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Fig 18. Lamp removal and replacement

Trace

Color

Signal

2

Red

SD Pin Voltage

3

Blue

Lamp Arc Current

4

Green

Lamp Voltage

The VDC pin of the IRS2158D (IC2) is also connected to the DC bus through the divider formed

by R23 and R24, with CVDC to COM to remove ripple and noise. This provides brown out protection for

the ballast which works by sensing the rectified AC line voltage at the positive output of BR1. When the

VDC voltage falls below 3 V the IRS2158D shuts down. The values of R23 and R24 are selected so that

this condition occurs when the AC line input drops below the minimum level at which the ballast can

maintain sufficient DC bus voltage for the lamp to remain ignited and run at the correct power. When the

ballast has shut down the voltage at the VDC pin will rise because the voltage at BR1 will no longer be full

wave rectified as there is no load and C2 is sufficient to maintain a DC level. To prevent the ballast from

immediately re-starting there is a 2 V hysteresis such that the VDC pin must rise above 5 V in order for the

IRS2158D to restart. This will only happen after the AC line has been restored to a sufficient level.

Without the brownout protection provided by the VDC pin if a brownout did occur the ballast

would shut down in the event of any hard switching at VS because voltage spikes would appear at the CS

pin. This would cause the fault counter to count up until the ballast shut down. In that event the ballast

would shut down and remain off until the line is switched off and then back on again. This is undesirable

and with the implementation of the VDC pin of the IR2158D can easily be avoided so that the ballast will

automatically recover from a Brownout event and switch the light back on.