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HID lamp with an AC square-wave voltage. The full-bridge begins oscillating at the

programmed frequency immediately when the IC comes out of UVLO Mode and turns

on. The full-bridge is typically driven at a low frequency to prevent acoustic resonances

from damaging the lamp. The full-bridge frequency is programmed with the external

capacitor at the CT pin. CT charges up and down linearly through internal sink and

source currents between a fixed voltage window of 2V and 4V. CT reaching 4V initiates

the toggling of LO1/HO1, and LO2/HO2 respectively (see Figure 5.5). The dead-time is

fixed internally at 1.0us typical. During the dead-time, all full-bridge MOSFETs are off

and the mid-points of each half-bridge are floating or unbiased. Should an external

transient occur during the dead-time due to an ignition voltage pulse, each half-bridge

mid-point (VS1 and VS2 pins) can slew high or low very quickly and exceed the dv/dt

rating of the IC. To prevent this, internal logic guarantees that the IGN pin is set to a

logic ‘low’ during the dead-time. No ignition pulses can occur until the dead-time has

ended and the appropriate full-bridge MOSFETs are turned on. This will guarantee that

the mid-points are biased to the output voltage of the buck or COM before an ignition

pulse occurs. The full-bridge stops oscillating only when the IC enters Fault Mode or

UVLO Mode.

Figure 5.5: Full-bridge timing diagram: CH1 is CT pin voltage, CH2 is LO1 voltage, CH3 is LO2

voltage and CH4 is VS1 pin voltage

5.5 Buck Control

The buck control circuit operates in critical-conduction mode or continuous-conduction

mode depending on the off-time of the buck output or the peak current flowing through

the buck MOSFET (MBUCK).

During normal lamp running conditions, the voltage

across the buck current sensing resistor, as measured by the CS pin, is below the

internal over-current threshold (1.2V typical). The buck on-time is defined by the time it

takes for the internal on-time capacitor to charge up to the voltage level on the PCOMP

pin or ICOMP pin, whichever is lower. During the on-time, the current in the buck