Applications Information: continued
7) Divider bypass Capacitor Crr
CS ´ (2.5V - 1.5V)
tCharge(t)
=
Since the feedback resistors divide the output voltage by a
factor of 4, i.e. 5V/1.25V= 4, it follows that the output ripple
is also divided by four. This would require that the output
ripple be at least 60mV (4 ´ 15mV) to trip the feedback com-
parator. We use a capacitor Crr to act as an AC short .
ICharge
Where ICharge is 264µA typical.
tCharge(t) = CS ´ 3787
The fault time is given by:
tFault = CS ´ (3787 + 1515 + 1.5 ´ 105)
Fault = CS ´ (1.55 ´ 105)
The ripple voltage frequency is equal to the switching fre-
quency so we choose Crr = 1nF.
t
8) Soft start and Fault timing capacitor CS.
For this circuit
CS performs several important functions. First it provides a
delay time for load transients so that the IC does not enter
a fault mode every time the load changes abruptly.
Secondly it disables the fault circuitry during startup, it
also provides soft start by clamping the reference voltage
during startup, allowing it to rise slowly, and, finally it
controls the hiccup short circuit protection circuitry. This
reduces the duty cycle to approximately 0.035 during short
circuit conditions.
tFault = 0.1 ´ 10-6 ´ 1.55 ´ 105 = 15.5µS
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
100µF is good. A ceramic surface mount capacitor should
also be connected between VCC and ground to filter high
frequency noise.
An important consideration in calculating CS is that itÕs
voltage does not reach 2.5V (the voltage at which the fault
detect circuitry is enabled) before VFB reaches 1.15V other-
wise the power supply will never start.
If the VFB pin reaches 1.15V, the fault timing comparator
will discharge CS and the supply will not start. For the VFB
voltage to reach 1.15V the output voltage must be at least
4 ´ 1.15 = 4.6V.
10) MOSFET Selection
The CS51031 drives 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.
If we choose an arbitrary startup time of 900µs, the value of
CS is:
A PFET with a continuous drain current (ID) rating greater
than the maximum output current is required.
CS ´ 2.5V
ICharge
t Startup
=
The Gate-to-Source voltage VGS and the Drain-to Source
Breakdown Voltage should be chosen based on the input
supply voltage.
900µs ´ 264µA
CSmin =
= 950nF @ 0.1µF
2.5V
The power dissipation due to the conduction losses is
given by:
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.
PD = IOUT2 ´ RDS(ON) ´ D where
RDS(ON) is the value at TJ = 100ûC.
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
given by:
CS ´ (2.4V - 1.5V)
The power dissipation of the PFET due to the switching loss-
es is given by:
tSlowDischarge(t)
=
IDischarge
PD = 0.5 ´ VIN ´ IOUT ´ (tr ) ´ fSW
Where IDischarge is 6µA typical.
tSlowDischarge(t) = CS ´ 1.5 ´ 105
Where tr = Rise Time.
The fast discharge time occurs when a fault is first detect-
ed. The CS capacitor is discharged from 2.5V to 2.4V.
11) Diode Selection
The flyback or catch diode should be a Schottky diode
because of itÕs 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.
CS ´ (2.5V - 2.4V)
tFastDischarge(t)
=
IFastDischarge
Where I FastDischarge is 66µA typical.
tFastDischarge(t) = CS ´ 1515
The diode power dissipation is given by:
PD = IOUT ´ VD ´ (1 - Dmin
)
The recharge time is the time for CS to charge from 1.5V to
2.5V.
7
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
