FAN6862_10 Datasheet, PDF (10/16 Pages) Fairchild Semiconductor – Highly Integrated Green-Mode PWM Controller

Operation Description
Startup Operation
Figure 22 shows a typical startup circuit and
transformer auxiliary winding for a FAN6862 application.
Before FAN6862 begins switching operation, it
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), FAN6862 begins switching and
the current consumed increases to 3mA. Then, the
power required is supplied from the transformer
auxiliary winding. The large hysteresis of VDD (8.5V)
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 23. PWM Frequency
Vo
V_FB
VFB.ZDC
(1.7V)
I_ds
Figure 22. Startup Circuit
Switching
Disabled
Switching
Disabled
Figure 24. Burst Mode Operation
Green-Mode Operation
The FAN6862 uses feedback voltage (VFB) as an
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 VFB decreases below VFB-N (2.2V), the
PWM frequency starts to linearly decrease from 65KHz
to 22.5kHz to reduce the switching losses. As VFB
decreases below VFB-G (2.1V), the switching frequency
is fixed at 22.5kHz and FAN6862 enters “deep” green
mode, where the operating current decreases to 2.5mA
(maximum), further reducing the standby power
consumption. As VFB decreases below VFB-ZDC (1.7V),
FAN6862 enters burst-mode operation. When VFB drops
below VFB-ZDC, FAN6862 stops switching 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 24.
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.
Figure 25. Frequency Hopping
© 2009 Fairchild Semiconductor Corporation
FAN6862 • Rev. 1.0.2
10
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