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FAN7602BN View Datasheet(PDF) - Fairchild Semiconductor

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Description
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FAN7602BN
Fairchild
Fairchild Semiconductor Fairchild
FAN7602BN Datasheet PDF : 17 Pages
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4. Burst-Mode Block
The FAN7602B contains the burst-mode block to reduce
the power loss at a light load and no load. A hysteresis
comparator senses the offset voltage of the Burst+ for
the burst mode, as shown in Figure 22. The Burst+ is the
sum of the CS/FB voltage and Plimit offset voltage. The
FAN7602B enters burst mode when the offset voltage of
the Burst+ is higher than 0.95V and exits the burst mode
when the offset voltage is lower than 0.88V. The offset
voltage is sensed during the switch off time.
D elay
C ircuit
+ Burst+
0 .9 5 V /0 .8 8 V
O ffset
3 C S/FB
Figure 22. Burst-Mode Block
5. Protection Block
The FAN7602B contains several protection functions to
improve system reliability.
5.1 Overload Protection (OLP)
The FAN7602B contains the overload protection func-
tion. If the output load is higher than the rated output cur-
rent, the output voltage drops and the feedback error
amplifier is saturated. The offset of the CS/FB voltage
representing the feedback information is almost zero. As
shown in Figure 23, the CS/FB voltage is compared with
50mV reference when the internal clock signal is high
and, if the voltage is lower than 50mV, the OLP timer
starts counting. If the OLP condition persists for 22ms,
the timer generates the OLP signal. This protection is
reset by the UVLO. The OLP block is enabled after the
soft-start finishes.
Clock
OLP 22ms
Timer
3 CS/FB
Soft-Start
50mV
voltage feedback loop is saturated, and the OLP initiates
if the LUVP condition persists more than 22ms.
Vin
1
LUVP
+
2V/1.5V
Figure 24. Line UVP Circuit
5.3 Latch Protection
The latch protection is provided to protect the system
against abnormal conditions using the Latch/Plimit pin.
The Latch/Plimit pin can be used for the output over-
voltage protection and/or other protections. If the Latch/
Plimit pin voltage is made higher than 4V by an external
circuit, the IC is shut down. The latch protection is reset
when the VCC voltage is lower than 5V.
5.4 Over-Voltage Protection (OVP)
If the VCC voltage reaches 19V, the IC shuts down and
the OVP protection is reset when the VCC voltage is
lower than 5V.
6. Output Drive Block
The FAN7602B contains a single totem-pole output
stage to drive a power MOSFET. The drive output is
capable of up to 450mA sourcing current and 600mA
sinking current with typical rise and fall time of 45ns and
35ns, respectively, with a 1nF load.
Figure 23. Overload Protection Circuit
5.2 Line Under-Voltage Protection
If the input voltage of the converter is lower than the min-
imum operating voltage, the converter input current
increases too much, causing component failure. There-
fore, if the input voltage is low, the converter should be
protected. In the FAN7602B, the LUVP circuit senses the
input voltage using the LUVP pin and, if this voltage is
lower than 2V, the LUVP signal is generated. The com-
parator has 0.5V hysteresis. If the LUVP signal is gener-
ated, the output drive block is shut down, the output
© 2007 Fairchild Semiconductor Corporation
FAN7602B Rev. 1.0.0
10
www.fairchildsemi.com
 

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