LT1158
16
1158fb
TYPICAL APPLICATIONS
APPLICATIONS INFORMATION
from a true short. This is done by using the current limit to
control cold fi lament current in conjunction with the self-
protection circuit of Figure 9. The reduced cold fi lament
current also extends the life of the fi lament.
A good guideline is to choose R
SENSE
to set I
SC
at ap-
proximately twice the steady state “on” current of the
lamp(s). t
SHUTDOWN
is then made long enough to guar-
antee that the lamp fi laments heat and drop out of current
limit before the enable capacitor discharges to the enable
low threshold. For a short-circuit, the enable capacitor
will continue to discharge below the threshold, shutting
down the top MOSFET. The LT1158 will then go into the
automatic restart mode described in Self-Protection with
Automatic Restart above.
The time constant for an incandescent fi lament is tens
of milliseconds, which means that t
SHUTDOWN
will have
to be longer than in most other applications. This places
increased SOA demands on the MOSFET during a short
circuit, requiring that a larger than normal device be used.
A protected high current lamp driver application is shown
in Figure 18.
Figure 10. High Effi ciency 3.3V Step-Down Switching Regulator (Requires No Heatsinks)
0.01μF
LT1158 F10
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
BOOST
T GATE DR
T GATE FB
T SOURCE
SENSE
+
SENSE
–
V
+
B GATE DR
BOOST DR
V
+
BIAS
ENABLE
FAULT
INPUT
GND
B GATE FB
LT1158
1N4148
10μF
24k
1000pF
0.05μF
1k
510Ω
1N4148
1
2
3
4
8
7
6
5
LT1431
1000μF
LOW ESR
500μF
LOW ESR
R
S
0.015Ω
L1
22μH
SHORT-CIRCUIT
CURRENT = 8A
200pF
+3.3V/6A
OUTPUT
5V TO 10V INPUT (USE LOGIC-LEVEL Q1, Q2)
8V TO 20V INPUT (USE STANDARD Q1, Q2
AND CONNECT BOOST DIODE TO PIN 1)
Q1
680k
0.1μF
s
100Ω
100Ω
Q2
VP0300
INSERT FOR
ZERO POWER
SHUTDOWN
100k
CMOS
ON/OFF
2N2222
+–
100k
Q1, Q2: IRLZ44 (LOGIC-LEVEL)
IRFZ44 (STANDARD)
L1: HURRICANE LAB
HL-KK122T/BB
R
S
: VISHAY/DALE TYPE LVR-3
VISHAY/ULTRONIX RCS01, SM1
ISOTEK CORP. ISA-PLAN SMR
CONSTANT OFF TIME CURRENT MODE CONTROL LOOP
FREQUENCY = WHERE t
OFF
≈ 10μs
1
t
OFF
( )
1 –
V
OUT
V
IN
4.99k
1%
1.62k
1%
+
+
+
