I'm sweeping the linear regulator input voltage on DC using Spice from 0V to maximum expected Vin. However, as the OTA that makes part of LDO is internally supplied by the output voltage, the simulation will never turn on the OTA and it will find a very stable point of operation in 0V. it is like the dilemma of chicken and egg.

LDO regulator

How I could force to start the OTA and calculate the minimum input voltage required to work properly?


  • 1
    \$\begingroup\$ You need to show your circuit and also explain why you think that if the sim doesn't currently work, why you would expect a real circuit to work. \$\endgroup\$
    – Andy aka
    Commented Feb 7, 2019 at 11:21
  • \$\begingroup\$ If I understand your goal, some regulators include a bootstrapping process that produces some weak, but useful starting point to get the process started, switching over to the output voltage once things stabilize. These usually aren't linear regulators, but they can be applied to producing the high side gate voltage boosts need, I think. \$\endgroup\$
    – jonk
    Commented Feb 7, 2019 at 12:12

1 Answer 1


Do the sweep from the highest voltage down to the lowest voltage. The simulator will be able to perform the dc operating point analysis correctly because it will use the high voltage rather than zero.

If your simulator won't support that option for a .dc simulation then use a very long transient simulation. Replace the dc voltage source with a PWL (piece-wise linear) source that starts at the maximum voltage then slowly ramps down.

  • \$\begingroup\$ I use the DC sweep from higher to lower values and it worked. However, in some cases the simulator might find a point higher than the actual which is also wrong. My solution is to help to pspice simulator using a .nodeset statement to the expected value. Thanks! \$\endgroup\$ Commented Feb 7, 2019 at 15:10

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