FAN302UL Fairchild Semiconductor, FAN302UL Datasheet - Page 14

• 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
V
V
over-voltage exceeding the IC voltage rating. When the
V
the protection is triggered. This protection is typically
caused by an open circuit of the secondary-side
feedback network.
Input Voltage Sensing and Brownout Protection
The FAN302UL indirectly senses input voltage using the
VS pin current while the MOSFET is turned on. Since
the VS pin voltage is clamped at 0.7V when the
MOSFET is turned on, the current flowing out of the VS
pin is approximately proportional to the input voltage, as
shown in Figure 38. Current flowing out of the VS pin is
calculated by:
DD
DD
DD
I
VS ON
Over-Voltage Protection
voltage exceeds 26.5V due to abnormal conditions,
over-voltage protection prevents IC damage from
.
Figure 37. Auto-Restart Mode Operation

Figure 38. Latch-Mode Operation
(
N
N
A
P
V
DL

0.7)
R
1
VS
1

R
0.7
VS
2

N V
N R
A
P
VS
DL
1
(3)
14
FAN302UL modulates the minimum ON time of the
MOSFET such that it reduces as input voltage
increases, as shown Figure 40. This allows smaller
minimum ON time for high-line condition, ensuring Burst
Mode operation occurs at almost the same power level,
regardless of line voltage variation. The minimum ON
time is also related to the bundle frequency of Burst
Mode operation.
The VS current is also used for brownout protection.
When the current out of the VS pin while the MOSFET
is on is smaller than 47μA for longer than 10ms, the
brownout protection is triggered.
Over-Temperature Protection (OTP)
The temperature-sensing circuit shuts down PWM
output if the junction temperature exceeds 140°C (t
V
V
output over-voltage conditions. Figure 41 shows the V
OVP protection method. When abnormal system
conditions occur that cause V
period of debounce time; PWM pulses are disabled and
FAN302UL enters Latch Mode until V
V
conditions are usually caused by an open circuit of the
secondary-side feedback network or abnormal behavior
by the VS pin divider resistor.
S
S
DD-LH
Figure 40. Minimum On Time vs. VS Pin Current
Over-Voltage Protection (OVP)
over-voltage protection prevents damage due to
Modulation
Protection
Brownout
Minimum
On Time
.
By that time, PWM pulses revive. V
Figure 39.VS Pin Current Sensing
V
DL
Monitoring
Current
Control
Block
PWM
Block
V
S
S
to exceed 2.8V, after a
R
VS2
C
DD
VS
R
drops to under
VS1
S
www.fairchildsemi.com
over-voltage
N
N
P
A
OTP
).
S