# Example circuit in datasheet for LED driver doesn't make much sense to me

Oct 28, 2022
4
I'm wanting to do a multiplexed LED project, maybe using the TM1681 LED driver. The datasheet has a confusing suggestion and I was hoping someone could tell me whether they think it makes sense. https://html.alldatasheet.com/html-pdf/1133642/TITAN/TM1681/713/4/TM1681.html

The datasheet is in Chinese but on page 4 it says that the row source current is 7 mA while the com sink current 350 mA. On the second half of page 11, it suggests that we might want to use transistors to power the LEDs but why does it put them on the com pins? At 7 mA per row, even if all 32 rows are on, that's 224 mA which is still below the 350 mA capacity of each com pin. Is there any point in that configuration?

In the other examples it has transistors on both rows and com pins but this particular one seemed pointless to me, offering no increase in current over direct drive. I will probably use transistors on the rows and the columns.

Edit: I just noticed, in the examples where transistors are used on both the rows and the columns, for some reason they swap the row and column pins around so the rows are controling the sinking of current through transistors and the coms are controling the source of current through transistors. What did they do that for?

Last edited:

#### kellys_eye

Jun 25, 2010
5,497
When multiplexing LEDs it is common to overdrive them to achieve the required light output - given that the strobing process is a very short mark-space ratio, the actual driving current pulse can be many times the rated, steady-state, value of 7mA.

Oct 28, 2022
4
When multiplexing LEDs it is common to overdrive them to achieve the required light output - given that the strobing process is a very short mark-space ratio, the actual driving current pulse can be many times the rated, steady-state, value of 7mA.
Thanks for replying. I realise that but that's not achieved by the transistors in the design on page 11 since the rows are the bottleneck and the transistors are on the coms which already have room for more current.

#### kellys_eye

Jun 25, 2010
5,497
The additional use of row/column driver transistors is specified for different 'power level' displays - i.e. if you use a larger segment (dot) type display you will need to increase the current handling capability of the driver lines of the controlling IC hence the use of additional transistors. In medium power applications only either the column or rows need these additional devices - in the case of the 'very large' displays you need to use buffer devices on BOTH the row and column lines hence the application note showing how thee differing power consumption displays are actually connected.

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