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SN65C3232E: About C1~C4 value

Ryoma Ohashi
Expert 4280 points
Part Number: SN65C3232E

Tool/software:

Hi team,

Can this device used with below C1~C4 values?

  • C1, C2, C3, C4：All 1μF @ Vcc=5V
  • C1: 0.1uF, C3~C4: 1uF @Vcc=3V~5.5V

Regards
Ohashi

  • +1
    TI__Expert 8357 points

    Ohashi-san,

    The values listed in table 9-1 on page 11 of the datasheet are our only recommended values.

    The values you have listed are ones that I cannot suggest. This app note explains more on how the charge pump works, the mathematics behind it, and the overall complexity of choosing these values.

    Regards,

    Ethan

  • 0 in reply to Ethan Sempsrott
    TI__Expert 4280 points

    Hi Ethan,

    Thanks for your support. Got it that the table 9-1 value is the only recommended values. And that the C1~4 affects the V+ ripple. 

    Regarding to the app note, I have recieved additional question from the customer. In the app note, it say's "Comparing Figure 9 with Figure 7, the amplitude of the ripples is close and the frequency is higher (20µs) due to the faster charging and discharging capability of larger capacitors."

    1. Does this means that with larger cap and higher ripple frequency, the RS232C signal become sharp?
    2. Related to Q1, what will be the impact of variable frequency and ripple of V+?

    Regards
    Ohashi

  • +1 in reply to Ryoma Ohashi
    TI__Expert 8357 points

    Hi Ohashi,

    I can certainly answer these questions, but please note that when the recommended charge pump values are not used from the datasheet, we cannot guarantee correct operation. 

     

    1. It depends on the ratio between C1/C3 and C2/C4. If C3 is larger than C1, then the voltage peak (Vp) decreases (and vice versa). This can be seen in Figure 8 as the line is less "sharp". When C1 equals C3 and they both increase, the voltage peak (or sharpness) stays roughly the same, but the charge/discharge frequency increases.

    2. The impact of the frequency and ripple on the V+ line is that it will add slight noise to the driver output on the bus. 

    Best,

    Ethan