LT1158

15

from a true short. This is done by using the current limit to

control cold filament current in conjunction with the self-

protection circuit of Figure 9. The reduced cold filament

current also extends the life of the filament.

mately twice the steady state “on” current of the lamp(s).

lamp filaments 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 filament is tens of

longer than in most other applications. This places in-

creased 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.

0.01

µF

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

B GATE DR

BOOST DR

BIAS

ENABLE

FAULT

INPUT

GND

B GATE FB

LT1158

+

1N4148

10

µF

24k

1000pF

0.05

µF 1k

510

Ω

1N4148

1

2

3

4

8

7

6

5

LT1431

+

1000

µF

LOW ESR

500

µF

LOW ESR

0.015

Ω

L1

22

µH

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

+

0.1

µF

×

100

Ω

100

Ω

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

VISHAY/ULTRONIX RCS01, SM1

ISOTEK CORP. ISA-PLAN SMR

CONSTANT OFF TIME CURRENT MODE CONTROL LOOP

FREQUENCY =

1

)

1 –

4.99k

1%

1.62k

1%

Figure 10. High Efficiency 3.3V Step-Down Switching Regulator (Requires No Heatsinks)