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THVD1450: Output voltage ripple

Part Number: THVD1450
Other Parts Discussed in Thread: MAX3221E

Hi team,

I'm designing an RS485 system using two THVD1450,  capacitance of the capacitor pump is designed according to the datasheet, 0.33uF at 5V. Waveforms at V+ and V- pins are pretty smooth, but ripple at output pins, A and B, is large. What's the reason for this? Can I decrease the ripple by appling bigger capacitor value for the pump? Thanks!

  • The THVD1450 datasheet does not mention a charge pump, and it would work at 3.3 V. Are you doing something with RS-232?

    Please show the schematic of your circuit.

  • Clemens,

    I'm sorry I confused THVD1450 with MAX3221E, this is an RS232 system. Two MAX3221E are connected together for communication, the SCH for those two are the same. Thanks!

  • The sawtooth shape consists of two parts. The steep rise happens when the flying capacitor's charge is dumped into the output capacitor; its size depends on the relative sizes of the two capacitors; its slope depends on the various parasitic resistances. The slow fall happens when the output capacitor is discharged by the load; its slope depends on the capacitance and the load current.

    To reduce the ripple of a regulated charge pump, you can

    • use low-ESR (i.e., ceramic) capacitors (you are already doing this), and minimize the trace lengths,
    • make the output capacitors larger,
    • make the flying capacitors smaller (which reduces the maximum load current),
    • reduce the load current.