TLC5940

The three solve overlapping problems differently. The TLC5940 is a constant-current sink with 12-bit grayscale plus per-channel dot correction and needs an external IREF resistor and a GSCLK; it is the right choice when you need matched, calibrated current through LEDs with a series voltage above VCC. The PCA9685 is an I2C PWM controller with open-drain totem outputs and no constant-current stage, so you still need series resistors and it cannot drive high-voltage strings. The TLC5947 is TI's newer 24-channel part with an internal oscillator, no external IREF and simpler wiring — usually a better buy for new designs unless you need the EEPROM dot correction or the 17 V headroom.

Use Imax = 31.5 × 1.24 V / R(IREF). For 20 mA that is 31.5 × 1.24 / 0.020 = 1953 ohm, so use 1.96 kΩ ±1 percent; for 60 mA use 640 ohm ±1 percent; for 120 mA use 320 ohm. TI warns that Imax must stay between 5 mA and 120 mA because the output becomes unstable below 5 mA — to run dimmer LEDs, program 5 mA or higher and reduce brightness with the 12-bit grayscale and 6-bit dot correction instead. Use a 1 percent thin-film resistor and keep it close to the pin, since the reference current is small and noise-sensitive.

They do different jobs. SCLK clocks the serial data shift register, so it only runs while you are loading new grayscale or dot-correction values, up to 30 MHz. GSCLK is the continuous grayscale timing clock that drives the internal PWM counter through all 4096 steps of each frame — the frame period is 4096 GSCLK cycles, so at a 4 MHz GSCLK you get roughly 977 Hz refresh. BLANK terminates a GSCLK frame, resets the counter and latches the data. Driving GSCLK from a stable oscillator or MCU timer is what gives you flicker-free, camera-friendly dimming.

It trims the constant-current level of each channel by 6-bit (64-step) resolution to compensate for LED forward-voltage and brightness spread, channel-to-channel mismatch inside the driver, and driver-to-driver variation across a daisy chain. Program it once at manufacture: raise VPRG to select dot-correction mode, clock in 96 bits (16 channels × 6 bits) on SIN with SCLK, pulse XLAT with DCPRG set appropriately, and the values are written to the integrated EEPROM, which survives power cycles and is rated for 50 writes. You can read them back through SOUT for verification.

Both are reported on the open-drain XERR pin. LED open detection compares the output voltage against a 0.3 V to 0.4 V threshold during a dedicated detection phase and flags a broken or disconnected LED on any channel. The thermal error flag trips when the junction reaches 150 °C to 170 °C, which tells you the driver is being over-driven or has insufficient copper under the PowerPAD. XERR goes low after a BLANK edge and stays low until you read the error status registers through SOUT, so in a daisy chain you must clock out the status of every device to clear the line. Wire XERR to a microcontroller input with a pull-up.

Yes, that is one of its main advantages. The outputs are open-drain constant-current sinks rated to 17 V on the LED supply (18 V absolute maximum), completely independent of the 3 V to 5.5 V VCC. So you can put several white LEDs in series between 12 V and the output pin, with the driver sinking the programmed current. Check the headroom: with a 1 V to 3 V output voltage at the sink transistor you can drop roughly 9 V to 10 V across the LED string, which is about three white LEDs at 3.2 V. Above that you need a 24 V rail or a boost converter.

Test the functions that clones usually break. Program a dot-correction value through VPRG, power cycle, and read it back through SOUT — a genuine part retains it in EEPROM, clones often lose it or fail the write entirely. Then measure the constant sink current on all sixteen outputs with the outputs at 1 V and compare: TI specifies ±1 percent to ±4 percent device-to-device matching at R(IREF) = 640 ohm over -20 °C to 85 °C, while remarked parts scatter widely. Finally check the laser marking, the HTSSOP-28 PowerPAD and the date code. We electrically test samples from every reel.

MOQ is one reel of TLC5940PWPR (2000 pcs, HTSSOP-28 with PowerPAD) for volume, or 100 pcs loose for prototyping; the PDIP-28 comes in tubes of 25 and the VQFN-32 on reel. Shenzhen stock ships in one to two business days. TI authorized-channel lead time is ten to eighteen weeks, and because the TLC5940 is a mature part, allocation is rare but the PDIP version is frequently short — plan production around the HTSSOP.