I would like to make a automatic switching of source for my buck converter when the input voltage is below 5v.

enter image description here

In the very simplified circuit image above as you can see the input voltage can be 3.3V or 6V or 8.4V. The buck converter that outputs 3.3V has a minimum input voltage of 4.5V, this is a problem when the source voltage is 3.3V. So instead if the source voltage is 3.3V (or anything below 4.5V) it must get its power from the buck-boost converter that constantly output 7.5V.

How do i create a switch that switches when the voltage is at a certain threshold. Also it would be nice to create a hysteresis to prevent it from going crazy

  • \$\begingroup\$ Do you need this 7.5V elsewhere (apart from feeding the 3.3V buck converter)? If not, why not use a 3.3V buck-boost with e.g. an input voltage range of 2.5V to 12V (using e.g. TPS63060) \$\endgroup\$ – Huisman Dec 24 '19 at 10:28
  • \$\begingroup\$ @Huisman yes it is being used elsewhere to power motors and would like to isolate the buck down. Also when at 3.3v the motors are turned off and the microcontroller powered by the buck down should still be powered to send out an error. The buck-boost will still boost up the 3.3v and that is what im after since the buck down wont be able to take in only 3.3v since its rated at 4.5v minimum \$\endgroup\$ – Jake quin Dec 24 '19 at 10:52

What about this: -

enter image description here

It seems like it would do what you want (or at least my interpretation of your requirements).

  • When the supply is 3.3 volts, the output from the buck-boost (7.5 volts) powers the buck regulator.
  • When the supply is 6 volts, the buck will receive power from the 7.5 volts or, if the 7.5 rail is turned off it will receive power from the 6 volts rail.
  • When the supply is 8 volts, that 8 volts will supply the buck.
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  • \$\begingroup\$ Since the buck - boost converter powers inductive loads will the buck converter be affected? \$\endgroup\$ – Jake quin Dec 24 '19 at 15:47
  • \$\begingroup\$ The devil is always in the details but, as a 1sr level assessment, a buck converter can handle reasonable amounts of ripple at its supply input and, if in doubt, the diodes I added will tend to allow the buck convertor’s input reservoir capacitor to reduce significantly any up stream ripple and noise but, the devil can still be in the detail so it’s up to you to ensure the input filtering is sufficient (in the absence of any schematic you might be thinking of). \$\endgroup\$ – Andy aka Dec 24 '19 at 16:33
  • \$\begingroup\$ Are you done with this question now @Jakequin \$\endgroup\$ – Andy aka Feb 23 at 16:37

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