A LED display (LED matrix display system, LED signage) is a flat-panel display that uses an array of light-emitting diodes as pixels for a video display. The resolution of LED displays continues to increase as pixel pitch becomes smaller to achieve better visual effects. Higher pixel density requires higher power consumption and introduces thermal issues. The traditional common-anode LED display can no longer meet the energy-saving requirements in high-resolution LED displays. In response, the industry has recently proposed a new common-cathode LED display technology suitable for high-resolution LED display.

The LED forward voltage varies by color. Typically, 1.8 V to 2.2 V for red LED and 2.8 V to 3.4 V for both blue and green LEDs. Figure 1-1, Figure 1-2, and Figure 1-3 show three types of LED driver solutions:

• traditional common-anode
• traditional common-cathode
• new common-cathode

Figure 1-1 Traditional Common-Anode LED Display

Figure 1-2 Traditional Common-Cathode LED Display

Figure 1-3 New Common-Cathode LED Display

The traditional common-anode LED display (shown in Figure 1-1) uses a single power rail for all three LED colors. The red LED required an additional external resistor in series to limit the voltage across the red LED. The disadvantage is extra power loss due to the resistor. An approach to eliminate the resistor is using common-cathode LED display with red, blue, and green LEDs powered by separate rails, as shown in Figure 1-2 and Figure 1-3.

The traditional common-cathode LED display (drive common-cathode LEDs) shown in Figure 1-2 has two power supplies. One is dedicated to the red LED. The other is for both the blue and green LEDs with ground as a common reference. This power supply is called a common-cathode power supply since the reference is ground (cathode).

The new common-cathode LED display (drive common-anode LEDs) shown in Figure 1-3 uses one supply connected to a common anode for all three LEDs and a second supply that generates a bias voltage for the red LED. This power supply is called a common-anode power supply because the reference is positive to the supply (anode). The key challenge is to generate the 1-V bias supply for the red LED.

Common-anode RGB LEDs share a single electrical connection for the anode of all three LEDs and are powered by a single power supply. Common-cathode RGB LEDs have two power supplies – one for the red LED and one for the blue and green LEDs with ground as a common reference. One supply is dedicated to the red LED and the other supply powers both the blue LED and the green LED. The improved common-cathode LED display uses one supply connected to a common anode for all three LEDs and a second supply that generates a bias voltage for the red LED. The key challenge is to generate the 1-V bias supply for the red LED.

For a video or image display application, red LEDs can typically occupy between 40% and 50% of the driver current. With the common-cathode LED driving method, the LED display surface temperature can be significantly reduced by more than ten degrees. This method enables color uniformity and improved LED life. In addition, power consumption can be reduced between 30% and 75%.

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.

Figure 1-4 Power System to Drive Common-Cathode LEDs

Figure 1-5 Power System to Drive Common-Anode LEDs

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.

Figure 1-6 Flyback or LLC and Synchronous Buck Converter Solution

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.