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inductor charges up to a peak level, depending on the inductance value, and the

secondary winding output of the buck inductor is at some negative voltage level,

depending on the ratio between the primary and secondary windings. The secondary

winding output is measured by the ZX pin, which clamps the negative voltage to a diode

drop below COM using the internal ESD diode, and limits the resulting negative current

flowing out of the pin with an external resistor, RZX. When the voltage on the internal

on-time capacitor exceeds the voltage on the PCOMP pin or ICOMP pin, the on-time has

ended and the buck output turns off.

The secondary winding output of the buck inductor transitions to some positive voltage

level, depending on the ratio between the primary and secondary windings, and causes

the ZX pin to exceed the internal 2V threshold. The current in the buck inductor begins

to discharge into the lamp full-bridge output stage. When the inductor current reaches

zero, the ZX pin decreases back below the 2V threshold. This causes the internal logic

of the buck control to start the on-time cycle again. This mode of operation is known as

critical-conduction mode because the buck MOSFET is turned on each cycle when the

inductor current discharges to zero. The on-time is programmed by the voltage level on

the PCOMP pin, and the off-time is determined by the time it takes for the inductor

current to discharge to zero, as measured by a negative-going edge on the ZX pin. The

resulting shape of the current in the inductor is triangular with a peak value determined

by the inductance value and on-time setting.

Figure 5.6: Buck control timing diagram (critical conduction mode): CH1 is TOFF pin voltage, CH2 is

ZX pin voltage, CH3 is Buck output voltage and CH4 is current through buck inductor LBUCK

During lamp warm-up or a short-circuit condition at the output, the inductor current will

charge up to an excessive level that can saturate the inductor or damage the buck

MOSFET. To prevent this condition, the buck current sensing resistor (RBCS) is set

such that the voltage at the CS pin exceeds the internal over-current threshold (1.2V

typical) before the inductor saturates. Should the CS pin exceed 1.2V before the internal