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on-time capacitor reaches the voltage level on the PCOMP pin or ICOMP pin, the on-

time will end and the buck output will turn off. The off-time is determined by a negative-

going edge on the ZX pin, or, if the maximum off time is reached as programmed by the

time it takes for the CTOFF on the TOFF pin to charge up to an internal threshold of 2V.

If the maximum off-time is reached before the inductor current discharges to zero, then

the inductor will begin charging again from some value above zero. This mode of

operation is known as continuous-conduction mode and results in a continuous DC

current in the inductor with a ripple bounded above by the over-current threshold and

below by the maximum off time setting (see Figure 5.7). Continuous-conduction mode

also allows for a higher average current to flow through the buck inductor before

saturation occurs than with critical-conduction mode.

Figure 5.7: Buck control timing diagram (continuous 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

5.6 Constant Power Control

During the general mode of operation and after the lamp has ignited, the IC regulates

the lamp output power to a constant level. To achieve this, the IC measures the lamp

voltage and lamp current at the VSENSE and ISENSE pins, multiplies the voltage and

current together using an internal multiplier circuit to calculate power, and regulates the

output of the multiplier circuit to a constant reference voltage by increasing or decreasing

the buck on-time. If the lamp power is too low then the output of the multiplier will be

below the internal reference voltage. The operational trans-conductance amplifier (OTA)

will output a sourcing current to the PCOMP pin that will charge up the CPCOMP to a

higher voltage. This will increase the on-time of buck and increase the output current to

the lamp for increasing the output power. If the lamp power is too high, then the

opposite will occur. The OTA will output a sinking current to the PCOMP pin that will

discharge the CPCOMP to a lower voltage. This will decrease the buck on-time and

CS = 1.2V