FAN6861 Datasheet, PDF (10/15 Pages) Fairchild Semiconductor – Low-Cost, Highly Integrated, Green-Mode PWM Controller for Peak Power Management

Operation Description
Startup Operation
Figure 17 shows the typical startup circuit and
transformer auxiliary winding for FAN6861 application.
Before FAN6861 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 17.5V (VDD-ON), FAN6861 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 (8V)
provides more holdup time, which allows using small
capacitor for VDD. The startup resistor is typically
connected to AC line for a fast reset of latch protection.
Figure 18. PWM Frequency
Figure 17. Startup Circuit
Figure 19. Burst Mode Operation
Green-Mode Operation
The FAN6861 uses feedback voltage (VFB) as an
indicator of the output load and modulates the PWM
frequency, as shown in Figure 18, such that the
switching frequency decreases as load decreases. In
heavy load conditions, the switching frequency is
65KHz. Once VFB decreases below VFB-N (2.85V), the
PWM frequency starts to linearly decrease from 65KHz
to 22kHz to reduce the switching losses. As VFB
decreases below VFB-G (2.2V), the switching frequency
is fixed at 22.5kHz and FAN6861 enters into deep
green mode, where the operating current reduces to
2.2mA (maximum), further reducing the standby power
consumption. As VFB decreases below VFB-ZDC (1.9V),
FAN6861 enters into burst-mode operation. When VFB
drops below VFB-ZDC, FAN6861 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 19.
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 20.
Figure 20. Frequency Hopping
© 2009 Fairchild Semiconductor Corporation
FAN6861 • Rev. 1.0.1
10
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