feedback voltage to rise. Once V
above
alternately enables and disables switching, reducing
switching loss in standby mode, as shown in Figure 24.
FB
FB-ZDC
DD
decreases below V
DD
. The startup resistor is typically connected to the
FB
22
(1.7V), FAN6862H(HR) enters Burst Mode.
V
capacitor (C
FB-ZDC
drops below V
shows
Figure 22. Startup Circuit
,
FB
switching
decreases below V
DD-ON
DD
a
). When V
), switching begins and the
typical
FB-ZDC
FB-G
resumes.
, FAN6862H(HR) stops
(2.1V), the switching
DD
startup
FB
DD
reaches a turn-on
decreases below
(8.5V) provides
FB-N
Burst
circuit
(2.6V), the
FB
FB
) as an
Mode
rises
and
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.
changes the switching frequency between 93.5kHz and
106.5kHz with a period of 4.4ms, as shown in Figure 25.
V
(1.7V)
V
FB.ZDC
I
V
FB
ds
O
Figure 24. Burst-Mode Operation
Figure 25. Frequency Hopping
An internal frequency hopping circuit
Figure 23. PWM Frequency
Switching
Disabled
Switching
Disabled
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