1308AI Linear Technology Corporation, 1308AI Datasheet - Page 12

• Manufacturer Part Number: 1308AI
• Description: Lt1308b - Single Cell High Currentmicropower 600khz Boost Dc/dc Converter
• Manufacturer: Linear Technology Corporation

LT1308A/LT1308B
APPLICATIONS
Soft-Start
In some cases it may be undesirable for the LT1308A/
LT1308B to operate at current limit during start-up, e.g.,
12
Figure 15. 5V SEPIC Start-Up from 9V Input into 10Ω Load
1V/DIV
2A/DIV
5V/DIV
2V/DIV
2A/DIV
5V/DIV
V
V
V
SHDN
V
SHDN
OUT
OUT
I
I
SW
L1
Figure 14. 5V Boost Converter of Figure 1.
Start-Up from 3V Input into 5Ω Load
U
Figure 16. 5V to 12V Boost Converter with Soft-Start Components Q1, C4, R3 and R4.
500μs/DIV
500μs/DIV
INFORMATION
V
U
5V
IN
+
C1: AVX TAJ476M010
C2: TAIYO YUDEN TMK432BJ106MM
D1: IR 10BQ015
L1: MURATA LQH6C4R7
Q1: 2N3904
C1
47μF
W
R3
33k
C4
33nF
SHUTDOWN
1308 F14
1308 F15
Q1
33k
R4
U
SHDN
V
V
IN
C
SOFT-START
COMPONENTS
R
47k
LT1308B
C
C
100pF
4.7μH
C
L1
GND
SW
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-
grammable soft-start can be implemented with 4 discrete
components. A 5V to 12V boost converter using the
LT1308B is detailed in Figure 16. C4 differentiates V
causing a current to flow into R3 as V
When this current exceeds 0.7V/33k, or 21μA, current
flows into the base of Q1. Q1’s collector then pulls
current out the V
slope of V
With C4 = 33nF, V
waveforms for Figure 16’s circuit are pictured in Figure
17. Without the soft-start circuit implemented, the inrush
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
detailed in Figure 18, while the time required to reach final
output voltage increases to about 15ms. C4 can be
adjusted to achieve any output slew rate desired.
FB
Δ
D1
V
10k
Δ
OUT
t
OUT
=
100k
33
is limited as follows:
0 7
k C
C
.
11.3k
•
pin, creating a feedback loop where the
OUT
330pF
V
1308 F16
4
/t is limited to 640mV/ms. Start-up
C2
10μF
V
12V
500mA
OUT
OUT
increases.
1308abfa
OUT
,