LT1931 Linear Technology, LT1931 Datasheet - Page 7

• Manufacturer Part Number: LT1931
• Description: 1.2MHz/2.2MHz Inverting DC/DC Converters in ThinSOT
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
CURRENT
CURRENT
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
LOAD
LOAD
Figure 5. Transient Response of Inverting Converter with 22 F
Tantalum Output Capacitor and No Phase Lead Capacitor
AC COUPLED
AC COUPLED
AC COUPLED
AC COUPLED
20mV/DIV
I
I
L1A
L1A
0.5A/DIV
0.1V/DIV
0.5A/DIV
200mA
100mA
200mA
100mA
Figure 4. Transient Response of Inverting Converter
with 220pF Phase Lead Capacitor
+ I
+ I
V
V
OUT
OUT
L1B
L1B
OUT
U
Figure 7. R
being far from its final value. The
V
SS
U
100 s/DIV
50 s/DIV
33nF/68nF
V
1N4148
5V
IN
R
15k
D2
C
SS
SS
SS
V
and C
W
OUT
+
SS
CURRENT
C1: TAIYO YUDEN X5R JMK212BJ475MG
C2: TAIYO YUDEN X5R LMK212BJ105MG
C3: TAIYO YUDEN XR5 JMK325BJ226MM
D1: ON SEMICONDUCTOR MBR0520
L1: SUMIDA CLS62-100
PROBE
at SHDN Pin Provide Soft-Start to LT1931 Inverting Converter
C1
4.7 F
1931 F04
1931 F05
U
V
SHDN
IN
LT1931
10 H
L1A
GND
NFB
SW
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 . 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 s. 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
as shown in Figure 7, can be used to limit inrush current
to a lower value. Figure 8 shows the relevant waveforms
with R
at V
to reach final value increases to 1ms. In Figure 9, C
AC COUPLED
V
IN
R1
29.4k
R2
10k
SHDN
0.5A/DIV
Figure 6. Start-Up Waveforms for 5V to – 5V Application
(Figure 1). No Soft-Start Circuit. V
500 s; Input Current Peaks at 800mA
2V/DIV
, is limited to a peak value of 0.5A as the time required
SS
V
1 F
C2
OUT
5V
0V
I
IN
= 15k and C
C4
220pF
D1
SS
10 H
L1B
= 33nF. Input current, measured
500 s/DIV
LT1931/LT1931A
1931 F07
C3
22 F
V
–5V
OUT
OUT
Reaches – 5V in
1931 F06
SS
and C
SS
7
SS
is
,