8.2.2.2 Detailed Design Procedure

In linear LED driver applications, the input voltage variation contributes to most of the thermal concerns. The resistor current, as indicated by Ohm’s law, depends on the voltage across the external resistors. The TPS92611-Q1 controls the driver current I_(DRIVE) to attain the desired total current. If I_(P) increases, the TPS92611-Q1 device decreases I_(DRIVE) to compensate, and vice versa.

While in low-dropout mode, the voltage across the R_(P) resistor may be close to zero, so that almost no current can flow through the external resistor R_(P).

When the input voltage is high, the parallel-resistor current I_(P) is proportional to the voltage across the parallel resistor R_(P). The parallel resistor R_(P) takes the majority of the total string current, generating maximum heat. The device must prevent current from draining out to ensure current regulation capability.

In this case, the parallel resistor value must be carefully calculated to ensure that 1) enough output current is achieved in low-dropout mode, 2) thermal dissipation for both the TPS92611-Q1 device and the resistor is within their thermal dissipation limits, and 3) device current in the high-voltage mode is above the minimal output-current requirement.

TI recommends to add capacitors C₁ and C₂ at SUPPLY and OUT. TI recommends C₁ of 1 µF and 100 nF close to the SUPPLY pin, and C₂ of 10 nF close to the OUT pin. A larger capacitor for C₁ or C₂ is helpful for EMC and ESD; however, it takes a longer time to charge up the capacitor and could affect PWM dimming performance. Current setting by a sense resistor is as described in Equation 1.

Equation 5.

LED-string maximum forward voltage = 3 × 2.5 V = 7.5 V.

Parallel resistor R_(P) is recommended to consume 1/2 of the total current at maximum supply voltage.

Equation 6.

Total device power consumption is maximum at 16 V input and LED minimal forward voltage.

Equation 7.

Resistor R_(P) maximum power consumption is at 16-V input.

Equation 8.

Users must consider the maximum power of both of the device and the parallel resistor.

For PWM scenarios, a non-power bipolar transistor with a base current-limiting resistor of 10 kΩ can modulate the output current together with the device PWM function.