NCP1587GDR2G datasheet(11/16 Pages) ONSEMI

Part #	NCP1587GDR2G

Description	Low Voltage Synchronous Buck Controller

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NCP1587G

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11

Blue curve: Gain-Frequency

Red curve: Gain-Frequency

(Phase margin = 61.417 degree, Gain margin = 9.347 dB)

Figure 13. Closed-loop Voltage Loop-gain of the NCP1587G

DESIGN EXAMPLE II: Type III Compensation

(Oscon Cap. with small ESR; Do not place RC, CC, CP)

Switching Frequency

Fsw = 275 KHz

Output Capacitance

RESR = 7 mW/Each

Output Capacitance

Cout = 2×560 mF

Output Inductance

Lout = 1 mH

Input Voltage

Vin = 12 V

Output Voltage

Vout = 1.6 V

Choose the loop gain crossover frequency;

Fco + 15 Fsw + 55 KHz

The corner frequency of the output filter is calculated below;

FLC +

1

2

p

1 mH

1120 mF

+ 4.7 KHz

Check the ESR zero frequency;

FESR +

1

2

p

RESR CO

FESR +

1

2

p

7mW

560 mF +

40.6 KHz

Choose CC1 for the soft start

CC1 + 33 nF

The compensation capacitor (CC1) is related to the loop

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;

1st zero;

FZ1 +

FLC

10 +

470 Hz

RC1 +

1

2

p

Fz1 CC1

+

1

2

p

470 Hz

30 nF +

11.3 kW

RC1 should be much larger than 2/gm in order to get the

stable system with transconductance amplifier.

Ù choose

RC1 = 12.1 kW

2nd zero;

Choose R3 for the crossover frequency. R3 should be

much larger than 2/gm for the stable system.

R3 + 10 kW

Fz2 + FLC + 4.7 KHz

C20 +

1

2

p

Fz2 R3

+

1

2

p

4.7 KHz

10 kW +

3.4 nF

Choose C20 = 3.3 nF

Poles of the compensation network are calculated as follows;

1st pole;

Choose R4 to cancel the output capacitor ESR zero.

FP1 + FESR + 40.6 KHz

R4 +

1

2

p

FP1 C20

+

1

2

p

40.6 kHz

3.3 n +

1.2 kW

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