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8 channel source drive

eagle22

Feb 1, 2013
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Can someone please tell me where to find information on how a 8 channel source drive works, and how to read its datasheet. i am a beginner and i am trying to troubleshoot an existing project.
 

Harald Kapp

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What part are you considering? There are so many...
We can help you understand the datasheet provided you show us the one in question. No two datasheets are created equal, so it will be utterly useless to explain one datasheet while you are looking at another one.
 

Harald Kapp

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O.K., I can find the datasheet.
Where's the question/problem? Which part needs explaining?
 

eagle22

Feb 1, 2013
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does the output voltage vary depending on the input voltage used to switch it on? Also, is the current output per output or for the the driver. For instance is i have 7 segments connected and the datasheet shows output of 350ma, is that 350 per segment or 50 (350/7) per segment?
 

gorgon

Jun 6, 2011
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All your questions is answered in the datasheet, bur may need some decoding.

From the datasheet you will find that the drivers need at least 2.4V input voltage to turn on full. The 350mA is the max current for each output, BUT you can't drive all outputs at the same time. From the graphs at page 2, 5 and 6, you will see that the max power dissipation is about 1.5W as best case with 25 deg ambient temperature.

This will result in a possible continuous current for all outputs at the same time to about 100mA. This will be close to the limits for the chip. Reducing the number of active outputs, and/ or duty cycle will increase the possible output current up to max 350mA per channel.

If you design for long life of operation, it is normal to be a bit conservative when loading the chip. You should also take ambient temperature into consideration.
 

eagle22

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i only need 60 ma , 7 v per output, so i thought it would work. if i measure the current from the 2981 to a 1 ohm resistor before the leds, i only get about 30ma, which is half of what i need. could the device be faulty. i know i have 3.5v from the max 7219 to the udn2981.
 

Harald Kapp

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What is the forward voltage of your LED? What is the (measured) output voltage of the IC?
Due to the darlington structure of the output, the output voltage will be at least 1.6V-1.8V less than Vs (the parameter is Vcesat) . So if Vs=7V, Vout is only 5.2V-5.4V. This may not be enough to drive 60mA through your LED.
 

eagle22

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Here is the schematic, i was trying to modify one for my application.
It was actually from this forum thread "MAX7219 and higher voltage".
my display is not shown but each segment is 6 leds (3 paralllel rows of 2 led in series), For a total required voltage of 6.9v and 60ma. The leds are bright blue at 3.45v / 20ma each. FYI, i had since replaced the ISET to the max7219 resistor with a 5ohm to make sure the UDN2981 is getting enough voltage to operate.
 

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Harald Kapp

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How do you meaasure the current? You do realize that the display is multiplexed, do you? Which means that the current is only flowing for short periods. If you use a multimeter, it will show an average current, not the peak value. To see the peak current, you will need an oscilloscope.
 

eagle22

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I do not have an oscilloscope. is there any other way to measure or do you know where to get a reasonably priced oscilloscope. Otherwise i have begun slowly checking every point of the circuit.
 

Harald Kapp

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Remove the MAX7219.
Apply static signals to the inputs of the ULN2803 and the UDN2981 such that only one LED is lit. Now measure the current. It is DC only. The value measured should be the same as the peak value when multiplexing.

This is an inexpensive scope type instrument. Note that it is not a full featured oscilloscope, but useful for quickly looking at waveforms.
 
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