ncp3170 ON Semiconductor, ncp3170 Datasheet - Page 20

• Manufacturer Part Number: ncp3170
• Description: Ncp3170 Synchronous Pwm Switching Converter
• Manufacturer: ON Semiconductor

To compensate the converter we must first calculate the
current feedback
F
L
M
V
Next the DC gain of the plant must be calculated.
F
G
L
I
M
V
V
calculated as follows:
A
F
I
L
M
V
V
equation:
Vo
VREF
Y
OUT
OUT
SW
SW
SW
OUT
25.1 +
OUT
OUT
IN
IN
OUT
IN
OUT
The un−scaled gain of the converter also needs to be
The amplitude ratio can be calculated using the following
1.254 W +
G +
1.358 W +
0.0088 W
0.0088 W
A +
M +
= Switching Frequency
= Output inductor value
= Current feedback
= Input Voltage
= Switching Frequency
= DC gain of the plant
= Output inductor value
= Output current
= Current feedback
= Input voltage
= Output voltage
= Un−scaled gain
= Switching Frequency
= Output Current
= Output inductor value
= Current feedback
= Input Voltage
= Output Voltage
= Output voltage
I
3.0 A
3.3 V
VO
OUT
= Regulator reference voltage
= Amplitude ratio
Y +
)
)
V
12 V ) 1.83
M*0.5*M
IN
1.358*0.5*1.359
VREF
V
L
4.7 mH
L
) 1.83
1
OUT
OUT
OUT
4.7 mH 500 kHz
1
FSW
³ 0.242 +
V
V
W
V
V
V
FSW
OUT
OUT
IN
@ F
W
500 kHz
V
IN
12 V
SW
3.3 V
3.3 V
500 kHz
12 V
L
@ L
I
OUT
OUT
0.8 V
3.3 V
OUT
3.0 A ) 3.3 V
4.7 mH
) V
FSW
4.7 mH
O
M *
500 kHz
(eq. 34)
(eq. 36)
(eq. 37)
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V
2
O
1.358 *
)
V
O
20
V
2
IN
3.3 V
M
2
frequency as shown in Equation 38:
CO
C
FZ
A
C
F
power stage has created or open loop response of the system.
The next step is to close the loop by considering the feedback
values. The closed loop crossover frequency should be less
than 1/10 of the switching frequency, which would place the
maximum crossover frequency at 50 kHz.
P
OUT
OUT
The ESR of the output capacitor creates a “zero” at the
The two equations above define the bode plot that the
)
ESR
ESR
3.3 V
2.885 kHz +
12 V
2
1.359
FZ
723 kHz +
F
P
= Un−scaled gain
= Output capacitor
= Current mode pole frequency
ESR
+
2p
2p
= Output capacitor ESR
= Output capacitor
= Output capacitor zero ESR frequency
+
(eq. 35)
2p
2p
A
1.358 W
1
CO
C
1
0.005 mW
OUT
ESR
1
³
44 mF
1
C
44 mF
OUT
³
(eq. 38)
(eq. 39)