LT1931/LT1931A
W U U
APPLICATIO S I FOR ATIO
U
VOUT
20mV/DIV
AC COUPLED
VOUT
2V/DIV
IL1A + IL1B
0.5A/DIV
AC COUPLED
IIN
0.5A/DIV
AC COUPLED
200mA
100mA
LOAD
CURRENT
5V
0V
VSHDN
100µs/DIV
1931 F04
500µs/DIV
1931 F06
Figure 4. Transient Response of Inverting Converter
with 220pF Phase Lead Capacitor
Figure 6. Start-Up Waveforms for 5V to –5V Application
(Figure 1). No Soft-Start Circuit. V Reaches –5V in
OUT
500µs; Input Current Peaks at 800mA
VOUT
0.1V/DIV
regulatortriestochargeuptheoutputcapacitorasquickly
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
atVIN,islimitedtoapeakvalueof0.5Aasthetimerequired
to reach final value increases to 1ms. In Figure 9, CSS is
AC COUPLED
IL1A + IL1B
0.5A/DIV
AC COUPLED
200mA
100mA
LOAD
CURRENT
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
C2
CURRENT
PROBE
L1A
10µH
L1B
1µF
10µH
V
IN
5V
D1
+
V
IN
SW
C1
4.7µF
V
OUT
–5V
R1
29.4k
C4
220pF
R
LT1931
SS
15k
C3
22µF
V
SHDN
NFB
SS
GND
R2
10k
D2
1N4148
C
SS
C1: TAIYO YUDEN X5R JMK212BJ475MG
C2: TAIYO YUDEN X5R LMK212BJ105MG
C3: TAIYO YUDEN XR5 JMK325BJ226MM
D1: ON SEMICONDUCTOR MBR0520
L1: SUMIDA CLS62-100
33nF/68nF
1931 F07
V
OUT
Figure 7. R and C at SHDN Pin Provide Soft-Start to LT1931 Inverting Converter
SS
SS
1931fa
7
Linear [ Linear ]
