ncp3101c ON Semiconductor, ncp3101c Datasheet - Page 20

• Manufacturer Part Number: ncp3101c
• Description: Ncp3101c Wide Input Voltage Synchronous Buck Converter
• Manufacturer: ON Semiconductor

used to calculate the transconductance output compensation
network as follows:
C
F
gm
R
R
C
CO
C
f
F
R
CO
C
C
R
Calculating Soft−Start Time
To calculate the soft start delay and soft start time, the
following equations can be used.
C
C
I
The time the output voltage takes to increase from 0 V to a
regulated output voltage is t
cross
SS
PO
LC
C
43.3 nF +
C
1
2
R
5.05 kW +
C
OUT
C
OUT
P
C
P
C
The cross over combined compensation network can be
C
C
ESR
ESR
+
+
2 * 2.35 kHz * 43.3 nF *
F
2 * F
C
309 pF + 820 mF *
t
3.59 ms +
1
PO
SSdelay
P
*
18.9 kHz
LC
+ C
R
= Compensation capacitor
= Pole frequency
= Transconductance of amplifier
= Top of resistor divider
= Bottom of resistor divider
= Crossover frequency
= Compensation resistor
= Compensation resistor
= Soft start current
= Compensation capacitance
= Output capacitor ESR
= Output capacitance
= Output inductor and capacitor frequency
= Output capacitor ESR
= Output capacitor
= Compensation pole capacitor
= Compensation pole capacitor
= Compensation capacitor
* C
2
R
1
* R
OUT
+
2
C
1
*
C
0.309 nF ) 43 nF * 0.83 V
* gm ³
*
*
P
10 kW ) 31.6 kW
2
R
2
CO
) C
) f
C
* 2 * p
ESR
I
cross
1
5.05 kW * 2 * p
SS
10 kW
ss
C
2
2
as shown in Equation 50:
10 mA
* 0.9 V
) 27 kHz * 12 mW * 820 mF
1
12 mW
³
* CO
ESR
* 3.4 mS
* C
OUT
³
(eq. 46)
(eq. 47)
(eq. 48)
(eq. 49)
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20
C
C
D
I
t
V
to the bottom of the ramp is considered t
delay time is the addition of the current set delay and t
which in this case is 3.2 ms and 3.59 ms respectively, for a
total of 6.79 ms.
Calculating Input Inrush Current
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 32 and calculated as:
SS
SS
P
C
ramp
The delay from the charging of the compensation network
The input inrush current has two distinct stages: input
1.31 ms +
Figure 32. Input Charge Inrush Current
Vcomp
t
SS
Vout
IPK
+
= Soft−start current
= Compensation pole capacitor
= Compensation capacitor
= Duty ratio
= Soft−start interval
= Peak−to−peak voltage of the ramp
Figure 31. Soft Start Ramp
0.309 nF ) 43 nF * 27.5% * 1.1 V
C
P
) C
C
I
SS
* D * V
TIME
V
10 mA
ramp
900 mV
³
ss
delay
. The total
(eq. 50)
ss
delay
,