This is WLED-display backlight driver circuit. This circuit can be used to power WLEDs by replacing the voltage-feedback network with a series current-sense resistor, R3, and the WLED strings. This circuit uses The TPS6108x that drive some series WLEDs in parallel for backlighting larger displays. Here is the circuit:
The feedback of this circuit is The voltage across the current-sense resistor. This feedback provides regulation. This circuit uses 1.2-V feedback voltages. The power lost of this circuit because of R3 is Plost= Iwled*Iwled*R3 = 1.2V*Iwled. The TPS6108x converters is equipped with an SS pin that can provide variable soft startup for boosted voltage regulation applications. Beside that, The SS pin can be used to lower the FB-pin reference voltage and to reduce sense-resistor power loss in a WLED current-regulation application.
The FB-pin reference voltage can be lowered by connecting a resistor, R1, from the SS pin to GND. WLED current can be calculated by following equation:
Analog dimming is provided by A second resistor, R2, that is connected with the FET and Q1 in series connection and parallel connection with R1. Those connection will lower the regulated FB-pin voltage across the sense resistor. [Source: Texas Instruments Application Note]