cs51031 ON Semiconductor, cs51031 Datasheet - Page 8

• Manufacturer Part Number: cs51031
• Description: Hysteretic Pfet Buck Controller
• Manufacturer: ON Semiconductor

fast discharge time and the recharge time. It is dominated by
the slow discharge time.
for the CS capacitor to discharge from 2.4 V to 1.5 V and is
given by:
where I
detected. The CS capacitor is discharged from 2.5 V to 2.4 V.
where I
to 2.5 V.
where I
but will also increase the Soft−Start time.
9) Input Capacitor
the input supply and reduces the noise and ripple voltage on
The fault time is the sum of the slow discharge time the
The first parameter is the slow discharge time, it is the time
The fast discharge time occurs when a fault is first
The recharge time is the time for CS to charge from 1.5 V
The fault time is given by:
For this circuit
A larger value of CS will increase the fault time out time
The input capacitor reduces the peak currents drawn from
t Fault + 0.1
t Fault + CS
Discharge
FastDischarge
Charge
t SlowDischarge(t) +
t FastDischarge(t) +
t SlowDischarge(t) + CS
t Charge(t) +
t FastDischarge(t) + CS
is 264 mA typical.
t Fault + CS
is 6.0 mA typical.
t Charge(t) + CS
is 66 mA typical.
10 *6
( 3787 ) 1515 ) 1.5
CS
CS
CS
( 1.55
1.55
( 2.5 V * 1.5 V )
I FastDischarge
I Charge
I Discharge
( 2.5 V * 2.4 V )
( 2.4 V * 1.5 V )
3787
1.5
10 5 + 15.5 ms
10 5 )
1515
10 5
10 5 )
http://onsemi.com
CS51031
8
the V
the V
output short circuit. A low ESR capacitor of at least 100 mF
is good. A ceramic surface mount capacitor should also be
connected between V
noise.
10) MOSFET Selection
pin swings from GND to V
depends on the operating conditions but for input voltages
below 7.0 V a logic level FET should be used.
greater than the maximum output current is required.
Source Breakdown Voltage should be chosen based on the
input supply voltage.
given by:
where
switching losses is given by:
where t
11) Diode Selection
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 CS51031 drives a P−Channel MOSFET. The V
A P−Ch FET with a continuous drain current (I
The Gate−to−Source voltage V
The power dissipation due to the conduction losses is
The power dissipation of the P−Ch FET due to the
The flyback or catch diode should be a Schottky diode
The diode power dissipation is given by:
CC
CC
r
remains above the UVLO voltage in the event of an
= Rise Time.
and V
R DS(ON) is the value at T J + 100°C
P D + 0.5
P D + I OUT
P D + I OUT 2
C
pins. This capacitor must also ensure that
CC
V IN
and ground to filter high frequency
V D
C
. The type of P−Ch FET used
R DS(ON)
I OUT
( 1.0 * D min )
GS
( t r )
and the Drain−to
D
f SW
D
) rating
GATE