MB3775 Datasheet, PDF (20/25 Pages) Fujitsu Component Limited. – SWITCHING REGULATOR CONTROLLER

MB3775
3. Measures for ensuring system stability when a low ESR smoothing capacitor is used
When a low ESR smoothing capacitor is used in the DC/DC converter, only the L and C are apparent even in the high-frequency region,
and the phase is delayed by almost 1805. Consequently, the system phase margin and stability are reduced. On the other hand, a
low ESR capacitor is needed to reduce the amount of output ripple. This is contrary to the system stability explained above.
To solve this problem, phase compensation can be used. This method increases the phase margin by advancing the phase when the
phase margin is reduced by a low ESR capacitor.
The three suggestions listed below are recommended for DC/DC converters using the MB3775.
(1) As shown in Fig. 35, a capacitor is connected in parallel with the output feedback resistor to advance the phase. Use the
formula below as a guideline for the capacitance.
1
C1
2πfR2
Unstable Frequency (See Fig. 32)
Fig. 35 - External circuit example1 to advance the phase
C1
R2
+ IN
+
- IN
-
R1
VREF
V0+
FB
CP
Fig. 36 - DC/DC Converter +5 V output
60
40
AV
20
VCC=10V
RL=25Ω
0 ^CP=0.1µF
Smoothing Capacitor
22µF OS-CON
-20
C1=4700pF
R1=1.8kΩ
R2=5.6Ω
-40
10
100
180
ϕ
90
66°
0
-90
1K
10K
Frequency f(Hz)
-180
100K
20