Because of their modularized design, LED displays scale easily.

Each LED module consists of one power module board, an LED controller board and one or more LED panels. For more information, see LED signage. For common-cathode LED driving displays that use dual power supplies, the common approach is dual outputs flyback or LLC. Figure 1-4 and Figure 1-5 show power solutions corresponding to Figure 1-2 and Figure 1-3.

Both methods described in in Figure 1-4 and Figure 1-5 are widely used for common-cathode LED driving displays. However, both have the disadvantage of high cost and large size as a result of large magnetics requirements (dual outputs transformer). In addition, dual outputs have cross-regulation challenges for wide dynamic load adjustment in LED display. To solve these problems, the new method proposed in Figure 1-6 shows a flyback or LLC and synchronous buck converter structure to drive common-anode LEDs.

In Figure 1-6, the output that comes directly from the flyback or LLC drives the blue and green LEDs. Another output crossing between the flyback or LLC output and the buck output drives the red LED. This approach uses a synchronous buck topology to generate a 1-V floating ground for the red LED. The buck converter operates only to sinki current instead of to source current in steady state. The sinking current converter has been widely used in some applications, such as the TEC driver. Refer to the Low-Power TEC Driver Application Note and the TEC driver reference design for 3.3-V inputs Design Guide.

This application note focuses only on analysis and implementation of the synchronous buck converter topology to design sinking current applications. It discusses some TI devices such as TPS548B22 (synchronous buck converter with integrated switch), TPS548D22 (synchronous buck converter with integrated switch), TPS549D22 (synchronous buck converter with integrated switches and PMBus™), and TPS53819A (synchronous buck controller with external switches and PMBus™) .

Principle of Synchronous Buck with Sinking Current Application analyzes in detail the operations of the synchronous buck topology with sinking current operation. It presents simulations to enable a comparison of the behaviors between sourcing current buck and sinking current buck.