I need the smallest power source possible to get 5V to power ATTINY402 and one 100K pullup and turn ON a MOSFET with a 2K resistor for 5 seconds once a day (to switch ON 1W LEDs run by a dedicated LED driver like AL5890)

So the continuous power consumption will be around 0.5mA with peak of 3mA for 5 seconds.

The power supply does not needs to be an isolated one as it will be enclosed in a sealed container and requires no user interactions.

The power supply needs to be inductor less and should not require an EMI filtering cap.

Since the current consumption is so low the Power Factor and Efficiency should not matter that much right? So its OK if the they are low.

The preferred size is less then or equal to SOP8 package. If there are no external components then that is preferable.

If this power supply requires a DC source like a bridge rectifier then it is already in our circuit so that should not be calculated in the size consideration.


simulate this circuit – Schematic created using CircuitLab

What I have considered so far:

  1. Capacitor dropper circuit:
    The reason I did not choose this is because the capacitor needs to mains rated and they are pretty big so I have discarded that thought.

  2. Resistor dropper Circuit:
    Placed as the last resort if nothing else is available.

  3. SR086 Based power Supply:
    Requires a lot of external components and pretty big 470uF electrolytic capacitor at the output.

    1. Small Transformer: I could not find a very small mains rated 60Hz transformer that could fulfill my requirements.

Suggestion: 5. MP100L:
This looks promising but it requires an X capacitor at the mains to keep the EMI under Class B (Which is the requirements).

  1. SMJR-N-1-24: I used this LED driver in my previous circuit and it provides 5V for my ATTINY as it has an internal 5V auxilary supply inside. So I was thinking maybe there is a chip that has only the 5V power supply circuitry inside. SMJR Auxiliary Power Supply Specs

Ideally I need a small chip that can give me 5V 2.5mA power from 120VAC. With minimal external components.

  • \$\begingroup\$ Have you seen these EEVBlog videos which deal with efficient unisolated mains power supplies: youtube.com/watch?v=_kI8ySvNPdQ and youtube.com/watch?v=oIAATOQe3to The topic of the videos might look like they're relevant to your question but I think they are. You could consider a high voltage regulator like suggested in the videos. Asking for product recommendations is actually Off topic here btw. \$\endgroup\$ – Bimpelrekkie May 15 '20 at 12:50
  • \$\begingroup\$ Thanks I will take a look. \$\endgroup\$ – IMFROST May 15 '20 at 13:06
  • \$\begingroup\$ My mistake, the R4 is 20K pull down resistor. Forgot to change the value. \$\endgroup\$ – IMFROST May 15 '20 at 13:53

There are LDOs available for that purpose. Some examples, I know of: - NCP785A, NCP786A from On Semiconductor - TPS7A78 from Texas Instruments - LR8 from Microchip

The devices from On Semi for example only need a diode and two caps to deliver 10mA of stabilized power at the rated voltage, eg. 3.3V, 5V, 12V, 15V, directly from the mains with 85VAC to 260VAC or from a DC source with 25V to 450V. So, they should work everywhere. The LDO is 4.1mm x 2.5mm in size.

  • \$\begingroup\$ That is exactly why asked this question in the first place. To see if there might be a solution that I may have missed. Thanks \$\endgroup\$ – IMFROST May 15 '20 at 15:18
  • \$\begingroup\$ Those are linear regulators, they are decidedly not LDO. The 5V NCP785A datasheet mentions 50V minimum input. \$\endgroup\$ – marcelm Feb 13 at 21:29

For what your asking, your perfectly in the use cases for a capacitive or resistive dropper, There may be bucket brigade chips that could work, but they will need external capacitors and some other passives.

With a capacitive dropper you would be using a 47-86nF X2 capacitor (depending on how much margin you want for aging), (peak voltage / current = impedance), thermal dissipation would be about 0.5W, which is plenty cool for this size capacitor, looks to be as small as 10x6x12mm from most suppliers for this size capacitor

For a resistive dropper it would be something around 47K, again about 0.5W, so probably use a 2 or 3W resistor for both the wattage and the voltage rating.

For both of these, afterwards you need to rectify the output, and use say a small linear regulator / LDO to bring it to exactly 5V,

If you drop the requirement of no external components, to just as compact as reasonable, then there are other options. but they likely won't be smaller than either of these.

  • \$\begingroup\$ Please share other options that requires external components. Thanks \$\endgroup\$ – IMFROST May 15 '20 at 13:09

Why not just use another LED driver?


simulate this circuit – Schematic created using CircuitLab

R5 provides a 5.6 V drop and D1/C2 protects U3 from dropouts (at, e.g., zero crossings). Increase C2 as needed to support peak loads.

  • \$\begingroup\$ It would constantly be sinking 10mA weather we need it or not, and the LED driver would be dissipating 1.15W whether we need it or not. \$\endgroup\$ – IMFROST May 15 '20 at 14:00
  • \$\begingroup\$ So what's your main priority? Small size or low power? \$\endgroup\$ – Dave Tweed May 15 '20 at 15:13
  • \$\begingroup\$ I am leaning towards @Lexi 's solution as I was looking for something like that and just did not knew such Linear Regulators existed. \$\endgroup\$ – IMFROST May 15 '20 at 15:17
  • \$\begingroup\$ Fair enough. Those do look pretty good. \$\endgroup\$ – Dave Tweed May 15 '20 at 15:21

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