Driving TLC5940 and TLC5955 LED Drivers From an FPGA

Constant-current LED drivers move current regulation and grayscale timing closer to the LEDs. The FPGA sends brightness words, controls shift and latch signals, and supplies any required grayscale clock. This tutorial uses TI’s TLC5940 and TLC5955 as concrete examples while keeping the controller architecture reusable.

Know what the driver provides

The TLC5940 uses an external grayscale clock and exposes BLANK, XLAT, error, and programming-mode behavior. The TLC5955 adds 16-bit grayscale, per-color brightness controls, dot correction, error detection, and display-repeat features. The exact bit packing and edge timing come from the selected datasheet—not from a generic SPI assumption.

A reusable shift-and-latch engine

typedef enum logic [2:0] {IDLE, LOAD, CLK_HI, CLK_LO, LATCH, DONE} state_t;

always_ff @(posedge clk) begin
    case (state)
        IDLE: if (start) begin
            shift_reg <= frame_data;
            bit_count <= FRAME_BITS-1;
            state <= LOAD;
        end
        LOAD: begin
            sin   <= shift_reg[bit_count];
            sclk  <= 1'b0;
            state <= CLK_HI;
        end
        CLK_HI: begin
            sclk  <= 1'b1;
            state <= CLK_LO;
        end
        CLK_LO: begin
            sclk <= 1'b0;
            if (bit_count == 0) state <= LATCH;
            else begin bit_count <= bit_count - 1'b1; state <= LOAD; end
        end
        LATCH: begin
            lat   <= 1'b1;
            state <= DONE;
        end
        DONE: begin
            lat   <= 1'b0;
            done  <= 1'b1;
            state <= IDLE;
        end
    endcase
end

Parameterize frame width and bit order, then wrap the engine with a device-specific formatter. Register all outputs and constrain the interface as real external timing paths.

Initialization sequence matters

• Hold outputs blank while clocks and configuration stabilize.
• Program current range, brightness control, and dot correction before grayscale data.
• Load a known all-zero frame and latch it.
• Start the grayscale clock or display-repeat function.
• Release blanking only after a complete valid frame is active.

Electrical and thermal checks

The current-setting resistor, LED supply, output compliance voltage, simultaneous-channel current, package power dissipation, and PCB copper determine safe operation. Do not choose LED current from RTL. Calculate it from the device equations and verify worst-case thermal conditions.

Official datasheets

• TI TLC5940 datasheet
• TI TLC5955 datasheet