FA5540 Fuji Electric holdings CO.,Ltd, FA5540 Datasheet - Page 27

• Manufacturer Part Number: FA5540
• Description: Fuji Switching Power Supply Control Ic
• Manufacturer: Fuji Electric holdings CO.,Ltd

10. Method for using each pin
winding.
Rev.0 July-2007
(1) No.1 pin (ZCD)
Function
Usage
(ⅰ) Detection of timing to make MOSFET ON.
(ⅱ) Latch protection
・Connection
・Operation
(ⅱ) Latch protection by an external signal.
(ⅰ) Detection of turn-on timing
・Connection
・Operation
・Complement
This pin is connected to a transformer auxiliary
winding through CR circuit with R
CZCD. (Fig.10)
When ZCD pin voltage drops lower than 62mV,
MOSFET is turned on.
The auxiliary winding voltage swings + and –
direction widely along with switching. A clamp
circuit is equipped to protect IC from this
voltage. If the auxiliary winding voltage is plus,
it passes a current shown in Fig. 11 and if
minus, shown in Fig.12. And then it clamps
ZCD pin voltage.
Be careful about polarity of an auxiliary
case of primary over-voltage.
pulse is shutdown and maintained as it is.
Since
protection by an external signal is 6.4V (min.)
as described in function (ⅱ), the resistor
R
not to exceed 6.4V in ordinary operation. At
the same time the resistor R
adjusted for ZCD pin current not to exceed
the absolute maximum rating.
The MOSFET voltage oscillates just before
TURN ON due to the resonance effect
between
resonant capacitor Cd. CZCD is adjusted for
MOSFET to turn on at the bottom of this
resonance
several 10kΩ and CZCD is several 10pF.
However CZCD is unnecessary if good
timing is obtained.
Fig. 14 shows the connection example in
When ZCD pin voltage exceeds 7.2V (typ.)
and continues for more than 40µs (FA5540) /
54µs (FA5541/42) (typ.), latch protection
operates.
Once latch protection operates, IC output
It can be reset by decreasing Vcc lower than
UVLO off-threshold voltage.
Pull up ZCD pin by an external signal.
ZCD
must be adjusted for ZCD pin voltage
the
transformer
(Fig.13).
threshold
Generally
voltage
inductance
ZCD
of
R
ZCD
must be
FA5540/5541/5542
ZCD
latch
and
and
is
Fig.14 Primary side over voltage protection circuit
Fig.11 Clamp circuit (positive auxiliary winding
Fig.12 Clamp circuit (negative auxiliary winding
27
30kΩ
30kΩ
30kΩ
VCC
ZCD
Vds
Fig. 10 ZCD pin circuit
Fig.13 Vds waveform
6
1
Fuji Electric Device Technology
60Ω
9.2V
60Ω
60Ω
-0.83V
9.2V
voltage)
voltage)
1
1
1
ZCD
ZCD
ZCD
current
current
C
Clamp
Clamp
R
ZCD
ZCD
Cd