I am looking at using the IS31FL3731 IC in a project soon, but I am confused as to why the data sheet says that the circuit can only use 32 RGB LEDs when it can use 144 regular LEDs. The data sheet says that the remaining LEDs in the Charlieplexed array not being used by the RGB LEDs can be used, but only for LEDs that are not red. Is this because the forward voltage for red LEDs is lower than the forward voltage for green and blue ones? If this is the case then could this be compensated for by matching the forward voltage of the red LED to that of the green and blue LEDs using a resistor?


if you look closely at the matrixes you will see that they are not regular square crosspoint matrixes, but instead charlieplexed. because of this each logical row and column has only 8 positions which limits the number of RGB leds that can be fitted.

thus there are only 16 places in each matrix where common-cathode or common-anode RGB leds will fit, charlieplexing also has voltage concerns where different led colours are mixed.

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  • \$\begingroup\$ I can see why the middle column would not be able to be used, but why can C1-1 to C1-3 and C9-6 to C9-8 not be used for 2 additional RGB LEDs? \$\endgroup\$ – user3242816 Oct 6 '19 at 19:26
  • \$\begingroup\$ chearlieplex works because the threshold behaviour of the leds (no light before 1.6V etc) blocks the undesired current paths (because the undesired paths are longer than the desired paths which are all one step), this workls great when all th LEDS are the same voltage threshold (ie same colour) when colours are mixed gaps are needed in the matrix to prevent paths that involve wrong colour LEDs from lighting up instead of the desired led... or maybe it's just that the data sheet author never thought of that. \$\endgroup\$ – Jasen Oct 6 '19 at 19:53

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