STEP 1: Determine the reference current setting resistor, R_(IREF), by using Equation 8.

Equation 8.

where

• V_(IREF) = 1.235 V (typical)
• I_(IREF) = 100 µA (recommended)

TI recommends 100 µA current for reference current, I_(IREF) if the ICTRL resistor is placed in the same board with TPS92633-Q1. The calculated result for R_(IREF) is 12.3 kΩ when I_(IREF) = 100 µA.

STEP 2: Design the ICTRL resistor, R_(ICTRL), for setting the regulation voltage, V_{(CS_REG)} by using Equation 9.

Equation 9.

where

• V_{(CS_REG)} = 100 mV (recommended)
• I_(IREF) = 100 µA (recommended)

TI recommends 100 mV for reference voltage across current sensing resistor, R_(SNSx) if the ICTRL pin is not used for driving off-board binning resistor or NTC resistor. The calculated result for R_(ICTRL) is 680 Ω when V_{(CS_REG)} = 100 mV.

STEP 3: Determine the current sensing resistor, R_(SNSx), by using Equation 10.

Equation 10.

where

• V_(IREF) = 1.235 V (typical)
• R_(ICTRL) = 680 Ω
• R_(IREF) = 12.3 kΩ
• I_{(OUTx_Tot)} = 140 mA

According to design requirements, output current for each channel is same so that the R_(SNS1) = R_(SNS2) = R_(SNS3) = 0.717 Ω. Two resistors in parallel are required to achieve equivalent 0.717-Ω resistance because 0.717 Ω is not a standard decade resistance value.

STEP 4: Design the current distribution between I_(OUTx) and I_(RESx), and calculate the current sharing resistor, R_(RESx) by using Equation 11. 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.

Equation 11.

where

• V_(SUPPLY) = 12 V (typical)
• I_{(OUTx_Tot)} = 140 mA

The calculated result for R_(RESx) resistor value including R_(RES1), R_(RES2) and R_(RES3) is 75 Ω when V_(OUTx) is typical 3 × 2.2 V = 6.6 V.

STEP 5: Design the single-LED short-circuit threshold voltage and calculate the value of R_{(SLS_REF)} resistor for setting single-LED short-circuit threshold by using Equation 12.

The total forward voltage for three LEDs in serial is 3 × 2.5 V = 7.5-V maximum and 3 × 1.9 V = 5.7-V minimum. Once anyone of three LEDs is defective with short-circuit behavior, the total forward voltage for remaining two LEDs in serial is 2 × 2.5 V = 5-V maximum and 2 × 1.9 V = 3.8-V minimum. So the 5.3 V is selected to be threshold for single-LED short-circuit, V_{(SLS_th_falling)}.

Equation 12.

where

• V_(IREF) = 1.235 V (typical)
• R_(IREF) = 12.3 kΩ
• N_(OUT) = 4
• N_{(SLS_REF)} = 1

The calculated result for R_{(SLS_REF)} is 13.3 kΩ for V_{(SLS_th_falling)} is 5.34 V.

STEP 6: Design the threshold voltage of SUPPLY to enable the LED open-circuit and single-LED short-circuit diagnostics, and calculate voltage divider resistor value for R1 and R2 on DIAGEN pin.

The maximum forward voltage of LED-string is 3 × 2.5 V = 7.5 V. To avoid the open-circuit fault or single-LED short-circuit reported in low-dropout operation conditions, additional headroom between SUPPLY and OUTx needs to be considered. The TPS92633-Q1 device must disable open-circuit detection and single-LED short-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 13.

Equation 13.

where

• V_{(OPEN_th_rising)} = 210 mV (maximum)
• V_{(CS_REG)} = 100 mV
• V_IL(DIAGEN) = 1.045 V (minimum)
• R₂ = 10 kΩ (recommended)

The calculated result for R1 is 64.9 kΩ when V_(OUTx) maximum voltage is 7.5 V and V_{(CS_REG)} is 100 mV.

STEP 7: Design the threshold voltage of SUPPLY to turn on and off each channel of LED, and calculate voltage divider resistor value for R3 and R4 on PWM input pin.

The minimum forward voltage of LED-string is 3 × 1.9 V = 5.7 V. To make sure the current output on each of LED-string is normal, each LED-string needs to be turned off when SUPPLY voltage is lower than LED minimum required forward voltage plus dropout voltage between INx to OUTx and V_{(CS_REG)}. The voltage divider resistor, R3 and R4 value can be calculated by Equation 14.

Equation 14.

where

• V_(DROPOUT) = 300 mV (typical)
• V_{(CS_REG)} = 100 mV
• V_IH(PWM) = 1.26 V (maximum)
• R₄ = 10 kΩ (recommended)

The calculated result for R3 is 38.3 kΩ when V_(OUTx) minimum voltage is 5.7 V and V_{(CS_REG)} is 100 mV.