# How the MAX232 works?

Ok, so I understand what the MAX232 does, and I understand why it does it and why I need it. However, my question is about the lower level of how it uses the external capacitors.
Going off the schematic below: How are capacitors C1, C2, C3, and C4 being used? Are C1 and C2 being used as charge pumps? If so, then what are C3 and C4 doing? From what I know about capacitors it looks like C3 and C4 are being used as decoupling caps like C5 is, right? I really don’t need to know all this because I know how to assemble and use everything properly, I am just really interested in how it actually works. Thanks!

As shown in the diagram, there are TWO “charge pump” circuits. C1 and C3 are used a a voltage-doubling setup, while C2 and C4 are used voltage-inverter setup. (When you think about it, a voltage inverter is another type of voltage doubler. The voltage difference between the most negative output and the most positive output is double that of the input.) (in other words, over all the max232 circuits provide voltage QUADRUPLING - the input is 0 to 5V (5V total) and the output is -10 to +10 (20V total), so it requires two stages of charge pump.)

But from what I can find about voltage doublers they double an AC voltage to DC and the MAX232 only tkaes DC current. So what kind of charge pump or doubler is the MAX232 using?

Also, if it is a charge pump then why is the negative lead of C3 connected to Vcc?

Thanks again.

It has an oscillator to convert the 5VDC to AC before it doubles and inverts.

-j

That would explain a lot! Thank You!

Now here’s something interesting. This schematic (below) replaces C3 and C4 with a single cap connecting V- and V+. Would this work properly?

http://www.sparkfun.com/commerce/images/tutorials/BeginningEmbedded/4-UART/UART1.jpg

Also, after doing more research it seems that the external capacitors are only there to increase voltage to around (+/-)5.5V and then it is the drivers that actually convert the voltage to around (+/-)12V. Although this isn’t true for all max232 chips, but it is for most. This is to allow communication with LVTTL (3.3V) without having to add more circuitry.

I found this out by looking at the datasheet for a generic max232 chip (Sipex SP3232). Sipex’s documentation is AMAZING! Not only do they include diagrams of some of the internals of the chip, but they explain all the stages of the charge pumps, drivers, and receivers. I haven’t been able to find that in any Maxim or TI datasheet.

Would this work properly?

I don’t think so. If the data sheet says use two capacitors, from each pin to ground, then it’s always wise to stick to that arrangement! As shown, the smoothing effect of the capacitor(s) would be diminished. Imagine both positive and negative supplies rippling up and down in phase, relative to 0V.