LT1308BCF View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1308BCF' PDF : 20 Pages View PDF
LT1308A/LT1308B
APPLICATIONS INFORMATION
VOUT
1V/DIV
IL1
2A/DIV
VSHDN
5V/DIV
500μs/DIV
1308 F14
Figure 14. 5V Boost Converter of Figure 1.
Start-Up from 3V Input into 5Ω Load
VOUT
2V/DIV
ISW
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.,
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 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 VOUT, 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 current out the
VC pin, creating a feedback loop where the slope of VOUT
is limited as follows:
ΔVOUT = 0.7V
Δt 33k • C4
With C4 = 33nF, VOUT/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 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.
12
L1
VIN
5V
4.7μH
D1
VOUT
12V
500mA
+ C1
47μF
VIN
SW
SHUTDOWN SHDN
LT1308B
10k
FB
VC
GND
100k
330pF
C2
10μF
C4
33nF
R4
33k
Q1
RC
47k
11.3k
R3
CC
33k
100pF
C1: AVX TAJ476M010
C2: TAIYO YUDEN TMK432BJ106MM
D1: IR 10BQ015
L1: MURATA LQH6C4R7
Q1: 2N3904
SOFT-START
COMPONENTS
1308 F16
Figure 16. 5V to 12V Boost Converter with Soft-Start Components Q1, C4, R3 and R4
1308abfb
Share Link: 
