AN4291 datasheet(13/53 Pages) STMICROELECTRONICS

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AN4291

Design concept

of the voltage divider from flowing through the LED during the dimming off-time. The

ADC maximum voltage is 1.25 V (the ADC gain is configured to x1).

•

The D4 diode is active during the OFF period, when the voltage across the inductor

goes high.

•

The U2 circuit (PM8834 MOSFET driver) is used to generate the correct gate voltage in

case the STLUX385A is powered from 3.3 V.

•

The D6 diode is used to protect the circuit if the LED chain opens or is accidentally

removed during the normal operation.

3.2

Solving the equations

The TOFF value introduced in the previous equation has to be converted into a number that

could be used by the STLUX385A. In particular, TOFF is implemented in the SMED as a

timer. This timer is a 16-bit register and is clocked according to the SMED clock (CLK = 96

Mhz). The timer register value is equal to:

Equation 5

where:

•

TOFF(SMED): timer value

•

CLK: SMED operating frequency

The STLUX385A monitors the voltage drop caused by the LED and it is calculated as the

difference between the power voltage (VLED_PW) and the voltage measured after the LED

string (VCOMx). Both VLED_PW and VCOMx are read by the ADC unit.

Equation 6

where:

ADCLED_VAL: the sensed LED drop voltage and calculated as VLED_PW - VCOMx

VCOMx: ADC value read from middle point of each channel (LED-inductor)

VLED_PW: ADC value read from the main power

P: is the resistor partition gain (R19+R25+R37)/R37 = (53400)/1200= 44.5 Ω

ADCFS: ADC top voltage = 1.25 V

ADCTOP: is the step value of ADC (2

10) = 1024

Equation 1 can be rewritten as follows:

Equation 7

OFF SMED

()

OFF

CLK

LED

ADC

LED_VAL

ADC

TOP

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

OFF SMED

()

MAX

AVR

–

() L

ADC

TOP

ADC

LED_VAL

ADC

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

⎝⎠

⎛⎞

CLK

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