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I decided to build step up regulator to power DHT22 (temperature sensor) from two AA bateries. The DHT22 needs 3V (or 3.3V acording to some datasheets) for operation.

I picked up MCP1625 (from Microchip datasheet here) step up regulator because it can accept low voltage and keeps high efficiency.

I used reference design

design

  • According to datasheet I used 10uF for input filtering.
  • I used little smaller resistor on feedback voltage divider resulting in 3.25 V output. Which should be fine.
  • I used two alkaline batteries with voltage 2.8 V
  • For second test I used Nimhd Eneoloop batteries 2.5 V.

Now, my problem is this:

  • Without load, I get on output 3.25 V
  • With only small load 3mA I see drop to 2.98V. At this point the current on input is 4.62mA, voltage is 2.8V (no drop here)
  • I did test with resistor as load to test whether batteries are able to provide more current then its taken by converter. They are capable to deliver more than 10mA.

My question is why my regulator is not taking more current from batteries?

  • Could it be caused by bad PCB design? (For example to thin lines for Vn, Vout)

  • Could it be caused by sloppy soldering? (My first SMD soldering project)

  • Something else?

Any advise would be appreciated.

Than you.

Unfortunately, I cant post my PCB because of restriction for new users and two links in post :-(.

List of components

  • L Ferrocore DLG-0403-4R7 (4.7uH, 1.15A, 110mOhm)
  • Cin, Cout Samsung MLCC, 10uF, 6.3V, X7R (CL21B106KQQNNNE)
  • Rs 1.6MOhm 5%, 1MOhm 1%

EDIT 2016-05-25 I fixed link to datasheet, editet inductor specs, PCB and photo in comments.

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    \$\begingroup\$ That inductor ... isn't REALLY 11 ohms DC resistance ,surely? \$\endgroup\$ – Brian Drummond May 24 '16 at 23:05
  • \$\begingroup\$ @BrianDrummond. That HAS to be 11 milliohms! As for the rest it seems ok. He does have to trade volts for milliamps since he is using a boost converter. \$\endgroup\$ – Sparky256 May 25 '16 at 0:21
  • \$\begingroup\$ @MightyPolo. Your Microchip link is no good. Links to pdf's work better. \$\endgroup\$ – Sparky256 May 25 '16 at 0:23
  • \$\begingroup\$ If you put links to PCB pictures in a comment, one of us will happily edit them into your post for you. \$\endgroup\$ – uint128_t May 25 '16 at 1:28
  • \$\begingroup\$ Hi! PCB is here: dropbox.com/s/rlpy438apxjn5ti/stepup_convertor3_pcb.png?dl=0. Real photo here:goo.gl/photos/MYdZ8cUk737PqxxW7. Thank you \$\endgroup\$ – MightyPolo May 25 '16 at 8:04
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Did you clean the board properly using IPA for example ? Flux or even flux remover residue left between the pins could disturb the IC stability or accuracy. As your feedback loop is using very high resistance values, it will be very sensitive to any parasitic effect.

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  • \$\begingroup\$ I cleaned the board again carefully and indeed, it worked for me! \$\endgroup\$ – MightyPolo Jun 10 '16 at 13:26
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Try Using a Larger Power Input Capacitor (4.7uf) as high a 100uf will not hurt anything. The Specs say if the input voltage exceeds the output voltage it will not regulate. I wonder if the coil is creating feedback to your power source which is mimicking a larger supply voltage.

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