FUJITSU MB3788 - Manual (Page 16)

MB3788
16
■ ■ ■ ■ 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.)
L
Tr
R C
R L V IN D
C
20
0
-20
-40
-60
10 100 1k 10k 100k
0
-90
-180
10 100 1k 10k 100k
(2)
(1)
(1): R C = 0
Ω
(2): R C = 31 m Ω
(2)
(1)
Gain
(dB)
Frequency f (Hz) Frequency f (Hz)
Phase
(deg)
(1): R C = 0
Ω
(2): R C = 31 m Ω
Fig. 7 Basic Voltage-Drop Type DC/DC Converter Circuit
Fig.8 Gain - Frequency Characteristic Fig.9 Phase - Frequency Charecteristic