MB3782 Datasheet, PDF (23/28 Pages) Fujitsu Component Limited. – Switching Regulator Controller

MB3782
s APPLICATIONS
3. Concerning Equivalent Series Resistance and Stability of Smoothing Capacitors
In DC/DC converters, the equivalent series resistance value (ESR) of smoothing capacitors has a major influence
on loop phase characteristics.
The ESR is a means by which phase characteristics approximate phase relationships to ideal capacitors in high-
frequency bands (see Graph 1-1), thus improving system stability. At the same time, the use of smoothing
capacitors with low ESR reduces system stability, so that care must be taken when using semiconductor
electrolytic capacitors (OS capacitors) or tantalum capacitors with low ESR.
L
Tr
Rc
VIN
D
RL
C
Figure 1 Basic circuit for step-down voltage DC/DC converter
Frequency vs.Gain
20
Frequency vs.phase
0
0
– 20
2
– 40
1 : Rc = 0 Ω
2 : Rc = 31 mΩ
1
– 60
10
100
1k
10 k
Frequency f (Hz)
100 k
2
– 90
1 : Rc = 0 Ω
1
2 : Rc = 31 mΩ
– 180
10
100
1k
10 k
100 k
Frequency f (Hz)
Graph 1 Frequency vs. gain and phase
23