CS51031
Applications Information: continued
7) Divider bypass Capacitor Crr
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 .
The ripple voltage frequency is equal to the switching fre-
quency so we choose Crr = 1nF.
8) Soft start and Fault timing capacitor CS.
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.
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 V
FB
reaches 1.15V other-
wise the power supply will never start.
If the V
FB
pin reaches 1.15V, the fault timing comparator
will discharge C
S
and the supply will not start. For the V
FB
voltage to reach 1.15V the output voltage must be at least
4
´
1.15 = 4.6V.
If we choose an arbitrary startup time of 900µs, the value of
C
S
is:
t
Startup
=
C
S
´
2.5V
I
Charge
t
Charge(t)
=
C
S
´
(2.5V - 1.5V)
I
Charge
Where I
Charge
is 264µA typical.
t
Charge(t)
= C
S
´
3787
The fault time is given by:
t
Fault
= C
S
´
(3787 + 1515 + 1.5
´
10
5
)
t
Fault
= C
S
´
(1.55
´
10
5
)
For this circuit
t
Fault
= 0.1
´
10
-6
´
1.55
´
10
5
= 15.5µS
A larger value of C
S
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
100µF 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 7V 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:
P
D
= I
OUT
2
´
R
DS(ON)
´
D where
R
DS(ON)
is the value at T
J
= 100ûC.
C
S
min =
900µs
´
264µA
= 950nF
@ 0.1µF
2.5V
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 C
S
capacitor to discharge from 2.4V to 1.5V and is
given by:
C
S
´
(2.4V - 1.5V)
t
SlowDischarge(t)
=
I
Discharge
Where I
Discharge
is 6µA typical.
t
SlowDischarge(t)
= C
S
´
1.5
´
10
5
The fast discharge time occurs when a fault is first detect-
ed. The C
S
capacitor is discharged from 2.5V to 2.4V.
C
S
´
(2.5V - 2.4V)
I
FastDischarge
The power dissipation of the PFET due to the switching loss-
es is given by:
P
D
= 0.5
´
V
IN
´
I
OUT
´
(t
r
)
´
f
SW
Where t
r
= Rise Time.
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.
The diode power dissipation is given by:
P
D
= I
OUT
´
V
D
´
(1 - D
min
)
t
FastDischarge(t)
=
Where I
FastDischarge
is 66µA typical.
t
FastDischarge(t)
= C
S
´
1515
The recharge time is the time for C
S
to charge from 1.5V to
2.5V.
7
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
