AN2835

The selected solution

Doc ID 15073 Rev 1

9/21

Figure 7.

Average current sense circuit

2.3

Ignition circuit

A high voltage is required to ignite the HID lamp. The ignition voltage depends on the type of

HID lamp. For a MHL (metal halide lamp) it is about 3-5 kV. For a hot lamp re-striking needs

about 20 kV. Immediate re-ignition of a hot lamp is not advised. Therefore, a cooling down

period for hot lamps is recommended. The ignition circuit is shown in Figure 8. The pulse

transformer (T1) is used to give the ignition pulse. The LIC01 is specially designed for high

voltage pulse generation purposes. At the beginning of turn-on S, with LIC01 in off-state,

Figure 9) of LIC01. Once LIC01 breaksdown, C1 discharges and the crowbar current

occurs. LIC01 is latched to on-state. The LIC01 is turned off when discharging a current

lower than the holding current (IH). Then LIC01 returns to the off-state. In such a case, a

voltage pulse is generated on Lp. With the turn ratio 1:n of T1, the high voltage across C2 is

generated and remains constant for a very short time. Therefore the lamp obtains a high

voltage pulse to ignite. LIC01 returns to the off-state after C1 discharges and another

charging to C1 is restarted. After the lamp is ignited, S is turned off and there is no more

voltage pulse generation on Lp.

Figure 8.

Ignition circuit

Figure 9.

Electrical characteristics of LIC01