AN4460 datasheet(13/54 Pages) STMICROELECTRONICS

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AN4460

Design concept

turn-on current generated by MOSFET Q2. The STLUX385A detects the peak current

by comparing CPP3 with an internal, DAC generated, voltage.



The R19, R25, Q4 and R37 are used to feed the voltage of the LED chain cathode to

the STLUX385A's ADC. The MOSFET Q4 is used to prevent the small biasing current

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 Equation 4 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 



ADCFS: ADC top voltage = 1.25 V



ADCTOP: is the step value of ADC (210) = 1024

TOFF SMED



TOFF CLK



VLED

ADCLED_VAL ADCFS P



ADCTOP

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

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