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I am trying to figure out the best possible circuit for MCP16251/2 feeding from 2 AA batteries in series with an output of 3.3v. MCP is extremely efficient boost converter with very low quiescent current which is ideal for my low-power battery application, however my problem is that two fresh AA batteries in series can potentially output higher voltage at the beginning of their lifespan than the Output setting of 3.3v used in my MCP16251/2 set-up. What are the potential consequences of having this overvoltage on the input pin? Unfortunately the datasheet does not mention anything for such scenario except for:

"Output Quiescent Current: < 4 µA typical, device is not switching (VOUT > VIN, excluding feedback divider current) For VIN > VOUT, VOUT will not remain in regulation." ...and I am not exactly sure how to interpret that. Will the chip just act as a pass-through or act funky in any way?

I suspect that the converter just won't regulate any voltage and act as pass-through, but just curious if this set up can damage the boost converter in any way? Any suggestions - theoretical or based on experience would be highly appreciated!

Attached is my current schematics based on somewhat improved reference design:

mcpschema

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  • \$\begingroup\$ It might be cleaner to sidestep the question and use a buck-boost instead. \$\endgroup\$
    – user16324
    Jan 30, 2021 at 13:11

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The device data sheet says this on the front page: -

• Maximum Input Voltage ≤ VOUT < 5.5V

So, you have to be careful.

But the device data sheet also says this (note 2, bottom of page 3): -

For VIN > VOUT, VOUT will not remain in regulation.

I take this is a favourable indication that if you can live with the output voltage rising a little above the natural regulated voltage on fresh batteries, you will be OK with this device.

However, there will be side issues because when the battery voltage exceeds Vout, the synchronous MOSFET inside the chip will switch off and then you will have one diode drop between input voltage and output AND, this is likely to cause the synch MOSFET to turn on and route full input voltage to the output.

The problem here (although it may not manifest itself as a real operational problem) is that the output voltage might oscillate between 3.3 volts (the regulated desired output voltage) and the fresh battery terminal voltage. That might mean your output voltage is a DC level of (say) 3.35 volts but changing from 3.3 volts to 3.4 volts at several kHz.

If you can live with this then that's OK but, it may behave itself - difficult to tell.

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  • \$\begingroup\$ Interesting - I never thought of potential Vout oscillating between 3.3v and 3.4v, but it makes sense, and may cause some issues down the road since I will have RF transition going out from this device. Do you think the two 10uF ceramic caps in parallel as well as an 100uf+ electrolytic cap can help mitigate this issue? \$\endgroup\$
    – bobnecat
    Feb 1, 2021 at 1:13
  • \$\begingroup\$ I wish the producer of this devise was much more clear of Vin>Vout scenario since they do list application of 1 to 3 cell alkaline in the input. I guess the 2+ cells is more relevant towards 5V output setting but nevertheless they could have made it more clear. \$\endgroup\$
    – bobnecat
    Feb 1, 2021 at 1:21
  • \$\begingroup\$ @bobnecat more capacitance might just slow down the oscillation but you might get lucky. I think we're done on this now unless you have any residual questions? \$\endgroup\$
    – Andy aka
    Feb 1, 2021 at 9:09
  • \$\begingroup\$ Thank you for your input! \$\endgroup\$
    – bobnecat
    Feb 2, 2021 at 5:30

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