JAJSJC6A December 2021 – June 2022 TPS92623-Q1
Step 1: Determine the current sensing resistor, R(SNSx) by using Equation 8.
According to design requirements, output current for each channel is same so that the calculated R(SNS1) = R(SNS2) = R(SNS3)= 1.15 Ω.
Step 2: Design the current distribution between I(OUTx) and I(RESx), and calculate the current sharing resistor, R(RESx), by using Equation 9. The R(RESx) value actually decides the current distribution for I(OUTx) path and I(RESx) path, basic principle is to design the R(RESx) to consume appropriate 50% total power dissipation at typical supply operating voltage.
The calculated result for R(RESx) resistor value including R(RES1), R(RES2)and R(RES3) is 117 Ω when V(OUTx) is typical 2 × 2.2 V = 4.4 V.
Step 3: Design the threshold voltage of SUPPLY to enable the LED open-circuit, and calculate voltage divider resistor value for R1 and R2 on DIAGEN pin.
The maximum forward voltage of LED-string is 2 × 2.5 V = 5 V. To avoid the open-circuit fault reported in low-dropout operation conditions, additional headroom between SUPPLY and OUTx must be considered. TheTPS92623-Q1 device must disable open-circuit detection when the supply voltage is below LED-string maximum forward voltage plus V(OPEN_th_rising) and V(CS_REG). The voltage divider resistor, R1 and R2 value can be calculated by Equation 10.
The calculated result for R1 is 43.4 kΩ when V(OUTx) maximum voltage is 5 V and V(CS_REG) is 156 mV.