FA5695 Datasheet, PDF (15/21 Pages) Fuji Electric – Fuji Switching Power Supply Control IC

FA5695
(6-2) Overcurrent detection protective circuit
The overcurrent detection protective circuit detects the inductor
current and protects MOSFET by turning off the OUT output when it
becomes higher than a set current level. With the overcurrent detection,
the voltage across the current detection resistance Rs connected to the
GND is fed to the IS terminal, and when the IS terminal voltage
compared by the overcurrent detection comparator becomes lower than
-0.6V, it is regarded as overcurrent state.
When the overcurrent is detected, the F/F for OUT output is reset to
make MOSFET turn off.
(7) Zero cross delay time setting circuit
Vds between the drain and the sources of the MOSFET starts
oscillating through resonance of L1 and the parasitic capacitor
component on the circuit just before the MOSFET turns on.
inductor
イcンurダreクnタt電流
MOVSdFEsT の Vds
Tzcd
T＜ 1/Fmax
Fig.11 when the switching frequency is
lower than the maximum frequency Fmax
When the proper value of Rt, the turn on timing of MOSFET can be
adjusted at the bottom of the voltage oscillation. This makes it possible
to minimize the switching loss and the surge current generated at the
turn-on. (Fig. 13)
When the Rt is smaller, the turn-on timing becomes earlier, and vice
versa. (Fig. 14)
Since the optimum value of this Rt changes depending on the circuit
and input/output conditions, tuning up is required so as to achieve an
optimum state while evaluating the operation with actual circuit.
(8) Restart timer
This IC utilizes self oscillation instead of the oscillator with fixed
frequency, and in the steady operation, it turns on MOSFET with a signal
from the zero current detector.
But in start up or light load condition, a trigger signal is required for
starting up or stable operation.
This IC is provided with a restart timer, and when the output of IC
continues turn off for 20μs or more, the trigger signal is automatically
generated.
This signal can realize stable operation even when starting up or the
load is light.
inductor
インダクタ電流
current
MO_VS_dF_sET の Vds
Tzcd
T= 1/Fmax
Fig.12 when the switching frequency is
limited to the maximum frequency Fmax
MOSFET
Vds
のVds
(9) Under Voltage Lock out (UVLO)
UVLO is equipped to prevent circuit malfunction when supply voltage
drops.
When the supply voltage rises from zero, the operation starts at 13V
(typ.).
When the supply voltage decreases after the operation stats, IC stops
the operation at 9V (typ.).
When UVLO is on and IC stops operation the OUT terminal becomes
LOW and cuts off the output. The current consumption of the IC
decreases to 80uA or less.
Tzcd(RTtzcd最適）
(with adequate Rt)
Fig.13 Vds waveform at turn on
(with adequate Rt)
(10) Output circuit portion
The output portion is of push-pull circuit and can directly drive the
MOSFET. The peak current of the output portion is 1.0A maximum for _MOSFET
sink and 1.0A maximum for source.
の^VdV^sds
Fuji Electric Co., Ltd.
AN-073E Rev.1.1
April-2011
Tzcd（Rtzcが小さい）
Tzcd
(Rt is too small)
Tzcd(Rt_Tz_zc_cが_d 大きい）
(Rt is too large)
Fig.14 Vds waveform at turn on
(with inadequate Rt)
http://www.fujielectric.co.jp/products/semiconductor/
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