What is different between ST3232 and ST202?

[TShim.8]

I knew that ST202 is compatible for ST3232. So I changed only ST202 into ST3232 without any circuit changing.

But ST3232 doesn't work.

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What is different between ST3232 and ST202, other than power supply voltage?

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I found the difference in typical application in datasheet. In ST202, V+ is connected VCC(5V) through a 0.1uF capacitor. But in ST3232, V+ is connected GND through a 0.1uF capacitor.

In fact, is it correct that V+ must be connected VCC in ST3232?

[Peter BENSCH]

The design should also work with the ST3232 if you take into account that other values are needed for the charge pump capacitors. The ST202 (obsolete) worked with 100nF capacitors, while the ST3232 requires much larger values to operate at 5V. You can find the values in the respective data sheet of ST3232EBxx/ECxx or ST3232Bxx/Cxx, each in table 9

It should not matter much that one of the capacitors is connected to 5V and the other to GND, because only the (approximately) doubled voltage is applied to V+ and is buffered by this capacitor.

Please note that the doubled voltage and thus the voltage values for Mark and Space differ: for ST202 it was between ±13.2V, for ST2323 ±7V.

Hope that answers your question?

Regards

/Peter

[TShim.8]

Thank you for your reply.

I use ST202 in 5V and use ST3232 in 3.3V.

In ST3232, capacitors (C1-C4) are 0.1uF.

I use non-polar chip capacitors.

In datasheet, direction of V- capacitor is different from others.

Should I use polarized capacitors? ​

[Peter BENSCH]

Ok, then both should be used with 0.1µF capacitors, non-polarized MLCC are fine.

As far as the capacitor at V- is concerned, a look back into history helps: the first RS-232 converters still had a lower switching frequency for the charge pump and used values of ≥1µF. At that time, such capacitors were not yet available in MLCC or were very expensive, so electrolytic capacitors were used.

Because V- actually carries a negative voltage compared to GND, such an electrolytic capacitor had to be connected with the correct polarity, i.e. as shown in the data sheet.

With today's ceramic MLCCs, the polarity does not matter; there are practically no polarised 0.1µF capacitors any more.

Regards

/Peter