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Author Topic: Best way to drive top side of 8x8 LED matrices when using TPIC6C595  (Read 771 times)
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In order to drive LED matricies you need control on both the top side and bottom side.  Bottom side control is pretty easy to accomplish using TPIC6x595 power shift registers as they can sink a pretty decent amount of current and are very easy to control since they are really just 595s with a big sink on them.  For top side control, it seems that a P-channel MOSFET or PNP transistor is the obvious choice.  I have a number of these, is this a good choice?

http://www.fairchildsemi.com/ds/ND/NDP6020P.pdf

The Arduino would select one FET's gate of 8 via a 74LS138 which holds all 8 outputs high (FET gates) except the selected outout.  The one low FET gate "opens" the selected FET and connects that row of the LED matrix to +5V.

How do I calculate the base resistor that is appropriate for this FET for +5V?

Thank you.
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Its a MOSFET, so not much current is needed.
You're really just protecting the driver from the FET's input gate capacitance.
With 1590 pF input capacitance, not much protection is needed.

74LS138 can only source 0.4mA of current, so I would add pullup resistors to turn the FET off fast/
It can sink 8mA, so  size the pullup to not exceed that.
5V/1K =5mA, should provide good performance.
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Its a MOSFET, so not much current is needed.
You're really just protecting the driver from the FET's input gate capacitance.
With 1590 pF input capacitance, not much protection is needed.

74LS138 can only source 0.4mA of current, so I would add pullup resistors to turn the FET off fast/
It can sink 8mA, so  size the pullup to not exceed that.
5V/1K =5mA, should provide good performance.

Thank you for the response.  If I read this right you are recommending a 1K pullup between the 138 pin and the base of the MOSFET, not a base resistor.  I was thinking that a base resistor might be necessary like a normal transistor.  There is some disagreement out there on this.  For example:

http://arduino.cc/forum/index.php?topic=80026.0

Grumpy Mike and johnwasser seem to disagree here.  This article:

http://talkingelectronics.com/projects/MOSFET/MOSFET.html

Says the following:

"However the 10k resistor on the gate of the MOSFET is not needed. Providing the voltage (up to 18v) on the gate rises and falls quickly, the MOSFET will not get hot. The critical period of time is the 0v to 3v section of the waveform as this is when the MOSFET is turning on."

Which seems to indicate to me that you can get away with not having a base resistor, but good design principles might indicate having one just to keep power usage the lowest possible by reducing dissipation of the MOSFET while in the ohmic region, no matter how short that is.

So, am I right on not needing a base transistor?  Just go with the 1K pullup and be done with it?  That works for me.
« Last Edit: September 10, 2012, 04:31:42 pm by JoeN » Logged

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I think in this case, where the pullup resistor is providing the current to charge the gate capacitance far in excess of what the LS138 is providing, that the Gate (not Base) current limit resistor can be skipped.
http://www.ti.com/lit/ds/symlink/sn74ls138.pdf
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Excellent.  Thank you for your help!
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