Applications Information: continued
to rise slowly and finally it controls the Hiccup short circuit
protection circuitry. This function reduces the PFET's duty
cycle to 2% of the CS period.
For this circuit
TFAULT = 0.1 × 10-6 × 1.55 × 105 = 0.0155
The most important consideration in calculating CS is that
it’s voltage does not reach 2.5V (the voltage at which the
fault detect circuitry is enabled) before VFB reaches 1.15V
otherwise the power supply will never start.
A larger value of CS will increase the fault time out time but
will also increase the soft start time.
8) Input Capacitor.
If the VFB pin reaches 1.15V the fault timing comparator will The input capacitor reduces the peak currents drawn from
discharge CS and the supply will not start. For the VFB volt-
age to reach 1.15V the output voltage must be at least 4 ×
1.15 = 4.6V.
If we choose an arbitrary startup time of 200µs we calculate
the value of CS 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. CIN should be a low ESR capacitor
of at least 100µf. A ceramic surface mount capacitor should
also be connected between VCC and ground to prevent
spikes.
CS × 2.5V
T =
ICHARGE
9) MOSFET Selection
200µs × 264µA
CS(min)
=
= 0.02µF
The CS51033 drive a P-channel MOSFET. The VGATE pin
swings from Gnd to VC. The type of PFET used depends on
the operating conditions but for input voltages below 7V a
logic level FET should be used.
2.5V
Use 0.1µf.
The fault time out time is the sum of the slow discharge
time the fast discharge time and the recharge time and is
obviously 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 2.4V to 1.5V and is
Choose a PFET with a continuous drain current (Id) rating
greater than the maximum output current. RDS(on) should be
less than
0.6V
RDS < =
167mΩ
IOUT(max)
given by:
CS × (2.4V-1.5V)
TSLOWDISCHARGE
=
ΙDISCHARGE
The Gate-to-Source voltage VGS and the Drain-to Source
Breakdown Voltage should be chosen based on the input
supply voltage.
Where IDISCHARGE is 6µA typical.
TSLOWDISCHARGE = CS × 1.5V × 105
The power dissipation due to the conduction losses is given
by:
PD = OUT2 × RDS(on) × D
The fast discharge time occurs when a fault is first detected.
The CS capacitor is discharged from 2.5V to 2.4V.
CS × (2.5V - 2.4V)
TFASTDISCHARGE
=
The power dissipation due to the switching losses is given
by:
ΙFASTDISCHARGE
r
Where IFASTDISCHARGE is 66µA typical.
PD = 0.5 × VIN × IOUT × (TR + TF) × FSW
TFASTDISCHARGE = CS × 1515
Where tr =Rise Time and tf= Fall Time.
The recharge time is the time for CS to charge from 1.5V to
2.5V.
10) Diode Selection.
CS × (2.5V-1.5V)
TCHARGE
=
The flyback or catch diode should be a Schottky diode
because of it’s fast switching ability and low forward volt-
age drop. The current rating must be at least equal to the
maximum output current. The breakdown voltage should
be at least 20V for this 12V application.
ΙCHARGE
Where ICHARGE is 264µA typical.
TCHARGE = CS × 3787
The diode power dissipation is given by:
The fault time out time is given by:
TFAULT = CS × (3787 + 1515 + 1.5 × 105)
TFAULT = CS × 1.55 × 105
PD = IOUT × VD × (1-DMIN
)
7
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
