FAN9611 Fairchild Semiconductor, FAN9611 Datasheet - Page 12

• Manufacturer Part Number: FAN9611
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
FAN9611 / FAN9612 • Rev. 1.1.4
This method is especially dangerous in power-factor-
corrector applications because the error amplifier’s
bandwidth is typically limited to a very low crossover
frequency. The slow response of the amplifier can
cause considerable overshoot at the output.
Functional Description
1. Detecting Zero Inductor Current
Each ZCD pin is internally clamped close to 0V (GND).
Any capacitance on the pin is ineffective in providing
any delay in ZCD triggering. The internal sense circuit is
a true differentiator to catch the valley of the drain
waveforms. The resistor between the auxiliary winding
of the boost inductor and the ZCD pin is only used for
current limiting. The maximum source current during
zero current detection must be limited to 0.5mA. Source
and sink capability of the pin is about 1mA, providing
sufficient margin for the higher source current required
during the on-time of the power MOSFETs.
The R
2. 5V Bias Rail (5VB)
This is the bypass capacitor pin for the internal 5V bias
rail powering the control circuitry. The recommended
capacitor value is 220nF. At least a 100nF, good-quality,
high-frequency, ceramic capacitor should be placed in
close proximity to the pin.
The 5V rail is a switched rail. It is actively held LOW
when the FAN9611/12 is in under-voltage lockout. Once
the UVLO turn-on threshold is exceeded at the VDD pin,
the 5V rail is turned on, providing a sharp edge that can
be used as an indication that the chip is running.
Potentially, this behavior can be utilized to control the
inrush current limiting circuit.
R
ZCD
(ZCD1, ZCD2)
ZCD

Figure 16. Zero-Current Detect Circuit
0
resistor value can be approximated by:
.
5
1
mA

V
2
O

N
N
BOOST
AUX
(1)
12
FAN9611/12 employs closed-loop soft-start where the
reference voltage of the error amplifier is slowly
increased to its final value. When the current and power
limits of the converter are properly taken into
consideration, the output voltage of the converter
follows the reference voltage. This ensures that the
error amplifier stays in regulation during soft start and
the output voltage overshoot can be eliminated.
3. Maximum On-Time Control (MOT)
Maximum on-time, MOT, (of the boost MOSFET) is set
by
FAN9611/12 implements input-voltage feedforward. The
maximum on-time is a function of the RMS input
voltage. The voltage on the MOT pin is 1.25V during
operation (constant DC voltage). The maximum on-time
of the power MOSFETs can be approximated by:
where V
measured at the VIN pin (must be divided down, see the
VIN pin description).
t
ON MAX
,
Figure 18. Maximum On-Time Control (MOT)
a
resistor
INSNS,PK

R
MOT
is the peak of the AC input voltage as

to
Figure 17. 5V Bias
120 10
analog


12

1.25
2.4
ground

V
INSNS PK
2
1
(AGND).
,
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The
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