Fujitsu MB3788 manual DC/DC Converter a V φ Characteristic Measurement Diagram

MB3788

(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).

		VOUT	VO+
	CNF		AV - φ characteristic between VOUT and VIN
	+	-IN	R2
			VIN
FB		+IN
	-
			R1
	VREF/2
	Error amplifier
Fig. 10 DC/DC Converter AV - φ Characteristic Measurement Diagram

Aluminum electrolytic capacitor gain - frequency and phase - frequency characteristics (DC/DC converter +5 V output)

60

V

CC

=

10 V

RL = 25 Ω

40

CP = 0.1 ∝

F

180

AV

20

ϕ⇒

90

VO+

Gain

+

(dB)

62

°

Phase

0

0 (deg)

-

GND

-20

-90

Aluminum electrolytic capacitor

-40

-180

220 ∝F (16 V)

Rc ≈ 1.0 Ω: fOSC = 1 kHz

101001 k 10 k 100 k

Frequency f (Hz)

Fig. 11 Gain - Frequency Characteristic

OS capacitor gain - frequency and phase - frequency characteristics (DC/DC converter +5 V output)

60
			VCC = 10 V
40	AV		RL = 25 Ω			180
			CP = 0.1	∝	F
20						90			VO+
		ϕ⇒
Gain
(dB)							Phase	+
0						0
		27					(deg)	-	GND
-20						-90

OS capacitor

22 ∝F (16 V)

-40-180Rc ≈ 1.2 Ω: fOSC = 1 kHz 101001 k 10 k 100 k

Frequency f (Hz)

Fig.12 Phase - Frequency Characteristic Curves

17