NCP1579
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8
APPLICATION SECTION
Input Capacitor Selection
The input capacitor has to sustain the ripple current
produced during the on time of the upper MOSFET, so it
must have a low ESR to minimize the losses. The RMS value
of this ripple is:
Iin
RMS
+ I
OUT
D (1 * D)
Ǹ
,
where D is the duty cycle, Iin
RMS
is the input RMS current,
& I
OUT
is the load current. The equation reaches its
maximum value with D = 0.5. Loss in the input capacitors
can be calculated with the following equation:
P
CIN
+ ESR
CIN
Iin
RMS
2
,
where P
CIN
is the power loss in the input capacitors &
ESR
CIN
is the effective series resistance of the input
capacitance. Due to large dI/dt through the input capacitors,
electrolytic or ceramics should be used. If a tantalum must
be used, it must by surge protected. Otherwise, capacitor
failure could occur.
Calculating Input Start-up Current
To calculate the input start up current, the following
equation can be used.
I
inrush
+
C
OUT
V
OUT
t
SS
,
where I
inrush
is the input current during start-up, C
OUT
is the
total output capacitance, V
OUT
is the desired output voltage,
and t
SS
is the soft start interval.
If the inrush current is higher than the steady state input
current during max load, then the input fuse should be rated
accordingly, if one is used.
Calculating Soft Start Time
To calculate the soft start time, the following equation can
be used.
t
ss
+
(C
p
) C
c
)*DV
I
ss
Where C
c
is the compensation as well as the soft start
capacitor,
C
p
is the additional capacitor that forms the second pole.
I
ss
is the soft start current
DV is the comp voltage from zero to until it reaches
regulation
Vcomp
880 mV
Vout
DV
The above calculation includes the delay from comp
rising to when output voltage starts becomes valid.
To calculate the time of output voltage rising to when it
reaches regulation; DV is the difference between the comp
voltage reaching regulation and 0.88 V.
Output Capacitor Selection
The output capacitor is a basic component for the fast
response of the power supply. In fact, during load transient,
for the first few microseconds it supplies the current to the
load. The controller immediately recognizes the load
transient and sets the duty cycle to maximum, but the current
slope is limited by the inductor value.
During a load step transient the output voltage initial
drops due to the current variation inside the capacitor and the
ESR. ((neglecting the effect of the effective series
inductance (ESL)):
DV
OUT−ESR
+ DI
OUT
ESR
COUT
where V
OUT- ESR
is the voltage deviation of V
OUT
due to the
effects of ESR and the ESR
COUT
is the total effective series
resistance of the output capacitors.
A minimum capacitor value is required to sustain the
current during the load transient without discharging it. The
voltage drop due to output capacitor discharge is given by
the following equation:
DV
OUT−DISCHARGE
+
DI
OUT
2
L
OUT
2 C
OUT
(V
IN
D * V
OUT
)
,
where V
OUT- DISCHARGE
is the voltage deviation of V
OUT
due to the effects of discharge, L
OUT
is the output inductor
value & V
IN
is the input voltage.
It should be noted that ΔV
OUT- DISCHARGE
and
ΔV
OUT- ESR
are out of phase with each other, and the larger
of these two voltages will determine the maximum deviation
of the output voltage (neglecting the effect of the ESL).
Inductor Selection
Both mechanical and electrical considerations influence
the selection of an output inductor. From a mechanical
perspective, smaller inductor values generally correspond to
smaller physical size. Since the inductor is often one of the
largest components in the regulation system, a minimum
inductor value is particularly important in
space-constrained applications. From an electrical
perspective, the maximum current slew rate through the
output inductor for a buck regulator is given by:
SlewRate
LOUT
+
V
IN
* V
OUT
L
OUT
This equation implies that larger inductor values limit the
regulator’s ability to slew current through the output
inductor in response to output load transients. Consequently,
output capacitors must supply the load current until the
inductor current reaches the output load current level. This
results in larger values of output capacitance to maintain
