Mb3788 – FUJITSU MB3788 User Manual
Page 17
MB3788
17
(Reference Data)
The phase margin is halved by changing the smoothing capacitor from an aluminum electrolytic capacitor (Rc
= 1.0
Ω
) to a semiconductor electrolytic capacitor (OS capacitor: Rc = 0.2
Ω
) with a low ESR (Figures 11 and 12).
Fig. 10 DC/DC Converter A
V
-
φφφφ
Characteristic Measurement Diagram
Fig. 11 Gain - Frequency Characteristic
Fig.12 Phase - Frequency Characteristic Curves
+
-
A
V
-
φ
characteristic between V
OUT
and V
IN
Error amplifier
V
OUT
C
NF
FB
V
IN
R
2
R
1
V
O
+
-
IN
+IN
V
REF/2
62
°
A
V
ϕ⇒
101001 k 10 k 100 k
60
40
20
0
-20
-40
180
90
0
-90
-180
V
CC
= 10 V
RL = 25
Ω
CP = 0.1
µ
F
Gain
(dB)
Frequency f (Hz)
Aluminum electrolytic capacitor gain - frequency and phase - frequency characteristics (DC/DC converter +5 V output)
Aluminum electrolytic capacitor
220
µ
F (16 V)
Rc
≈
1.0
Ω
: f
OSC
= 1 kHz
V
O
+
+
-
Phase
(deg)
GND
A
V
ϕ⇒
101001 k 10 k 100 k
60
40
20
0
-20
-40
180
90
0
-90
-180
27
°
V
CC
= 10 V
RL = 25
Ω
CP = 0.1
µ
F
Gain
(dB)
Frequency f (Hz)
OS capacitor gain - frequency and phase - frequency characteristics (DC/DC converter +5 V output)
OS capacitor
22
µ
F (16 V)
Rc
≈
1.2
Ω
: f
OSC
= 1 kHz
V
O
+
+
-
GND
Phase
(deg)