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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
-
φ
φφ
φ
Characteristic Measurement Diagram
Fig. 11 Gain - Frequency Characteristic
Fig.12 Phase - Frequency Characteristic Curves
+
-
A
V
- φ characteristic between VOUT and VIN
Error amplifier
V
OUT
CNF
FB
V
IN
R2
R1
VO+
-
IN
+IN
VREF/2
62°
A
V
ϕ⇒
101001 k 10 k 100 k
60
40
20
0
-20
-40
180
90
0
-90
-180
VCC = 10 V
RL = 25 Ω
CP = 0.1 µF
Gain
(dB)
Frequency f (Hz)
Aluminum electrolytic capacitor gain - frequency and phase - frequency characteristics (DC/DC converter +5 V output)
Aluminum electrolytic capacitor
220 µF (16 V)
Rc ≈ 1.0 Ω: f
OSC = 1 kHz
V
O+
+
-
Phase
(deg)
GND
AV
ϕ⇒
101001 k 10 k 100 k
60
40
20
0
-20
-40
180
90
0
-90
-180
27°
VCC = 10 V
RL = 25 Ω
CP = 0.1 µF
Gain
(dB)
Frequency f (Hz)
OS capacitor gain - frequency and phase - frequency characteristics (DC/DC converter +5 V output)
OS capacitor
22 µF (16 V)
Rc ≈ 1.2 Ω: f
OSC = 1 kHz
V
O+
+
-
GND
Phase
(deg)