NCP1547 데이터 시트보기 (PDF) - ON Semiconductor

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NCP1547

1.5 A, 340 kHz, Buck Regulator with Synchronization Capability

ON Semiconductor 

NCP1547

them for some low cost designs. Electrolytic capacitors are

physically large, and not used in applications where the size,

and especially height is the major concern.

Ceramic capacitors are now available in values over 10 mF.

Since the ceramic capacitor has low ESR and ESL, a single

ceramic capacitor can be adequate for both low frequency

and high frequency noises. The disadvantage of ceramic

capacitors are their high cost. Solid tantalum capacitors can

have low ESR and small size. However, the reliability of the

tantalum capacitor is always a concern in the application

where the capacitor may experience surge current.

Output Capacitor

In a buck converter, the requirements on the output

capacitor are not as critical as those on the input capacitor.

The current to the output capacitor comes from the inductor

and thus is triangular. In most applications, this makes the

RMS ripple current not an issue in selecting output

capacitors.

The output ripple voltage is the sum of a triangular wave

caused by ripple current flowing through ESR, and a square

wave due to ESL. Capacitive reactance is assumed to be

small compared to ESR and ESL. The peak to peak ripple

current of the inductor is:

IP

*

P

+

VO(VIN * VO)

(VIN)(L)(fS)

VRIPPLE(ESR), the output ripple due to the ESR, is equal

to the product of IP−P and ESR. The voltage developed

across the ESL is proportional to the di/dt of the output

capacitor. It is realized that the di/dt of the output capacitor

is the same as the di/dt of the inductor current. Therefore,

when the switch turns on, the di/dt is equal to (VIN − VO)/L,

and it becomes VO/L when the switch turns off. The total

ripple voltage induced by ESL can then be derived from

VRIPPLE(ESL)

+

ESL(VLO)

)

ESL(VIN

*

L

VO)

+

ESL(VLIN)

The total output ripple is the sum of the VRIPPLE(ESR) and

VRIPPLE(ESL).

Figure 15. The Output Voltage Ripple Using Two 10 mF

Ceramic Capacitors in Parallel

Figure 16. The Output Voltage Ripple Using One

100 mF POSCAP Capacitor

Figure 17. The Output Voltage Ripple Using

One 100 mF OS−CON

Figure 18. The Output Voltage Ripple Using

One 100 mF Tantalum Capacitor

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