Electronic – Cheap 4 digit 7 segment display driver

My guess would be this:

To switch the display on:

• STB goes low
• Clock in the byte 00001101 (0x0D)
• STB goes high

To write the segments:

• STB goes low
• clock in the Write command: 00000000 (8 bits of zeros). This means: it's a write command, with auto-incrementing addresses, writing to address 0x00 on page 0x00.
• Now you can clock in the data to fill the memory. This consists of six bytes, one for each digit.
• STB goes high

To clock data (make sure the clock is slower than 1MHz):

• STB goes low
• Loop for 8 bits of the byte
  • Set data out bit
  • Clock goes low
  • Clock goes high
• End of Loop
• STB goes high

It appears that the reading of the datasheet whereby TLC5940 can sink only 120mA/chip is incorrect:

Rather, it seems that the correct reading of the "\$I_O\$ Output Current (dc)" value on page 2 of the datasheet is as maximum current per channel, which is also the interpretation arrived at by Sparkfun discussion to this driver. The ambiguity of the phrasing Output Current (dc) is resolved in favor of per channel reading owing to:

• The fact that these drivers employ current mirrors, which given sufficient (voltage) headroom (see figure 5 of the datasheet), no more than that, do not use very much power.

^{simulate this circuit – Schematic created using CircuitLab}

To clarify the above point: In the schematic above, the voltage that the TLC5940 sees, i.e. LED Vcc (or \$Vcc_{LED}\$ but be above certain threshold, cal that threshold \$V_{HEADROOM/TLC5940}\$, which is around 1.25V for \$I_{output}\$ 120mA, and is .35V or so for \$I_{output}\$ 20mA. Any voltage at \$V_{OUT/TLC5940}\$ over \$V_{HEADROOM/TLC5940}\$ must be dissipated by the device as heat. For maximum possible current in the TLC5940, you would want to match \$Vcc_{LED}\$ to \$V_{forward}\$ of that LED, so that the TLC5940 IC dissipates the minimum power necessary, since power dissipation rating is a limiting factor on how much current the device can sink at each channel.

(and back to the list)

• The fact that in recommended operating conditions the maximum current refers explicitly to channels:

• According to the power dissipation calculation, assuming \$d_{PWM}\$ of 100%, VCC=5.0V, supply current of 60mA, \$V_{OUT}\$ of 1.0V (test conditions), dot correction of 1, we arrive at 5.0V*0.06A + 1.0V * 0.06A * 1 * 1 * 16channels we get 1.26W of device power dissipation. The PDIP package is rated for at least 2.5W at 25 deg C.