FAN302UL Fairchild Semiconductor, FAN302UL Datasheet - Page 13

• Manufacturer Part Number: FAN302UL
• Description: This highly integrated PWM controller provides several features to enhance the performance of flyback converters
• Manufacturer: Fairchild Semiconductor

© 2011 Fairchild Semiconductor Corporation
FAN302UL • Rev. 1.0.2
Frequency Hopping
EMI reduction is accomplished by frequency hopping,
which spreads the energy over a wider frequency range
than the bandwidth of the EMI test equipment, allowing
compliance with EMI limitations. The internal frequency-
hopping circuit changes the switching frequency
progressively between 135kHz and 145kHz with a
period of t
Burst-Mode Operation
The power supply enters Burst Mode at no-load or
extremely light-load conditionS. As shown in Figure 36,
when V
and the output voltage drops at a rate dependent on
load current. This causes the feedback voltage to rise.
Once V
provide switching pulse. The feedback voltage then falls
and the process repeats. Burst Mode alternately
enables and disables switching of the MOSFET,
reducing the switching losses in Standby Mode. Once
FAN302UL enters Burst Mode, the operating current is
reduced from 3.5mA to 200μA to minimize power
consumption in Burst Mode.
V
R
DL
HV
Figure 34.V-I Characteristics of HV Pin
FB
FB
p
drops below V
, as shown in Figure 35.
exceeds V
Figure 35. Frequency Hopping
V
DL
FBH
I
HV
, the internal circuit starts to
FBL
, the PWM output shuts off

V
DL
R

HV
V
HV
13
Slope Compensation
The sensed voltage across the current-sense resistor is
used for current-mode control and pulse-by-pulse
current limiting. A synchronized ramp signal with
positive slope is added to the current-sense information
at each switching cycle, improving noise immunity of
current-mode control.
Protections
The FAN302UL self-protection functions include V
Over-Voltage Protection (V
Temperature
Protection (V
OVP and brownout protection are implemented as Auto-
Restart Mode, while the V
implemented as Latch Mode.
When an Auto Restart Mode protection is triggered,
switching is terminated and the MOSFET remains off,
causing V
voltage of 5V; the protection is reset, the internal startup
circuit is enabled, and the supply current drawn from the
HV pin charges the hold-up capacitor. When V
reaches the turn-on voltage of 16V, normal operation
resumes. In this manner, auto restart alternately
enables and disables MOSFET switching until the
abnormal condition is eliminated, as shown in Figure 37.
When a Latch Mode protection is triggered, PWM
switching is terminated and the MOSFET remains off,
causing V
voltage of 5V, the internal startup circuit is enabled
without resetting the protection, and the supply current
drawn from HV pin charges the hold-up capacitor. Since
the protection is not reset, the IC does not resume PWM
switching even when V
16V, disabling the HV startup circuit. Then, V
again down to 5V. In this manner, the Latch Mode
protection alternately charges and discharges V
there is no more energy in the DC link capacitor. The
protection is reset when V
allowed only after power supply is unplugged from the
AC line, as shown in Figure 38.
DD
DD
Figure 36. Burst-Mode Operation
to drop. When V
to drop. When V
S
OVP), and brownout protection. The V
Protection
DD
reaches the turn-on voltage of
S
DD
(OTP),
OVP and internal OTP are
DD
DD
DD
drops to 2.5V, which is
drops to the V
drops to the V
OVP), internal Over-
V
S
Over-Voltage
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DD
DD
DD
DD
turn-off
turn-off
drops
until
DD
DD
DD