CS51031 Datasheet, PDF (8/12 Pages) Cherry Semiconductor Corporation – Fast PFET Buck Controller Does Not Require Compensation

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 I_Discharge is 6.0 µA 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 I_{FastDischarge} is 66 µA 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 I_Charge is 264 µA 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 V_CC and V_C pins. This capacitor must also ensure that
the V_CC remains above the UVLO voltage in the event of an
output short circuit. A low ESR capacitor of at least 100 µF
is good. A ceramic surface mount capacitor should also be
connected between V_CC and ground to filter high frequency
noise.
10) MOSFET Selection
The CS51031 drives a P–channel MOSFET. The V_GATE
pin swings from GND to V_C. The type of PFET used
depends on the operating conditions but for input voltages
below 7.0 V a logic level FET should be used.
A PFET with a continuous drain current (I_D) rating greater
than the maximum output current is required.
The Gate–to–Source voltage V_GS 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 PFET due to the switching
losses is given by:
PD + 0.5 VIN IOUT (tr) fSW
where t_r = 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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