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I am trying to design a device that will take in power (fixed at power up, sometimes 2V, sometimes 12V) with a rise time of around 10 us, and output the same voltage with a rise time of anything between 10ms to 40ms, from 10% to 90%. Output current is up to 170 ma. The minimum rise time is really causing more problems than I thought it would. Options I've looked at:

  • Unity gain opamp (AD8397) with setpoint set from RC filter. This is the best option so far, the RC filter sets the rise time of the output, but it doesn't add impedance to the output. The opamp needs to be powered off the input voltage, and I don't like that there is still a voltage offset because opamps don't go all the way to the rail. Plus, good performance at 2 and 12V is tough.
    • Any ideas on how to improve the rail to rail performance?
  • Passive filter: This was how I originally wanted to go, but the inrush current ended up being way too high, and I want to keep the output resistance under 10 ohms ideally. This ends up needing 1.5 Henry inductors, more trouble than it is worth.
  • Buck regulator with ramp up? This would be an ideal system if it exists, just a PMIC that would switch a FET with increasing duty cycle up to fully on over 10+ ms. But I haven't seen a simple circuit that would do this.

Does anyone have ideas for a simple solution to slow down a rise time with minimal increase to impedance?

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  • \$\begingroup\$ Capacitive passive filter? \$\endgroup\$
    – George White
    Commented Jul 18, 2021 at 2:51
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    \$\begingroup\$ @GeorgeWhite to give a 10+ ms rise time, it would either add too much resistance or too much surge current \$\endgroup\$
    – pscheidler
    Commented Jul 18, 2021 at 4:09
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    \$\begingroup\$ If buck regulators with ramp-up would be your ideal solution as you say, these do exist so just buy them. \$\endgroup\$
    – Justme
    Commented Jul 18, 2021 at 7:08
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    \$\begingroup\$ @pscheidler the keyword to look for is "soft start" \$\endgroup\$
    – DamienWontContributeToAITheft
    Commented Jul 18, 2021 at 14:47
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    \$\begingroup\$ See also ti.com/lit/pdf/slva307 \$\endgroup\$
    – DamienWontContributeToAITheft
    Commented Jul 18, 2021 at 14:51

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Buck regulators with slow start exist, they are made by various manufacturers, just use parametric searches on manufacturer web pages.

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  • \$\begingroup\$ I don't know if an appropriate part really exists here, given the 2-12V range, and I worry about running the converter at 100% duty cycle so the output voltage equals the input. That's just not their normal mode of operation. \$\endgroup\$
    – pscheidler
    Commented Jul 18, 2021 at 14:41

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