7

LT1931/LT1931A

1931fa

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

APPLICATIO S I FOR ATIO

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

33

Ω. 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 500

µs. For some applications, this initial inrush

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

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

to a lower value. Figure 8 shows the relevant waveforms

20mV/DIV

AC COUPLED

0.5A/DIV

AC COUPLED

200mA

100mA

LOAD

CURRENT

100

µs/DIV

1931 F04

Figure 4. Transient Response of Inverting Converter

with 220pF Phase Lead Capacitor

0.1V/DIV

AC COUPLED

0.5A/DIV

AC COUPLED

200mA

100mA

LOAD

CURRENT

50

µs/DIV

1931 F05

Figure 5. Transient Response of Inverting Converter with 22

µF

Tantalum Output Capacitor and No Phase Lead Capacitor

5V

–5V

1931 F07

SW

L1A

10

µH

CURRENT

PROBE

L1B

10

µH

D1

GND

LT1931

C1

4.7

µF

15k

D2

1N4148

C3

22

µF

C2

1

µF

R2

10k

R1

29.4k

C4

220pF

NFB

SHDN

+

33nF/68nF

C1: TAIYO YUDEN X5R JMK212BJ475MG

C2: TAIYO YUDEN X5R LMK212BJ105MG

C3: TAIYO YUDEN XR5 JMK325BJ226MM

D1: ON SEMICONDUCTOR MBR0520

L1: SUMIDA CLS62-100

2V/DIV

0.5A/DIV

AC COUPLED

5V

0V

500

µs/DIV

1931 F06

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

500

µs; Input Current Peaks at 800mA