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MAX3232: Output signal oscillations DOUT2

Vladimir Usachev
Prodigy 50 points
Part Number: MAX3232

Hello TI.

Figure 1 shows the output signal of the MAX3232 chip on the DOUT2 pin. Output DOUT1 has a clear signal. the range of the DOUT2 signal voltage is 480 mV, the amplitude is 5.67 V. Figure 2 shows the connection diagram of the MAX3232 chip with capacitors. Is there a solution to this problem?

Figure 1

Figure 2

Best Regards, Vladimir.

  • 0
    TI__Guru 62920 points

    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

  • 0 in reply to Miguel Robertson
    Prodigy 50 points

    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.

  • 0 in reply to Vladimir Usachev
    TI__Guru 62920 points

    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