MB3785A Datasheet, PDF (24/29 Pages) Fujitsu Component Limited. – Switching Regulator Controller (4 Channels plus High-Precision, High-Frequency Capabilities)

MB3785A
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 17 and 19.) Conversely, if a low-ESR smoothing capacitor is used, system stability deterio-
rates. Therefore, use of a low-ESR semiconductor electrolytic capacitors (OS – CON) or tantalum capacitors calls
for careful attention.
Figure 17. Basic Circuit of Stepdown DC/DC Converter
L
Tr
VIN
D
RC
RL
C
Figure 18. Gain-Frequency Characteristic
20
0
–20
(2)
–40
(1) : RC = 0 Ω
(1)
(2) : RC = 31 mΩ
–60
10
100
1k
10 k
Frequency f (Hz)
100 k
Figure 19. Phase-Frequency Characteristic
0
(2)
–90
(1) : RC = 0 Ω
(1)
(2) : RC = 31 mΩ
–180
10
100
1k
10 k
100 k
Frequency f (Hz)
24