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[SOLVED] CD4050 appears to be leaking current from inputs to Vdd
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[SOLVED] CD4050 appears to be leaking current from inputs to Vdd

by despedes on Fri Aug 09, 2013 2:08 am

Hi,

I've been working on a sound project based on an Arduino. I need to be able to read audio data from a SD card and play it as PCM through one PWM port. This is fairy easy using the TMRpcm library, so I managed to get the thing working on the breadboard. I used a CD4050 chip as a level register to communicate with the SD card, and used my bench power supply for 3.3V.

I then soldered the board I changed the bench supply to the LM1117 powered by a 9V battery. However, the micro controller couldn't talk to the SD card. I thought that I had messed up something so I checked every connection, voltages, changed the voltage regulators (LM1117-3.3). Nothing worked.

After a couple hours troubleshooting, I found that the output of the LM1117-3.3 regulator was a little bit high (at 3.8V) when the Arduino was hooked up and returned to 3.3V once I disconnected it. I reproduced the level shifter circuit on the breadboard and found that the 5V input from the Arduino were leaking through the Vdd of the 4050, thus increasing the voltage of the 3.3V bus (to around 3.8V). I changed the chip for a new one and still, same deal. I even disconected the Vdd from 3.3V and could light up a LED throug Vdd and Vss in the 4050 by only having the Arduino inputs connected.

Has anybody experienced something like this? Maybe my batch of 4050 is bad? The supply and inputs inside the CD4050 should be isolated, right?

Thank you in advance for any comments, advice or anything you could give.
Last edited by despedes on Fri Aug 09, 2013 6:46 pm, edited 1 time in total.

despedes
 
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Re: CD4050 appears to be leaking current from inputs to Vdd

by zener on Fri Aug 09, 2013 2:43 am

Look at the data sheet for the 4050. The answer is probably there.

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Re: CD4050 appears to be leaking current from inputs to Vdd

by despedes on Fri Aug 09, 2013 10:01 am

Hi Zener!

Thanks for your post.

Unfortunately, the datasheet doesn't give any information regarding this issue.

By the way, I checked and rechecked the pinouts and compared my wiring to the schematics of the micro-SD breakout board from Adafruit. Everything seems to be in order.

despedes
 
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Re: CD4050 appears to be leaking current from inputs to Vdd

by zener on Fri Aug 09, 2013 6:11 pm

What you have there is a paradox: "That which cannot be but is." I have seen those many times. They are always interesting. You seem to have good troubleshooting skills. The LED test was a good idea. I would also do a diode test from the input to the Vdd. Technically it should not show any current but seems like it will in your case. I would confirm it acts the same in the bread board and out of the bread board. The TI datasheet shows the alleged internal circuitry, which should definitely not conduct in the way you are seeing. It is (slightly) interesting that the test conditions for the Input Current Max spec do not include a case where the Vin voltage is higher than Vdd, even thought that is how it would be in a level shifting application I think.

If it was me I would probably call TI and ask them about it. Or maybe they make you fill out a web form these days, I don't know. There may be an explanation for it. As it sits I am not sure if it is violating any specs on the datasheet.

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Re: CD4050 appears to be leaking current from inputs to Vdd

by despedes on Fri Aug 09, 2013 6:44 pm

Thanks Zener for your comments.

I finally managed to fix it. I just went to another store and bought Fairchild's 74HC4050. Everything works great now. No more leaking current through the Vdd pin.

I believe the CD4050s I bought before were just faulty.

Anyway, thanks again.

despedes
 
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Re: [SOLVED] CD4050 appears to be leaking current from inputs to Vdd

by zener on Fri Aug 09, 2013 7:32 pm

I am curious, who is the mfg of those CD4050 and what is the date code?

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Re: [SOLVED] CD4050 appears to be leaking current from inputs to Vdd

by despedes on Fri Aug 09, 2013 10:20 pm

This is the chip.

Image

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Re: [SOLVED] CD4050 appears to be leaking current from inputs to Vdd

by zener on Fri Aug 09, 2013 11:23 pm

Thanks. That looks like a fairly recent one.

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