IRPLCFL5U
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7.2. Open Filaments Protection
The open filament protection relies on the non-ZVS circuit of the IR2520D, enabled when pin VCO
reaches 4.6V. Should an open filament lamp fault occur, hard-switching will occur at the half-bridge
and the non-ZVS circuit inside the IR2520D will detect this condition, increase the frequency each
cycle and shut down when VCO decreases below 0.85V; both gate driver outputs will be latched
‘low’. This will prevent hard-switching and damaging of the half-bridge MOSFETs.
Fig. 7.3 shows the VCO pin and VS pin at the shutdown with open filament. As you can see, at start-
up the VCO pin charges from 0V up to 4.6V, at 4.6V the non-ZVS circuit is enabled, CVCO dis-
charges and the frequency increases. When the voltage at the VCO pin decreases below 0.85V we
have latched shutdown. Fig. 7.4 shows the voltage at the VCO pin (HB Voltage) at the shutdown
with a shorter time scale. The FMIN pin can be used as trigger as this pin transitions from 5V to
COM when the IC enters fault mode or UVLO-.
Fig. 7.3: 3 is the voltage in pin VCO and 2 is pin VS
