Freescale Semiconductor M68HC08 ireq = A ⋅ ek ⋅ ireqAD + q, rounditmin ⋅ ADmax ⁄ imax, itADmin =

Software Design

level adjustment (brightness linearization). The regulation error is then the input value for the PI regulator. The output value from the PI regulator is the HRP period for half bridge power stage.

Input required luminance level adjustment adapts the required luminance level to its exponential value. This is done because of the nonlinear function dependence of luminance on the lamp current. The transformation table is used. The table contains the required current values.

The values in the table can be calculated using the following expression.
ireq = A ⋅ ek ⋅ ireqAD + q	(EQ 5-1)

where:

•ireg is the required current in the exponential expression used for the control algorithm calculation

•iregAD is the required current measured by the AD converter

•A is the exponential curve gain

•q is the exponential curve offset

To calculate the coefficients A and q correctly, the following values must be known.

•	Minimum HRP frequency	fmin
•	Maximum HRP frequency	fmax
• Tube current measurement range		imax
• Minimum required tube current		itmin
• Maximum required tube current		itmax
•	ADC maximum value	ADmax
•	ADC minimum value	ADmin
• Minimum required tube current value converted to ADC range		itADmin
• Maximum required tube current value converted to ADC range		itADmin

•k determines function precision

Then:

itADmin =	round(itmin ⋅ ADmax ⁄ imax;0)
itADmax =	round(itmax ⋅ ADmax ⁄ imax;0)
A	itADmax – itADmin
	= ----------------------------------------------------
	ek ⋅ ADmax – ek ⋅ ADmin

q = itADmin – A ⋅ ek ⋅ ADmin

(EQ 5-2)

(EQ 5-3)

(EQ 5-4)

(EQ 5-5)

This is a time-consuming calculation, but it can be simplified by using the sheet “HRP_Setup” provided in the Excel file “DLB_Setup.xls”. This file can be downloaded along with this designer reference manual.

Dimmable Light Ballast with Power Factor Correction, Rev. 1

38 Freescale Semiconductor