Functional description

L6572

12/22

Doc ID 023882 Rev 1

current is set to the powerboost level. Otherwise, the ZVS control prevents the half-bridge

from operating below resonance. This condition does not lead to a latched turn-off.

During the powerboost phase, the current defined by the voltage across the RNO resistor is

internally increased by the fixed ratio XPB. During the first half of the powerboost phase, the

rated resonant current remains constant at the defined level, while during the second half,

the rated current decreases in 15 equal steps up to the value of the normal lamp operation.

The length of the powerboost phase is roughly defined by the duration of 4063232 period

times of the clock generator. There are 2097152 period times for the constant powerboost

level. The decrease to the normal operation level takes place in equal steps with a length of

131072 period times of the clock generator per step.

The duration of the powerboost phase is shortened to 1022 period times of the clock

generator when the lamp is already warm or when operated with reduced light output.

During normal operation, two possible rated resonant currents can be achieved related to

the output signal of the two-step dimming unit. During the operation of the lamp with nominal

light output, the nominal rated value is defined by the external resistor RNO. When operating

the lamp with reduced light output, the reduced rated value is defined by the external

resistor RDO.

The ratio between both possible rated values of the resonant current leading to reduced or

nominal light output is defined by the ratio RNO/RDO.

5.6

Thermal powerboost control

A cold lamp emits a lower light level even when driven by a correct rated current. To speed

up the lamp heating, a higher current is applied during few seconds of nominal operation

(thermal powerboost). The junction-temperature of the device is measured: if it is above

TPB the powerboost phase is entered as described in Section 5.5, otherwise the lamp is

directly operated with nominal light output.

5.7

Output stage

The output stage processes the trigger signals from the current regulation unit, the ignition

recognition unit and the UVLO block and controls the high-side and low-side driver.

Corresponding with the UVLO function the output stage controls the startup of the device.

5.8

Deadtime generation

The deadtime of the half-bridge is programmed by the external resistor RDT and it is equal

for both high-side and low-side driver. It is adjustable between 1

μs and 5 μs. A minimum

deadtime that avoids cross conduction of the power transistors is ensured by a minimum

resistance of the external resistor RDT.

The deadtime generator delays the switch on both of power transistors, while the switch off

signal from the output stage runs immediately to the drivers.