CAT4237

http://onsemi.com

9

Open LED Protection

In the event of an “Open LED” fault condition, the

CAT4237 will continue to boost the output voltage with

maximum power until the output voltage reaches

approximately 35 V. Once the output exceeds this level, the

internal circuitry immediately places the device into a very

low power mode where the total input power is limited to

about 4 mW (about 1 mA input current with a 3.6 V supply).

The SW pin clamps at a voltage below its maximum rating

of 60 V. There is no need to use an external zener diode

required to prevent any overvoltage damage in the open

LED condition.

Figure 21. Open LED Protection without Zener

VIN

CAT4237

L

Schottky 100 V

(Central CMSH1−100)

R1

15 W

4.7 mF

0.22 mF

FB

SW

GND

OFF ON

33 mH

SHDN

Figure 22. Open LED Switching Waveforms without

Zener

10 msec/div

SW PIN

10 V/div

Zener

INPUT VOLTAGE (V)

5.0

4.5

4.0

3.5

3.0

2.5

0

0.5

1.0

1.5

2.0

Zener

INPUT VOLTAGE (V)

5.0

4.5

4.0

3.5

3.0

2.5

30

35

40

45

50