FA5626 FUJI [Fuji Electric], FA5626 Datasheet - Page 23

• Manufacturer Part Number: FA5626
• Description: Fuji Switching Power Supply Control IC
• Manufacturer: FUJI [Fuji Electric]

(4)LAT pin
• To perform overheat protection using an NTC thermistor
As shown in Fig.22, thermistor TH1 is connected to the LAT
pin to perform overheat protection (latch shutdown).
Since the LAT pin source current is 70A (Typ.), select TH1
whose resistor Rth satisfies the following expression at the
desired overheat protection temperature. If temperature
setting for overheat protection is not feasible with TH1 only,
connect an additional resistor (R5) in series for adjustment.
• To perform latch shutdown using independent abnormality
detection signal
As shown in Fig. 23, NPN transistor Tr1 is connected to
LAT pin, and a detection signal is inputted to the base of
Tr1.
The polarity of the input signal must be such that the level
will go high at an error.
Note that, because a constant current flows from LAT pin,
there is no need of a circuit for clamping LAT pin voltage to
above latch shutoff threshold voltage when normal.
(5) Feedback
Fig.24 shows the circuit configuration of the FB pin.
A photo-coupler PC is connected as a feedback circuit that
monitors the output voltage and performs PWM control.
This signal gives threshold voltage for the current
comparator. Consequently, if noise is added to this signal,
the output pulses are disturbed. Capacitor C3 is generally
connected for protection against noise.
AN-071E Rev.1.2
April-2011
Rth @ LAT+R5  1.05V / 70A  15.0k
Fuji Electric Co., Ltd.
Fig. 23 Latch shutdown function by an external
Fig.22 Overheat protection function using a
Fig.24 FB pin circuit configuration
C3
thermistor
signal
2
FB
PC
FA5626
(6) Current sensing unit
As described in 8-(4) One-shot circuit, the minimum ON
width is set for this IC to minimize malfunction due to surge
current that occurs when the power MOSFET is turned on.
However, if the surge current that occurs at the time of
power ON is large, or noise is applied externally at the time
of power ON, malfunction might occur.
In such cases, add RC filters C6 and R7 as shown in
Fig.25.
Determine the CR filter constants according to the cutoff
frequency and time constant.
The cutoff frequency is given by:
fc = 1/(2    C6  R7)
This frequency must be greater than IC operation
frequency of 65 kHz.
Set the RC time constant to 500 nsec or smaller.
Note that, by the input bias current at IS pin, R7 is
subjected to offset with respect to the overload detection
threshold voltage. Do not connect an excessive value.
Otherwise, the overload detection value may vary
considerably.
Recommendations:
R7 = 1 k.
100pF C6  470pF.
To obtain an optimum effect in function, position capacitor
C6 as near IC as possible, and minimize the wiring length.
23
Fig.25 IS pin filter
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