Datasheet

LT1308A/LT1308B
12
1308abfb
APPLICATIONS INFORMATION
Soft-Start
In some cases it may be undesirable for the LT1308A/
LT1308B to operate at current limit during start-up, e.g.,
when operating from a battery composed of alkaline cells.
The inrush current may cause suffi ciency internal voltage
drop to trigger a low-battery indicator. A programmable
soft-start can be implemented with 4 discrete compo-
nents. A 5V to 12V boost converter using the LT1308B
is detailed in Figure 16. C4 differentiates V
OUT
, causing
a current to fl ow into R3 as V
OUT
increases. When this
current exceeds 0.7V/33k, or 21µA, current fl ows into
the base of Q1. Q1’s collector then pulls current out the
V
C
pin, creating a feedback loop where the slope of V
OUT
is limited as follows:
ΔV
OUT
Δt
=
0.7V
33k •C4
With C4 = 33nF, V
OUT
/t is limited to 640mV/ms. Start-up
waveforms for Figure 16’s circuit are pictured in Figure 17.
Without the soft-start circuit implemented, the inrush cur-
rent reaches 3A. The circuit reaches fi nal output voltage in
approximately 250µs. Adding the soft-start components
reduces inductor current to less than 1A, as detailed in
Figure 18, while the time required to reach fi nal output
voltage increases to about 15ms. C4 can be adjusted to
achieve any output slew rate desired.
Figure 14. 5V Boost Converter of Figure 1.
Start-Up from 3V Input into 5 Load
Figure 15. 5V SEPIC Start-Up from 9V Input into 10 Load
1308 F14
V
OUT
1V/DIV
I
L1
2A/DIV
V
SHDN
5V/DIV
500µs/DIV
1308 F15
V
OUT
2V/DIV
I
SW
2A/DIV
V
SHDN
5V/DIV
500µs/DIV
Figure 16. 5V to 12V Boost Converter with Soft-Start Components Q1, C4, R3 and R4
V
IN
SW
LT1308B
GND
V
C
FB
SHDN
+
330pF
100k
SHUTDOWN
SOFT-START
COMPONENTS
C1
47µF
C4
33nF
Q1
R4
33k
R3
33k
V
IN
5V
D1
L1
4.7µH
V
OUT
12V
500mA
C2
10µF
C
C
100pF
11.3k
R
C
47k
10k
C1: AVX TAJ476M010
C2: TAIYO YUDEN TMK432BJ106MM
D1: IR 10BQ015
L1: MURATA LQH6C4R7
Q1: 2N3904
1308 F16