Datasheet

V
AUX
C11
Q3
FLTR2
D11
DIM
V
CC
FLTR2
R18
C17
R17
Q4
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Design Guide
Figure 33. TRIAC Holding Circuit
12.6 Overvoltage Protection
The circuit described in Figure 34 provides over-voltage protection (OVP) in case of LED open circuit
failure. The use of this circuit is recommended for stand-alone LED driver designs where it is essential to
recover from a momentary open circuit without damaging any part of the circuit. In the case of an
integrated LED lamp (where the LED load is permanently connected to the driver output) a simple zener
diode or TVS based overvoltage protection is suggested as a cost effective solution. The zener diode/TVS
offers protection against a single open circuit event and prevents the output voltage from exceeding the
regulatory limits. Depending on the LED driver design specifications, either one or both techniques can be
used to meet the target regulatory agency approval
Figure 34. OVP Circuit
The OVP threshold is programmable and is set by selecting appropriate value of zener diode D11. The
capacitor C11 across the base of transistor Q3 is used to filter the voltage ripple present on the auxiliary
voltage and prevent false OVP tripping due to voltage spikes caused by leakage inductance.
The circuit operation is simple and based on biasing of transistor Q3 during fault conditions such that it
pulls down the voltage on the FLTR2 pin to ground. The bias current depends on how much overdrive
voltage is generated above the zener diode threshold. For proper circuit operation, it is recommended to
design for 4V overdrive in order to adequately bias the transistor. Therefore the zener diode should be
selected based on the expression,
(41)
21
SNOA555CApril 2011Revised May 2013 AN-2091 LM3448 - 230VAC, 6W Isolated Flyback LED Driver
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