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
saturated due to V
OUT
being far from its final value. The
APPLICATIO S I FOR ATIO
WUUU
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 R
SS
and C
SS
,
as shown in Figure 7, can be used to limit inrush current
to a lower value. Figure 8 shows the relevant waveforms
with R
SS
= 15k and C
SS
= 33nF. Input current, measured
at V
IN
, is limited to a peak value of 0.5A as the time required
to reach final value increases to 1ms. In Figure 9, C
SS
is
V
OUT
20mV/DIV
AC COUPLED
I
L1A
+ I
L1B
0.5A/DIV
AC COUPLED
200mA
100mA
LOAD
CURRENT
100µs/DIV
1931 F04
Figure 4. Transient Response of Inverting Converter
with 220pF Phase Lead Capacitor
V
OUT
0.1V/DIV
AC COUPLED
I
L1A
+ I
L1B
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
V
IN
V
IN
5V
V
SS
V
OUT
V
OUT
–5V
1931 F07
SW
L1A
10µH
CURRENT
PROBE
L1B
10µH
D1
GND
LT1931
C1
4.7µF
R
SS
15k
D2
1N4148
C3
22µF
C2
1µF
R2
10k
R1
29.4k
C4
220pF
NFB
SHDN
+
C
SS
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
Figure 7. R
SS
and C
SS
at SHDN Pin Provide Soft-Start to LT1931 Inverting Converter
V
OUT
2V/DIV
I
IN
0.5A/DIV
AC COUPLED
5V
0V
V
SHDN
500µs/DIV 1931 F06
Figure 6. Start-Up Waveforms for 5V to –5V Application
(Figure 1). No Soft-Start Circuit. V
OUT
Reaches – 5V in
500µs; Input Current Peaks at 800mA
