NCP1587 ONSEMI [ON Semiconductor], NCP1587 Datasheet - Page 11

• Manufacturer Part Number: NCP1587
• Description: Low Voltage Synchronous Buck Controller
• Manufacturer: ONSEMI [ON Semiconductor]

DESIGN EXAMPLE II: Type III Compensation
(Oscon Cap. with small ESR; Do not place R
Choose the loop gain crossover frequency;
The corner frequency of the output filter is calculated below;
Check the ESR zero frequency;
Choose C
gain magnitude, one zero position and the soft start. By
adjusting the value of this compensation capacitor, the
crossover frequency and the soft start time can be adjusted.
Zeros of the compensation network are calculated as follows;
Switching Frequency
Output Capacitance
Output Capacitance
Output Inductance
Input Voltage
Output Voltage
The compensation capacitor (C
1st zero;
F
F
F
LC
ESR
ESR
C1
+
for the soft start
+
+
2
2
2
F
co
p
+ 1
p
p
C
5
C1
1 mH
R
7 mW
1
1
ESR
+ 33 nF
F
1
sw
Figure 13. Closed-loop Voltage Loop-gain of the NCP1587
+ 55 KHz
1120 mF
C
C1
560 mF
O
F
R
C
L
V
V
) is related to the loop
sw
ESR
out
out
out
in
= 12 V
= 275 KHz
= 1 mH
= 1.6 V
= 2×560 mF
+ 4.7 KHz
+ 40.6 KHz
= 7 mW/Each
C
, C
C
http://onsemi.com
, C
P
)
11
Blue curve: Gain-Frequency
Red curve: Gain-Frequency
(Phase margin = 61.417 degree, Gain margin = 9.347 dB)
stable system with transconductance amplifier. Ù choose
R
much larger than 2/gm for the stable system.
Poles of the compensation network are calculated as follows;
C1
R
2nd zero;
Choose R3 for the crossover frequency. R3 should be
Choose C20 = 3.3 nF
1st pole;
Choose R4 to cancel the output capacitor ESR zero.
C1
= 12.1 kW
should be much larger than 2/gm in order to get the
R3 + 10 kW
F
C20 +
R4 +
R
z2
C1
+
+ F
+
+
+
2
2
2
2
F
LC
2
2
P1
F
p
p
Z1
+ 4.7 KHz
+ F
p
p
p
p
+
F
40.6 kHz
1
1
ESR
F
F
4.7 KHz
P1
10
F
1
470 Hz
LC
z2
1
z1
1
+ 40.6 KHz
1
+ 470 Hz
C20
R3
C
C1
3.3 n
10 kW
30 nF
+ 1.2 kW
+ 11.3 kW
+ 3.4 nF