FA5590 FUJI [Fuji Electric], FA5590 Datasheet - Page 20

• Manufacturer Part Number: FA5590
• Description: FUJI Power Supply Control IC
• Manufacturer: FUJI [Fuji Electric]

Von(max): Low voltage ON threshold voltage of UVLO
The value of R7 expressed with the formula is, however, at least
necessary and minimum condition to start the IC, and actually it should
be decided considering the starting up time required for each application
circuit.
This starting up time must be examined by measuring in actual circuit
operation.
During the steady operation, Vcc is supplied from the auxiliary winding
of the transformer. But there is some time delay until the auxiliary
winding voltage sufficiently rises after the IC starts switching operation.
To prevent Vcc from decreasing to the off threshold voltage of UVLO, it
is necessary to decide the capacitance of the C5 connected to Vcc.
Since this time delay differs depending on the circuit, it should be
decided after checking with actual circuit
It is also recommended to place the ceramic capacitor C9 (about 0.1uF)
to remove switching noise.
In some cases, the voltage oscillation of Vds just before MOSFET turns
on is applied to the OUT terminal through parasitic capacitors, etc. and
minus voltage may be added to the OUT terminal. If this minus voltage is
large, the parasitic element inside the IC is activated, and the IC may
malfunction.
If this minus voltage is expected to exceed -0.3V, Schottky barrier diode
should be connected between the OUT terminal and GND. With the
forward voltage of the Schottky barrier diode, the minus voltage can be
clamped.
For other terminals as well, care should be taken so that minus voltage
will not be applied in the same way.
12 Advice for design
Main power parts such as MOSFET, inductor, and diode in the main
switching circuit are operating with large voltage and current. For this
reason, if the IC or wires of input signals are located close to these main
power parts, malfunction may occur affected by noise generated there.
Special care should be taken to the following cases. (Bad examples)
- IC is placed under the main circuit parts such as inductor or just on the
back side of the main circuit parts in case of a double-sided board. (Fig.
29)
- IC is placed just beside the inductor, MOSFET or diode. (Fig. 30)
- Signal wires are placed under the inductor or near MOSFET or diode.
(Fig. 31)
AN-016E Rev.1.2
April-2011
(1) advice in pattern designing
(9) Minus voltage of each terminal
Fuji Electric Co., Ltd.
FA5590 10.6V(max)
FA5591 14V(max)
Fig.30 Bad example (2)
IC is placed just beside the
inductor,MOSFET
インダクタやMOSFETのすぐ近くに
ICが配置されている。
FA5590N,FA5591N
20
UVLO
UVLO
OFF
ON
Vcc
http://www.fujielectric.co.jp/products/semiconductor/
Fig.28 Protection circuit of OUT
Fig.29 Bad example (1)
pin against the negative voltage
Fig.27 Vcc voltage at startup
Fig.31 Bad example (3)
インダクタの下(基板の裏面の場合
IC is placed under the inductor
も含む）にICが配置されている。
インダクタの下やMOSFETのすぐ近く
を信号の配線が通過している。
Signal wires are placed under
the inductor or near MOSFET
7
OUT
Auxiliary winding voltage
SBD
Vcc must not drop
below UVLO OFF
補助巻線電圧
低下しないこと
UVLO OFFまで
時間t
Time t