NCP3126
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18
Assuming an output capacitance of 470 mF in parallel with
22 mF with a crossover frequency of 35 kHz, the
compensation values for common output voltages can be
calculated as shown in Table 6:
Table 6. COMPENSATION VALUES
V
in
(V)
V
out
(V)
L
out
(mF)
Cf
(nF)
Cc
(nF)
Rc
(kW)
Cp
(nF)
12 0.8 3.3 NI 180 0.357 2.7
12 1.0 3.3 0.180 120 0.442 2.7
12 1.1 3.3 0.180 120 0.475 2.2
12 1.2 4.7 0.180 120 0.787 2.2
12 1.5 4.7 0.180 120 0.909 1.8
12 1.8 6.8 0.180 100 1.5 1.2
12 2.5 6.8 0.220 100 1.87 1
12 3.3 8.2 0.220 100 2.05 1
12 5.0 10 0.220 100 2.5 1.2
5 0.8 3.3 NI 100 0.887 1.8
5 1.0 3.3 0.180 82 1.1 1.5
5 1.1 3.3 0.180 82 1.65 0.82
5 1.2 4.7 0.180 82 1.82 0.82
5 1.5 4.7 0.180 82 2.21 0.82
5 1.8 4.7 0.180 82 2.61 0.82
5 2.5 4.7 0.180 82 3.4 0.82
Calculating Soft−Start Time
To calculate the soft−start delay and soft−start time, the
following equations can be used.
t
SSdelay
+
ǒ
C
P
) C
C
Ǔ
0.9 V
I
SS
³
(eq. 45)
7.45 ms +
ǒ
2.83 nF ) 80 nF
Ǔ
0.9 V
10 mA
C
P
= Compensation pole capacitor
C
C
= Compensation capacitor
I
SS
= Soft−start current
The time the output voltage takes to increase from 0 V to
a regulated output voltage is t
ss
as shown in Equation 46:
t
SS
+
ǒ
C
P
) C
C
Ǔ
D V
ramp
I
SS
(eq. 46)
2.51 ms +
ǒ
2.83 nF ) 80 nF
Ǔ
27.5% 1.1 V
10 mA
C
P
= Compensation pole capacitor
C
C
= Compensation capacitor
D = Duty ratio
I
SS
= Soft−start current
t
SS
= Soft−start interval
V
ramp
= Peak−to−peak voltage of the ramp
V
900 mV
Vcomp
Vout
Figure 25. Soft−Start Ramp
The delay from the charging of the compensation network
to the bottom of the ramp is considered t
ssdelay
. The total
delay time is the addition of the current set delay and t
ssdelay
,
which in this case is 9 ms and 7.45 ms respectively, for a
total of 16.45 ms.
Calculating Input Inrush Current
The input inrush current has two distinct stages: input
charging and output charging. The input charging of a buck
stage is usually not controlled, and is limited only by the
input RC network, and the output impedance of the upstream
power stage. If the upstream power stage is a perfect voltage
source, then the input charge inrush current can be depicted
as shown in Figure 26 and calculated as:
IPK
Figure 26. Input Charge Inrush Current
I
ICinrush_PK
1 +
V
IN
CIN
ESR
(eq. 47)
120 A +
12
0.1
