Answer 3
As with all switched-capacitor circuits, the effective circuit impedance (in this case, the output impedance of the converter) is the inverse of the product of the switching frequency (in hertz) and the capacitor value (in farads).

An RS-232 driver is expected to have a current limit of around 10 mA. Let's assume you can tolerate a 1-V drop in output voltage at this current. This means that the converter must have an output impedance no higher than:

Given that the switching frequency is 100 kHz, then C1 must be at least:

If the converter is supplying current to multiple output drivers, then the value of C1 must be multiplied accordingly.

Contributor: David Tweed

Published July 2003