Supply microcontrollers with LNK304DN?

Posted by: - on 04/15/2010

Hi forum,

while searching for a cost-effective and reliable power supply for low power 8-bit microcontrollers, I stumbled over the LNK304DN series ICs. They look like perfectly suited to my demands. However, I possibly overlooked something important, so I'd like to ask you to review if it really fits the purpose:

Demand:
- Not necessarily galvanically isolated
- Input 250 VAC
- Output 3.3 or 5 V with less than 100 mA current (3.3 V with 70 mA, ie. 0.25 W would be perfectly ok)
- Energy efficient (no quiescent current or heat dissipation as in resistive and capacitive coupling designs)
- Minimal circuitry, low part count
- Reasonably priced

I see all of the above points perfectly met by the device, however I'm looking for a possibility to minimize the design the "Design Guide" suggests for a 0.25 W Buck Converter. Do you see any possibility to reduce external parts, the 400 V capacitor or the inductivity in some way?

Size is one of the main constraints I have, so any possibility to reduce it will be helpful.

The microcontroller powerered by the device will be constantly operating and needs a stable supply. If necessary, a low power linear regulator can be put in between, however I'd prefer not to use one if this can be avoided.

Please let me know if you think that my approach is fundamentally wrong and you would suggest a different device for this kind of application.

Thanks in advance,
Markus

Comments

A buck converter is pretty much as simple as it gets.

If we take DER-231 as an example:
http://www.powerint.com/sites/default/files/PDFFiles/der231.pdf
... the 400V capacitors and input choke L1 form the EMI filter. The only way you can shrink those is if your system already has an EMI filter, or if you don't care about conducted EMI. The buck choke L2 cannot be shrunk much; the calculation of its value is described in the App note.

Pls. read the application notes:
http://www.powerint.com/sites/default/files/product-docs/an37.pdf
as well as the datasheet:
http://www.powerint.com/sites/default/files/product-docs/lnk302_304-306.pdf

Adding a linear regulator is unnecessary. The SMPS can be designed for the accuracy, stability, and output ripple spec you need.

Hi Tucker,

thanks for the quick reply. I already presumed something along the lines of your answer. I'm aware of the "Design Guide" application note and the datasheet you mention. The DER-231 document, on the other hand, was previously unknown to me. Thanks for providing this kind of practical application note on the topic.

I attached my current design, a < 0.25 W buck converter with 5 V output along the description in the "Design Guide". Priority is on part count, while not running into a serious EMI problem.

As the large parts cannot be avoided as you already said, there is possibly a way to get smaller ones at least for the capacitors. Is it mandatory to use polar capacitors? Or would it be viable to use film/ceramics as well? (By now, the electrolytic ones seem in fact to have the smallest outline, but there might be non-polar capacitors that are still smaller).

Thanks in advance,
Markus

You can use ceramic caps for C2 and C3. For C4, bear in mind that it needs to be >400V rated - I don't know if ceramic caps are available that are smaller than an electrolytic.

The only possible issue with using ceramics for C2 and C3 is oscillation due to extremely low ESR - this can happen in flybacks; I am not sure if it can happen in a buck converter.