FAN6862L Fairchild Semiconductor, FAN6862L Datasheet - Page 10

• Manufacturer Part Number: FAN6862L
• Description: A highly integrated PWM controller, FAN6862R/L provides several features to enhance the performance of flyback converters
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FAN6862R/L • Rev. 1.0.0
Operation Description
Startup Operation
Figure
transformer auxiliary winding for a typical application.
Before FAN6862R/L begins switching operation, it
consumes only startup current (typically 8μA) and the
current supplied through the startup resistor charges
the V
voltage of 16V (V
and 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 FAN6862R/L uses feedback voltage (V
indicator of the output load and modulates the PWM
frequency, as shown in Figure 22, 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 FAN6862R/L enters “deep”
green mode, where the operating current decreases to
2.5mA (maximum), further reducing the standby power
consumption. As V
FAN6862R/L enters burst-mode operation. When V
drops 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 23.
DD
capacitor (C
21
shows
Figure 21. Startup Circuit
FB-ZDC
DD
FB
DD-ON
. The startup resistor is typically
FB-G
decreases below V
FB
DD
, switching stops and the output
a
). When V
decreases below V
), FAN6862R/L begins switching
FB
(2.1V), the switching frequency
rises above V
typical
DD
startup
reaches the turn-on
FB-ZDC
FB-N
FB-ZDC
circuit
(2.5V), the
, switching
FB
DD
) as an
(1.7V),
(8.5V)
and
FB
FB
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 24.
V
(1.7V)
V
FB.ZDC
+4.2kHz
+2.9kHz
22.5kHz
-4.2kHz
I
Vo
-2.9kHz
FB
ds
65kHz
Frequency
Figure 23. Burst-Mode Operation
Figure 24. Frequency Hopping
Figure 22. PWM Frequency
V
FB-ZDC
Switching
Disabled
V
FB -G
V
FB -N
Switching
Disabled
Frequency
PWM
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V
FB