LT1308A/LT1308B
U
W U U
APPLICATIONS INFORMATION
when operating from a battery composed of alkaline
cells. The inrush current may cause sufficiency internal
voltage drop to trigger a low-battery indicator. A pro-
grammablesoft-startcanbeimplementedwith4discrete
components. A 5V to 12V boost converter using the
VOUT
1V/DIV
IL1
2A/DIV
LT1308B is detailed in Figure 16. C4 differentiates VOUT
,
VSHDN
5V/DIV
500μs/DIV
1308 F14
causing a current to flow into R3 as VOUT increases.
When this current exceeds 0.7V/33k, or 21μA, current
flows into the base of Q1. Q1’s collector then pulls
currentouttheVC pin, creatingafeedbackloopwherethe
slope of VOUT is limited as follows:
Figure 14. 5V Boost Converter of Figure 1.
Start-Up from 3V Input into 5Ω Load
VOUT
2V/DIV
ΔVOUT
Δt
0.7V
33k •C4
=
ISW
With C4 = 33nF, VOUT/t is limited to 640mV/ms. Start-up
waveforms for Figure 16’s circuit are pictured in Figure
17.Withoutthesoft-startcircuitimplemented,theinrush
current reaches 3A. The circuit reaches final output
voltage in approximately 250μs. Adding the soft-start
components reduces inductor current to less than 1A, as
detailedinFigure18,whilethetimerequiredtoreachfinal
output voltage increases to about 15ms. C4 can be
adjusted to achieve any output slew rate desired.
2A/DIV
VSHDN
5V/DIV
500μs/DIV
1308 F15
Figure 15. 5V SEPIC Start-Up from 9V Input into 10Ω Load
Soft-Start
In some cases it may be undesirable for the LT1308A/
LT1308B to operate at current limit during start-up, e.g.,
L1
4.7μH
D1
V
OUT
V
IN
5V
12V
500mA
V
SW
IN
+
SHDN
SHUTDOWN
C1
47μF
LT1308B
330pF
11.3k
100k
10k
C2
10μF
FB
GND
V
C
C4
33nF
R4
33k
R
C
Q1
47k
C
C
R3
33k
100pF
SOFT-START
COMPONENTS
C1: AVX TAJ476M010
1308 F16
C2: TAIYO YUDEN TMK432BJ106MM
D1: IR 10BQ015
L1: MURATA LQH6C4R7
Q1: 2N3904
Figure 16. 5V to 12V Boost Converter with Soft-Start Components Q1, C4, R3 and R4.
1308abfa
12
Linear [ Linear ]
