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I'm having a doubt about the voltage rating of the capacitors of the RS3232 (a UART to RS232 transceiver).

typical application

For C3 and C4 (storing the +- voltage rails for the transceiver), the never exceed voltages are 7 and -7V, so 10V capacitors should be fine.

For C1 and C2, I'm not really sure what they do (I suspect they are used for the charge pumps), so I'm not sure about the voltage that will be on these pins, so I'm unsure about the voltage ratings.

Do you know what voltage rating I need for C1 and C2? (nb : supply voltage will be 4.5V)

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4 Answers 4

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Neither TI nor Analog Devices specify this in their datasheets.

When relevant info is missing, I usually check for application notes or reference designs. TI has some reference designs with schematics and BOMs (see below).

Following capacitors have been used in these reference designs:

  • CV+: 0.1μF/50V and 0.1μF/25V
  • CV-: 0.1μF/50V and 0.1μF/6.3V
  • C1+/1-: 0.1μF/50V and 0.1μF/6.3V
  • C2+/2-: 0.1μF/50V and 0.1μF/6.3V

To be on the safe side I would pick 100nF 50V X7R 10% 0603 ceramic caps (these are quite common parts), so I wouldn't have to worry about it.


DLP4500-C350REF

CV+, CV-, C1+/1-, C2+/2- are rated (see BOM): 0.1μF, CERAMIC, 50V, X7R, 0805

enter image description here


TIDM-TM4C129USBHS

Download the PDF and left-click on the components.

CV+:
enter image description here
enter image description here

CV-, C1+/1-, C2+/2-:
enter image description here
enter image description here

enter image description here


TIDC-SPPBLE-SW-RD

CV+, CV-, C1+/1-, C2+/2- are rated (see BOM): 100nF, CERAMIC X7R, 50V, +/-10%, 0603

enter image description here

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I believe RS232 signals are specified to work up to +/- 15VDC (source). Now the caps don't sit on the signal lines directly, but I would probably use 16V caps as a bare minimum, just to be safe. Using the same cap with 100nF at all these locations would be ideal for BoM. 100nF X5R 16V or 25V is by no means expensive or exotic, so why specify something worse just for the heck of it?

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The answer is not that straightforward.

The capacitor volatage rating will depend on what types of capacitors you intend to use.

So for instance, even if the chip has a maximum absolute limits for the voltage at +/- 7V, a 10V capacitor may not be good enough depending on what kind of capacitor it is.

If you use e.g. 100nF 10V ceramic capcitor, it won't have 100nF of capacitance at 10V, so you should not use a 10V rated capacitor. A 25V or 50V rated capacitors are better for this.

If you use a 0.1uF 10V electrolytic, then it can be fine from the voltage point of view, even if electrolytic capacitors may work poorly at low temperatures and might be a bad choise anyway.

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    \$\begingroup\$ "it won't have 100nF of capacitance at 10V" Doesn't this depend on dielectric? IIRC the NP0 ones are specified to be accurate across the whole specified voltage range. (Though of course using NP0 for plain old decoupling caps is pretty senseless.) \$\endgroup\$
    – Lundin
    May 24, 2023 at 11:58
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    \$\begingroup\$ @Lundin It seems I forgot to specify X7R somewhere. I'll edit it in. But I checked and I could not find a 100nF 10V NP0/C0G, that's too large. \$\endgroup\$
    – Justme
    May 24, 2023 at 12:36
  • \$\begingroup\$ Yeah I think there was some other post about that here recently and around 1uF seems to be the upper limit for NP0/C0G currently (and those are very expensive). More relevant in this case I think, is the definition of "X7R". I read some research paper about capacitance accuracy of those at some point and there was a lot of difference between different vendors. The bottom line is, as always, to design with margins. \$\endgroup\$
    – Lundin
    May 24, 2023 at 13:14
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One is used to double the supply voltage; this is where 7V comes from. The other is used to invert the boosted voltage (hence -7V). So they will have 3.3 and 6.6V respectively. Or I suppose, for up to 5.5V supply, 5.5 and 7, and there must be some internal limiting (clamping or regulation) that prevents the output from exceeding its rating. This could be before or after the boost stage, so a 3.5 to 5.5V rating might be needed on the boost capacitor.

10V is a safe assumption for all of them.

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    \$\begingroup\$ But a 100nF 10V X7R capacitor would not be a safe assumption. How much capacitance a 10V X7R cap has if you put 7V over it may not be enough. \$\endgroup\$
    – Justme
    May 24, 2023 at 10:55
  • \$\begingroup\$ @Justme The C(V) of a type 2 ceramic is unrelated to its voltage rating; voltage rating is merely the safe operating rating, which is more or less what's asked about here. C(V) is more strongly correlated with body size, which would be fine for 0402 and up, here. \$\endgroup\$ May 24, 2023 at 11:12

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