FA5695 Datasheet, PDF (20/21 Pages) Fuji Electric – Fuji Switching Power Supply Control IC

FA5695
Von(max): Low voltage ON threshold voltage of UVLO 14V(max.)
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.
(9) Minus voltage of each terminal
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.
Vcc
UVLO
ON
UVLO
OFF
UVLO OFFまで
V_低cc_下m_しus_なt _いno_こt _とdrop
below UVLO OFF
補助巻線電圧
Auxiliary winding voltage
時間t
Time t
Fig.27 Vcc voltage at startup
OUT
7
SBD
Fig.28 Protection circuit of OUT
pin against the negative voltage
12 Advice for design
(1) advice in pattern designing
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)
イICンisダplクacタeのd u下nd(e基r t板heのin裏du面ctoのr 場合
も含む）にICが配置されている。
Fig.29 Bad example (1)
ICイiンs ダplaクceタdやjuMstObSeFsEidTeのthすeぐ近くに
inIdCuがcto配r, 置MOさSれFEてTいる。
Fig.30 Bad example (2)
Fuji Electric Co., Ltd.
AN-073E Rev.1.1
20
April-2011
イ_Siン_gnダ_alク_wタ_ireの_s 下_areや_pM_laO_ceS_dFE_uT_ndの_erすぐ近く
を_th信_{e i}号_ndの_uc配_tor線_orが_ne通_ar過_Mし_Oて_SFい_Eる_T 。
Fig.31 Bad example (3)
http://www.fujielectric.co.jp/products/semiconductor/