I am with EE background and recently I had question in interview regarding “How to sense/know when device needs power?”.

So, basically I had a good interview on different topics and one of them was MOSFET.

I answered mostly all questions properly until this question comes up.

Interviewer asked me: “If you are making power supply for a laptop and when laptop goes in to sleep that time your supply should not provide full power (may be negligible). And when you on the laptop or started working on it at that time your supply should provide required power.”

How can you design circuit which sense/detects when device needs power?

Can any one of you help me on this? I would really appreciated any input.

  • \$\begingroup\$ Welcome. I do think it is called a under-voltage detector... \$\endgroup\$
    – user105652
    Jul 21, 2020 at 0:39

1 Answer 1


The assumption could be different for the design criteria.

" sleep mode does not inhibit full power" rather sleep mode plus full SoC inhibits all charger power.

The charger has OCP but does not regulate current. It is always done by the load. The laptop has 2 separate regulators for DC-to many DC for MOBO and shares the maximum current available to do both such that it never demands more than the designed max. expected. The battery charger will have an algorithm for CCC-CV-float- hysteresis and back to CC-CV in some efficient slow response .

So the charge controller takes whatever current is left over from the unit power demands using a current summing limiter.

For example, a 3.5A charger @ 19.5V might take up to 2.5A at max LCD brightness and CPU & HDD load, leaving the battery CC/CV circuit to use a maximum of 1A.

Current sensors may be high or low side and often limited to 50mV drop for efficiency with a precision gain amplifier to convert to voltage and compare with a Bandgap ref.


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