MAX3232: Output signal oscillations DOUT2

Hi Vladimir,

Some ripple is expected.  This is because the capacitors connected to the chip form a charge pump circuit which generates the higher-voltage V+/V- rails that are switched onto the DOUT ports to send data.  You can read more about it and how the cap values influence output ripple here:

e2e.ti.com/.../how-the-rs-232-transceiver-s-regulated-charge-pump-circuitry-works

It is strange that DOUT1 and DOUT2 would look different, though, since they share common V+/V- rails.  Is one driving high and one driving low?  How do the V+ and V- signals look?

Regards,
Max

Yes, in the starting condition DOUT2 driving high, DOUT1 driving low. The forms of these signals repeat the V+ V- signals shown in figure 3 and 4, respectively. Also, the RTS signal from the microcontroller is pulled to the ground. Is it possible to reduce the oscillation of these signals?

Figure 3 - V+

Figure 4 - V-

Best regards, Vladimir.

Hi Vladimir,

Yes, the capacitor values will influence the ripple frequency and magnitude.  Some equations and example waveforms are given in the blog I linked above.  A lower ratio of C1 to C3 results in less of an increase in the V+ voltage during a charging cycle.  The rate of discharge (and therefore the time between charging cycles, i.e., the ripple frequency) depends on the C3 value, so lower values would give higher-frequency ripple and higher values would give lower-frequency ripple).

Regards,
Max