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MAX3221: MAX3221 capacitors not configured for the VCC input

Glenn Varnon52
Prodigy 240 points
Part Number: MAX3221

I was working an issue on one of our PCA's to determine why the RS-232 interface was not working. As I troubleshot, I was thinking it was just a bad chip. So, I replaced the chip. It still did not function correctly. I decided to look at the datasheet to see if I was missing a connection or a component. 

As I reviewed the datasheet, I noticed that our design uses 0.1uf capacitors on the voltage doubler pins (positive and negative) and on the 5.5V and -5.5v supply generated by the charge pump. The VCC to the chip is 5.0vdc.

When I looked at page 13 of the datasheet, this seems to be an issue. C1 should be a 0.047uf cap and C2 to C4 should be a 0.33uf cap for that condition.

My issue is that it seems like the circuit has been in that condition since its invention and has been operating that way for several years. 

So, what are the effects of running the device in a 3.3vdc configuration in a +5vdc environment? What would be the effects? I assume I should be concerned.

  • 0
    Guru 298860 points

    A smaller input capacitor increases the input ripple (which can affect the other devices on that supply).
    A smaller output capacitor increases the output ripple.
    Smaller flying capacitors increase the output impedance, i.e., the output voltage might drop too much. For a regulated charge pump, they also increase the switching frequency and can decrease the ripple.

  • 0
    TI__Mastermind 38093 points

    Hi Glenn,

    Glenn Varnon52 said:
    So, what are the effects of running the device in a 3.3vdc configuration in a +5vdc environment? What would be the effects? I assume I should be concerned.

    From my understanding of your question, you are asking what are the effects of running the device with 3.3V capacitor configuration when supply voltage is 5V? That is, VCC = 5V, then C1 = C2 = C3 = C4 = 0.1uF. This shouldn't cause the device to fail. If anything, smaller charge pump capacitors will reduce ripple voltage height. I have attached a charge pump blog post that explains the effects of changing charge pump capacitance.

    https://e2e.ti.com/blogs_/b/analogwire/posts/how-the-rs-232-transceiver-s-regulated-charge-pump-circuitry-works

    This blog post also goes into details about some of the comments Clemen's have made. 

    Since you are using 3.3V capacitor configuration on a 5V system, can you lower your supply voltage down to 3.3V to see if the transceiver operates correctly? Do you have scope captures of the input/output waveforms?

    Regards,

    Tyler