CAT4238

http://onsemi.com

8

Open LED Protection

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

CAT4238 will continue to boost the output voltage with

maximum power until the output voltage reaches

approximately 40 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 6 mW (about 1.6 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 18. Open LED Protection without Zener

VIN

CAT4238

L

Schottky 100 V

(Central CMSH1−100)

15 W

4.7 mF

0.22 mF

FB

SW

GND

OFF ON

47 mH

SHDN

Figure 19. Open LED Switching Waveforms without

Zener

2 ms/div

Zener

INPUT VOLTAGE (V)

5.0

4.5

4.0

3.5

3.0

0

2.0

1.0

4.0

5.0

5.5

3.0

Zener

INPUT VOLTAGE (V)

5.0

4.5

4.0

3.5

3.0

35

40

45

50

5.5

55