Sizing source drive

Can anybody point me where to learn more about how a source drive ( transistor array) works, how to read the datasheet and properly size. i am trying to troubleshoot a design and i am a beginner.

Tell us more about what you are trying to accomplish, specifically. As for picking on, you are likely going to be looking at breakdown voltage and current output capability primarily. Of course it all depends upon exactly what you are trying to drive.

i am developing a larger 7 segment display which uses about 7 volts 60ma per segment. The source drive will have a 12v 2A regulated supply, the source drive would be turned on from a max7219. One question i have is, is the output voltage dependent upon the input voltage from the max7219? (i.e. will a 4 volts result in more output voltage than a 1v?)

Well, the MAX7219 expects to be a current limited supply to the load. By sticking another device in between the display and the MAX chip, you will defeat the whole purpose of it I think. You should try to find another current driver that uses a larger supply voltage.

I thought the source driver will take small current from the MAX7219 and "open" the path for the 12v to go to the segment. I thought the driver act as a series of transistors. I guess, then that comes back to my original question, Where can learn about how to properly use and size a source driver, no matter what the application.

Do you have a part number for the "source driver" you are considering. I looked at some at Jameco and they don't appear to have any mechanism to limit the output current, other than the device's specified limits. The MAX chip expects to be able to sense the actual load current in order to do what it does. With another device in between the load and the MAX chip there doesn't seem to be any direct way to accomplish that. I suppose you can just use the MAX chip to strobe the display and current limit the segments some other way, but it seems like a waste to ignore one of the chips main features.


You should look at Maxim application note on doing this.

Do you have another thread going on this also, with UDN2891 & ULN2803?

If not, and you come across it, post this link there.

MAX7219 high voltage App note AN1196.pdf (69 KB)

forgive me,maybe, i am not describing my problem well enough. My layout works, however the leds are not as bright as they are supposed to be. i can get digits to scroll ( using ledcontrol library with intensity-15) So the MAX is turning on the source driver, however i just tested and with a 12v input i am only getting 6.9v output.

yes, that was my post, after posting i thought it may be too specific, and thought i would go for a more general question.

The application note that Crossroads posted a link to will show you how to get more current out of the MAX7219. The app note is suspiciously silent about R1 in the segment driver circuit. Looks like you have to do your own current limiting to the segments.

The voltage output of the MAX chip is irrelevant it will only be high enough to exceed the Vf (forward voltage drop) of the segments, it is the current that matters. The reason your display is dim is that the MAX7219 can only supply 40mA and your display needs 60mA.

I guess that is where I am confused, I thought the max7219 is sending a low voltage, low current to the UDN2981 source drive. I though this low current will switch the source drive will allow the 12 volts higher amp current to the segments. I am wrong in this assumption? I thought the source drive acts as a transistor?

The UDN2981 would do as you describe, but without something to limit the current, it would dump 500mA into the segments. The MAX chip has a programmable set resistor that determines how much current it will deliver into its load. The UDN2981 doesn't have anything like it, that I can see. You can't just apply voltage to an LED like you would a light bulb. Without current limiting, they will burn out as soon as they turn on. The app note that Crossroads posted shows how to do this with a transistor, and limit the current with a resistor.

From the 2981 i was planning some resistors before the leds. This is why I was measuring the output of the 2981. I am getting no where near the results I need. I only need 60ma and 7 v per segment. Could I have a faulty unit? I figured I would get no output at all if faulty.

So you are saying that the output from the UDN2981 isn't enough to light them up the way you want? It's supposed to put out 500mA as I recall. Try testing The LED with 12V and a 100 ohm resistor outside of your circuit. Based upon the LED dropping about 7V, the 100 ohm resistor should put the current at 50mA.