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SN75C3221: Can the V- be controlled

Part Number: SN75C3221


I've got a circuit that needs a negative voltage reference, basically no current draw.  Is there  a way to control the V- pin on the SN75C3221?  We basically have two modes of operation in our circuit; 1 requires the RS-232 interface, the other needs a fixed negative voltage reference into a high impedance source.  Is there a way that I can know ahead of time (without measuring with a DVM) what the voltage on the V- pin will be?



  • Hi Jim,

    The negative charge pump rail (V-) will be regulated by the device to be more negative than -5 V. So, this could be considered the minimum amplitude. Beyond this minimum level, there will be some ripple as the charge pump switches in order to transfer charge onto the holding capacitors. The amplitude and frequency of this ripple will vary with different charge pump capacitance values, VCC levels, and load currents; for more details on this, I'd recommend referencing this blog:

    If you have any further questions, please just let me know.