AN4460 datasheet(14/54 Pages) STMICROELECTRONICS

14 / 54 page

Design concept

AN4460

14/54

DocID026129 Rev 1

Equation 1 can be rewritten as follows:

Equation 7

for example:

Equation 8

where:



IMAX: max.current imposed through the DAC setup (1/16)



IAVG: average current - imposed as 90% of IMAX (by design)



(IMAX - IAVG): current difference: (IMAX * 0.1)



I: is the inductance value = 470 10-6 H (by design)



CLK: Is the SMED CLK = 96 106 Hz

The average current IAVG is set by design to be equal to 90% of IMAX.The algorithm controls

the current to be centered in IAVG, with a ripple equal to ± 10% of IMAX. In Equation 8, the

current and the voltage across LED are unknown. The current (IMAX) is an input value

defined by the user. The voltage on the LED string is the difference between the main power

voltage and the VCOMx voltage sensed by two ADC channels. Considering 1.184 A IMAX

current and IAVG approximated to 1.066 (the real value is 1.0654), Equation 4 is expanded

into:

Equation 9

where K (196763 in this case) is a constant for the given LED current selected by the user.

The IMAX values accepted by the STEVAL-ILL057V1 board are bounded by the DAC limits

as shown in Table 1:

Table 1. Current, DAC, K relation table

Index

DAC value

Medium

current(1)

Maximum

current

K (dec)

13 (0x0D)

1065 mA

1184 mA

196.763E+03

12 (0x0C)

984 mA

1093 mA

181.628E+03

11 (0x0B)

901 mA

1002 mA

166.492E+03

10 (0x0A)

819 mA

911 mA

151.356E+03

TOFF SMED



2IMAX IAVR

–

 L



ADCTOP

ADCLED_VAL ADCFS P



------------------------------------------------------------------------



 CLK



TOFF SMED



2IMAX IAVR

–

 L



ADCTOP CLK



ADCLED_VAL ADCFS P



--------------------------------------------------------------------------------------------------------

TOFF CLOCK



1.184 1.066

–

 470 10

–

1024 96



x10



ADCLED_VAL 1.25 44.5



-----------------------------------------------------------------------------------------------------------------------------------=

196763

ADCLED_VAL

------------------------------------

ADCLED_VAL

------------------------------------

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