LT1931 データシートの表示（PDF） - Linear Technology

部品番号

コンポーネント説明

一致するリスト

LT1931

1.2MHz/2.2MHz Inverting DC/DC Converters in ThinSOT

Linear Technology 

APPLICATIO S I FOR ATIO

LT1931/LT1931A

VOUT

20mV/DIV

AC COUPLED

IL1A + IL1B

0.5A/DIV

AC COUPLED

LOAD 200mA

CURRENT 100mA

100µs/DIV

1931 F04

Figure 4. Transient Response of Inverting Converter

with 220pF Phase Lead Capacitor

VOUT

0.1V/DIV

AC COUPLED

IL1A + IL1B

0.5A/DIV

AC COUPLED

LOAD 200mA

CURRENT 100mA

50µs/DIV

1931 F05

Figure 5. Transient Response of Inverting Converter with 22µF

Tantalum Output Capacitor and No Phase Lead Capacitor

START-UP/SOFT-START

For most LT1931/LT1931A applications, the start-up in-

rush current can be high. This is an inherent feature of

switching regulators in general since the feedback loop is

saturated due to VOUT being far from its final value. The

VOUT

2V/DIV

IIN

0.5A/DIV

AC COUPLED

VSHDN

5V

0V

500µs/DIV

1931 F06

Figure 6. Start-Up Waveforms for 5V to – 5V Application

(Figure 1). No Soft-Start Circuit. VOUT Reaches – 5V in

500µs; Input Current Peaks at 800mA

regulator tries to charge up the output capacitor as quickly

as possible, which results in a large inrush current. Fig-

ure 6 shows a typical oscillograph of the start-up wave-

form for the application of Figure 1 starting into a load of

33Ω. The lower waveform shows SHDN being pulsed

from 0V to 5V. The middle waveform shows the input

current, which reaches as high as 0.8A. The total time

required for the output to reach its final value is approxi-

mately 500µs. For some applications, this initial inrush

current may not be acceptable. If a longer start-up time is

acceptable, a soft-start circuit consisting of RSS and CSS,

as shown in Figure 7, can be used to limit inrush current

to a lower value. Figure 8 shows the relevant waveforms

with RSS = 15k and CSS = 33nF. Input current, measured

at VIN, is limited to a peak value of 0.5A as the time required

to reach final value increases to 1ms. In Figure 9, CSS is

CURRENT

L1A

VIN

PROBE

10µH

C2

1µF

5V

+

C1

VIN

SW

4.7µF

RSS

15k

LT1931

R1

29.4k

VSS

SHDN

NFB

GND

R2

10k

D2

1N4148

CSS

33nF/68nF

C1: TAIYO YUDEN X5R JMK212BJ475MG

C2: TAIYO YUDEN X5R LMK212BJ105MG

VOUT C3: TAIYO YUDEN XR5 JMK325BJ226MM

D1: ON SEMICONDUCTOR MBR0520

L1: SUMIDA CLS62-100

C4

220pF

L1B

10µH

D1

VOUT

–5V

C3

22µF

1931 F07

Figure 7. RSS and CSS at SHDN Pin Provide Soft-Start to LT1931 Inverting Converter

1931fa

7

Share Link: