MB3817 Datasheet, PDF (21/29 Pages) Fujitsu Component Limited. – Switching Regulator Controller

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