8.2.1.2 Detailed Design Procedure

STEP 1: Determine the current setting resistor, R_(SNS) value by using Equation 2.

Equation 2.

where

• V_{(CS_REG)} = 98 mV (typical.)
• I_(LED) = 250 mA

STEP 2: Design the threshold voltage for SUPPLY to enable the LED open-circuit diagnostics and calculate the resistor divider value.

LED-string maximum forward voltage = 3 × 2.5 V = 7.5 V. To avoid the open-circuit fault reported in low-dropout operation conditions, additional headroom between SUPPLY and OUT needs to be considered. The TPS92613-Q1 device must disable open-circuit detection when the supply voltage is below LED-string maximum forward voltage plus maximum V_{(OPEN_th_rising)} and maximum V_{(CS_REG)}. The voltage divider resistor, R₁ and R₂ value can be calculated by Equation 3.

Equation 3.

where

• V_IL(DIAGEN) = 1.045 V (minimum)
• V_{(OPEN_th_rising)} = 335 mV (maximum)
• V_{(CS_REG)} = 102.5 mV (maximum)
• R₁ = 10 kΩ recommended

The calculated result for R₂ is 65.7 kΩ when V_(OUT) maximum voltage is 7.5 V.

STEP 3: Thermal analysis for the worst application conditions.

Normally the thermal analysis is necessary for linear LED-driver applications to ensure that the operation junction temperature of TPS92613-Q1 is well managed. The total power consumption on the TPS92613-Q1 itself is one important factor determining operation junction temperature, and it can be calculated by using Equation 4. Based on the worst-case analysis for maximum power consumption on device, consider either optimizing PCB layout for better power dissipation as Layout describes or adding an extra heat-sharing resistor as described in Single-Channel LED Driver With Heat Sharing.

Equation 4.

where

• V_{(CS_REG)} = 98 mV (typical)
• I_(Quiescent) = 250 µA (maximum)

In this application, the calculated result for maximum power consumption on the TPS92613-Q1 is 2.55 W at V_(SUPPLY) = 16 V and I_(LED) = 250 mA conditions.

TI recommends to add capacitors C_(SUPPLY) at SUPPLY and C_(OUT)at OUT. TI recommends one 1-μF capacitor plus one 100-nF decoupling ceramic capacitor close to the SUPPLY pin for C_(SUPPLY) and a 10-nF ceramic capacitor close to the OUT pin for C_(OUT). The larger capacitor for C_(SUPPLY) or C_(OUT) is helpful for EMI and ESD immunity; however, large C_(OUT) takes a longer time to charge up the capacitor and may affect PWM dimming performance.