AN4291 datasheet(14/53 Pages) STMICROELECTRONICS

14 / 53 page

Design concept

AN4291

14/53

DocID024519 Rev 1

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), the 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 STEVAL385LED4CH board are bounded by the DAC

limits as shown in the following table:

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

9 (0x09)

738 mA

820 mA

136.221E+03

8 (0x08)

648 mA

729 mA

121.085E+03

7 (0x07)

574 mA

638 mA

105.949E+03

6 (0x06)

492 mA

547 mA

90.814E+03

5 (0x05)

410 mA

456 mA

75.678E+03

OFF SMED

()

MAX

AVR

–

() L

ADC

TOP

CLK

ADC

LED_VAL

ADC

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

OF F CLOCK

()

1.184

1.066

–

() 470 10

–

1024

x10

ADC

LED_VAL

1.25

44.5

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

196763

ADC

LED_VAL

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

ADC

LED_VAL

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

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