CS51031 データシートの表示（PDF） - ON Semiconductor
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CS51031 Datasheet PDF : 10 Pages
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CS51031
The fault time is the sum of the slow discharge time the
fast discharge time and the recharge time. It is dominated by
the slow discharge time.
The first parameter is the slow discharge time, it is the time
for the CS capacitor to discharge from 2.4 V to 1.5 V and is
given by:
tSlowDischarge(t) + CS
(2.4 V * 1.5 V)
IDischarge
where IDischarge is 6.0 mA typical.
tSlowDischarge(t) + CS 1.5 105
The fast discharge time occurs when a fault is first
detected. The CS capacitor is discharged from 2.5 V to 2.4 V.
tFastDischarge(t)
+
CS (2.5 V * 2.4
IFastDischarge
V)
where IFastDischarge is 66 mA typical.
tFastDischarge(t) + CS 1515
The recharge time is the time for CS to charge from 1.5 V
to 2.5 V.
tCharge(t) + CS
(2.5 V * 1.5 V)
ICharge
where ICharge is 264 mA typical.
tCharge(t) + CS 3787
The fault time is given by:
tFault + CS (3787 ) 1515 ) 1.5 105)
tFault + CS (1.55 105)
For this circuit
tFault + 0.1 10*6 1.55 105 + 15.5 ms
A larger value of CS will increase the fault time out time
but will also increase the Soft−Start time.
9) Input Capacitor
The input capacitor reduces the peak currents drawn from
the input supply and reduces the noise and ripple voltage on
the VCC and VC pins. This capacitor must also ensure that
the VCC remains above the UVLO voltage in the event of an
output short circuit. A low ESR capacitor of at least 100 mF
is good. A ceramic surface mount capacitor should also be
connected between VCC and ground to filter high frequency
noise.
10) MOSFET Selection
The CS51031 drives a P−Channel MOSFET. The VGATE
pin swings from GND to VC. The type of P−Ch FET used
depends on the operating conditions but for input voltages
below 7.0 V a logic level FET should be used.
A P−Ch FET with a continuous drain current (ID) rating
greater than the maximum output current is required.
The Gate−to−Source voltage VGS and the Drain−to
Source Breakdown Voltage should be chosen based on the
input supply voltage.
The power dissipation due to the conduction losses is
given by:
PD + IOUT2 RDS(ON) D
where
RDS(ON) is the value at TJ + 100°C
The power dissipation of the P−Ch FET due to the
switching losses is given by:
PD + 0.5 VIN IOUT (tr) fSW
where tr = Rise Time.
11) Diode Selection
The flyback or catch diode should be a Schottky diode
because of it’s fast switching ability and low forward voltage
drop. The current rating must be at least equal to the
maximum output current. The breakdown voltage should be
at least 20 V for this 12 V application.
The diode power dissipation is given by:
PD + IOUT VD (1.0 * D min)
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