CAT4238 Datasheet, PDF (8/12 Pages) Catalyst Semiconductor – High Efficiency 10 LED Boost Converter

CAT4238
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
between Vout and the FB pin. A 50 V rated C₂ capacitor is
required to prevent any overvoltage damage in the open
LED condition.
V_IN
C₁
4.7 mF
OFF _ON
L
47 mH
Schottky 100 V
(Central CMSH1−100)
V_OUT
C₂
SW
VIN
CAT4238
SHDN
FB
GND
0.22 mF
V_FB = 300 mV
R₁
15 W
Figure 18. Open LED Protection without Zener
5.0
4.0
3.0
2.0
1.0
0
3.0
3.5
4.0
4.5
5.0
5.5
INPUT VOLTAGE (V)
Figure 20. Open LED Supply Current vs. V_IN without
Zener
55
50
45
40
35
3.0
3.5
4.0
4.5
5.0
5.5
INPUT VOLTAGE (V)
Figure 21. Open LED Output Voltage vs. V_IN without
Zener
2 ms/div
Figure 19. Open LED Switching Waveforms without
Zener
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