MB3775(1994) Просмотр технического описания (PDF) - Fujitsu

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MB3775
(Rev.:1994)

Switching Regulator Controller

Fujitsu 

MB3817

s EQUIVALENT SERIES RESISTOR AND STABILITY OF SMOOTHING CAPACITOR

The equivalent series resistance (ESR) of a smoothing capacitor in a DC/DC converter greatly affects the phase

characteristics of the loop depending on its value.

System stability is improved by ESR because it causes the phase to lead that of the ideal capacitor in high-

frequency regions. (See Figures 2 and 3) Conversely, if a low-ESR smoothing capacitor is used, system stability

deteriorates. Therefore, use of a low-ESR semiconductor electrolytic capacitors (ex. OS–CON) or tantalum

capacitors calls for careful attention.

• Figure 1 Basic circuit of stepdown DC/DC converter

Tr

L

RC

V IN

D

RL

C

• Figure 2 Gain-Frequency characteristic

• Figure 3 Phase-Frequency characteristic

20

0

−20

(2)

−40

(1) : RC = 0 Ω

(2) : RC = 31 mΩ

(1)

−60

10

100

1k

10 k

Frequency f (Hz)

100 k

0

(2)

−90

(1) : RC = 0 Ω

(1)

(2) : RC = 31 mΩ

−180

10

100

1k

10 k

100 k

Frequency f (Hz)

21

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