Maxing out the MAX7219
Electronegativity
Posts: 311
If anyone out there is planning to push the MAX7219 display driver near the limit of its capability you might want to consider adding extra bypass capacitors.
I have a current project that uses an SX28 to control 8 high brightness 7 digit displays through the MAX7219.
With the manufacturer's recommended circuit (1 X 10uF & 1 X 100nF capacitors), it runs fine for a minute or so then some of the segments flicker and fade out.
I figured the capacitors might be draining faster than they could recharge and started placing additional capacitors accross the Vin and ground pins of the MAX7219.
An additional 220uF was sufficient to take care of the problem.
I put another 100uF bypass capacitor on Vdd of the SX28 just to make sure.
The power supply is 8 rechareable AA batteries, an L7805 (5.0V 2.0A) regulator, and 100.1uf of capacitance on both the input and output.
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I wonder if this wire is hot...
I have a current project that uses an SX28 to control 8 high brightness 7 digit displays through the MAX7219.
With the manufacturer's recommended circuit (1 X 10uF & 1 X 100nF capacitors), it runs fine for a minute or so then some of the segments flicker and fade out.
I figured the capacitors might be draining faster than they could recharge and started placing additional capacitors accross the Vin and ground pins of the MAX7219.
An additional 220uF was sufficient to take care of the problem.
I put another 100uF bypass capacitor on Vdd of the SX28 just to make sure.
The power supply is 8 rechareable AA batteries, an L7805 (5.0V 2.0A) regulator, and 100.1uf of capacitance on both the input and output.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
I wonder if this wire is hot...
Comments
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·1+1=10
Swapping in a LD1084-50 regulator (5.0V 5.0A) for the LM7805 (5.0V 2.0A) made no difference.
I messed around with the refresh rate (intensity) but that didn't help.
Finally I switched in a 5.5k ISET resistor even though the data sheet says not to go below 9.53k, and it seems perfectly fine now.
I guess I am pushing the MAX7219 a little out of spec here, but the displays I am using are real power hogs (2 X Lumex LDQ-N516RI).
The displays can take more current than the driver can give, so I suppose it had nothing left by the time it came to scan the final segment.
The extra capacitors boosted it up so that the segment didn't become completely dark, but it was still dimmer than the others.
A higher current display driver would probably be a better solution but I like the sophisticated decoding that the MAX7219 has to offer.
Hasn't burned up yet...maybe it won't.
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I wonder if this wire is hot...