Overview

The TLC5940 is a 16-channel constant-current sink LED driver IC manufactured by Texas Instruments. Designed for high-density LED displays, RGB LED matrices, backlighting, and precision PWM control, it provides 12-bit (4096-step) grayscale pulse-width modulation (PWM) timing per channel alongside 6-bit (64-step) dot correction for individual channel brightness calibration.

The constant-current outputs sink up to 120 mA per channel (for VCC > 3.6V) and withstand up to 17V off-state voltage, making it suitable for driving long LED strings or high-power LEDs. All 16 channel currents are globally configured by a single external reference resistor (IREF). Multiple TLC5940 ICs can be easily daisy-chained using the serial data input (SIN) and serial data output (SOUT) pins.

Quick reference

Pin configuration

Functional description

The TLC5940 operates in two distinct data programming modes depending on the state of the VPRG pin:

1. Grayscale (GS) Mode (VPRG = Low): Shifts 192 bits (16 channels × 12 bits) of PWM duty cycle data into the serial shift register via SIN on SCLK rising edges. A high pulse on XLAT latches the data into the grayscale registers. The internal 12-bit counter increments on each GSCLK pulse, turning off each OUTn channel when the counter reaches its programmed 12-bit value.
2. Dot Correction (DC) Mode (VPRG = High): Shifts 96 bits (16 channels × 6 bits) of dot correction data into the registers to adjust individual LED channel gain between 0% and 100% of IMAX.

Output Current Calculation

The maximum output current IMAX across all channels is set by a single resistor RIREF connected between pin IEF (IREF) and GND:

$I_{\text{MAX}} = \frac{V_{\text{IREF}}}{R_{\text{IREF}}} \times 31.5 = \frac{1.24\,\text{V}}{R_{\text{IREF}}} \times 31.5 \approx \frac{39.06}{R_{\text{IREF}}}$

For example, a $R_{\text{IREF}} = 2.0\,\text{k}\Omega$ resistor yields an $I_{\text{MAX}} \approx 19.5\,\text{mA}$ per channel.

Absolute maximum ratings

Electrical characteristics

Typical application

                    +5V VCC
                       |
               +-------+-------+
               |               |
             100nF           10uF
               |               |
              GND             GND
               |               |
               +-------+-------+----------+
                       |                  |
                    +--+------------------+--+
                    | 28                 15  |
                    | VCC               IEF  |
  MCU SPI MOSI ---->| 7 SIN                  |==== RIREF (2.0k) ===> GND
  MCU SPI SCK  ---->| 8 SCLK                 |
  MCU GPIO (XLAT) ->| 9 XLAT         OUT0--4 |----> [LED Cathodes] ---> +5V/+12V
  MCU GPIO (BLANK)->| 10 BLANK       OUT5-15 |----> [LED Cathodes] ---> +5V/+12V
  MCU PWM (GSCLK) ->| 12 GSCLK               |
  MCU GPIO (VPRG) ->| 6 VPRG            SOUT |---> Next TLC5940 SIN
                    | GND               XERR |---> MCU Interrupt (Optional)
                    +--+------------------+--+
                       |
                      GND

Common mistakes

• Missing IREF Resistor: Leaving the IEF pin unconnected or shorted directly to GND will cause zero output current or excessive current that destroys the chip.
• Forgetting GSCLK Pulses: Unlike simple shift registers, TLC5940 REQUIRES a continuous clock signal on GSCLK (or 4096 pulses per PWM cycle) to increment the internal 12-bit PWM counter.
• Not Pulsing BLANK High at Counter Rollover: BLANK must be pulsed high every 4096 GSCLK cycles to reset the internal counter to 0 and re-enable outputs.
• Floating VPRG: VPRG must be driven low for normal Grayscale PWM operation. Leaving it floating puts the IC in an indeterminate mode.

Notes & further reading

• Arduino Library: The TLC5940 library by Alex Leone uses MCU hardware timers to generate GSCLK and BLANK signals automatically via interrupts.
• Power Dissipation: When driving 16 channels at 120 mA each, total IC dissipation can exceed 1.5 W. Ensure thermal pad (HTSSOP/QFN) is properly soldered to a ground plane heat sink.