Fujitsu MB3788 manual Basic Voltage-Drop Type DC/DC Converter Circuit

MB3788

■EQUIVALENT SERIES RESISTANCE OF SMOOTHING CAPACITOR AND STABILITY OF DC/DC CONVERTER

The equivalent series resistance (ESR) of the smoothing capacity in a DC/DC converter has a great effect on the loop phase characteristics.

The ESR causes a small delay at the capacitor with a series resistance of 0 (Figures 8 and 9), thus improving system stability. On the other hand, using a smoothing capacitor with a low ESR reduces system stability. Therefore, attention should be paid to using semiconductor electrolytic capacitors (such as OS capacitors) or tantalum capacitors with a low ESP. (Phase margin reduction by using an OS capacitor is explained on the next page.)

	Tr	L
		RC
VIN	D	RL
		C

Fig. 7 Basic Voltage-Drop Type DC/DC Converter Circuit

	20
	0
Gain	-20				(2)
(dB)
	-40	(1): RC = 0	Ω
		(2): RC = 31 mΩ		(1)
	-60
	10	100	1k	10k	100k

Frequency f (Hz)

0
				(2)
Phase -90
(deg)
(1): RC = 0 Ω				(1)
(2): RC = 31 mΩ
-180
10	100	1k	10k	100k
		Frequency f (Hz)

Fig.8 Gain - Frequency Characteristic Fig.9 Phase - Frequency Charecteristic

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