In my project, the input power of a device comes from a battery pack. Here it is:

enter image description here

Here is a simplified architecture inside the battery pack:

enter image description here

  • There is a charger that charges the battery when V_IN is supplied.
  • There is a converter that boost the battery voltage up to + 18 VDC.

We asked the PCM supplier of the battery pack the following condition:

  • When V_IN is connected, V_OUT should be floating.
  • When V_IN is disconnected, V_OUT should be +18 VDC

What we have observed on the first samples is the following:

  • When V_IN is disconnected, V_OUT = V_BATT
  • When V_IN is plugged, then disconnected, it takes 0.5 to 1 s before V_OUT = + 18 VDC

Here is an extract of the boost part from the PCM schematics:

enter image description here

  • Is it normal that the boost is bypassed when it is off?
  • Is there a way to make the transition from V_OUT = floating to V_OUT = +18 VDC so that it is immediate?

Thank you.


1 Answer 1


Note the path from the input of the boost through the inductor and diode. There's nothing the boost can do (as drawn) when off to block current from the input to the output. That's typical of a boost converter. It would need an additional switch to implement true disconnect. So if your cells are directly connected to the input of the boost, Vout will never be floating unless there's something you're not showing that can disconnect the boost input or output.

Since V_OUT floating isn't a possibility without a design change, the time to get from V_BATT minus a diode drop to +18 is the startup time of the boost converter. It won't be "immediate from floating" unless you add a disconnect switch on the output of the boost, keep the boost running, and turn on the output switch when you disconnect V_IN.

Depending on how immediate is immediate you could potentially use a boost with an output disconnect feature and optimize it for faster startup time.

Something like this as an example, but you can probably find choices from ADI/MPS/other vendors as well: TPS61376


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