The circuit I am currently working on has a common connector for 12V and up to 48V input - this is fixed due to retrofit of POE to a design.

The target board requires a 12V supply rail. With 12V in through a regulator, however, this voltage will drop and this cannot happen so, a solution would just be to put in a jumper across the input to regulator output (with the EN pin set so that Vin > 12V + Vregdrop). This is acceptable but in order to stop someone plugging in a 48V supply to a board with a jumper, an automatic switch would be ideal.

Now, the circuit I have designed here will do most of that for me. Vin is passed to 12V when it is 12V. If the input is above ~15V, the pass transistor M1 is turned off and the regulator is on. The issue comes with the startup of a power supply - there is a small peak at the start depending upon the input rise time. The power supplies can vary and that the current draw on the 12V rail is ~2A.

How can I delay this until it has reached a steady input voltage of ~12V or >15V?

12V/48V in to 12V out

  • \$\begingroup\$ You can use the RUN input as an enable and drive it high from a comparator when the input has reached the level you want. \$\endgroup\$
    – Finbarr
    Commented Feb 13, 2018 at 12:04
  • 2
    \$\begingroup\$ This is a X-Y problem. You started out reasonably enough explaning the problem you have. But, your question isn't about that problem but about some imagined solution to that problem. I'd rather step back, get more specs for the real problem, then suggest some ways to address it. Those suggestions may or may not include something like what you're asking about. However, that kind of answer would be out of scope for this question. \$\endgroup\$ Commented Feb 13, 2018 at 12:06
  • 4
    \$\begingroup\$ There are buck converters which can operate in passtrough mode; if the input voltage is too low, the high side MOSFET is just left on all the time, so that you get a neglible voltage drop. The LTM8064 unfortunately doesn't support this. Another approach is to use a buck/boost (e.g. cds.linear.com/docs/en/datasheet/8056fa.pdf) or SEPIC converter, which will continue to function with an input below, at or above the output voltage. \$\endgroup\$
    – jms
    Commented Feb 13, 2018 at 12:10

2 Answers 2


If you can live with 11.5 volts at your output you could use this: -

enter image description here

It drops only half a volt between input and output and you can set the FB pin for it to naturally produce 11.5 volts by adjusting the 1 Mohm and/or the 88.7 kohm resistor.

Or just use a buck-boost regulator capable of delivering a regulated 12 volt from a range of input voltages below 12 volt to above 48 volt.


You could use either a buck regulator with...

  • an integrated charge pump to keep the NMOS fully on at 100% duty cycle
  • a PMOS instead of a NMOS, which gives 100% duty cycle (example).

In your 12V case, the voltage drop is thus only due to the MOS' resistance plus inductor resistance. You should be able to drop less than 0.2V which shouldn't disturb your 12V circuit.


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