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I'm designing a 3V3 PSU for 12/6V (lead) battery powered (outdoor) application. Supposed load is 70mA but I plan sleep mode (50uA) with periodic wake-up (ATMega1284P with 32kHz (RTC) crystal). It means I need high efficiency under very light load. Hence I believe I need switching PSU IC designed for such scenario (TI calls it light-load support), mainly very low quiescent current (<30uA or so).

As Xbee module is to be powered from PSU there is Digi recommendation "*If you are using a switching regulator for your power supply, switching frequencies above 500 kHz are preferred. Power supply ripple should be limited to a maximum 250 mV peak to peak.*".

I found Texas Instruments TPS62120 and TPS62125 ICs. The former supports only max.75mA , the latter supports 300mA which seems perfect. Both chips offer efficiency 80-90% @ 70mA, 44% @ 50uA. But the TPS62125 is available on miniature WSON socket which not too nice for hand hot air soldering. Even TPS62120 is only 75mA it has more user friendly SOT package but output current is too near to required current. There are other TPS62xxx ICs (160, 177, ...) but they are not so good at light loads.

Can anybody confirm the idea is correct or is another recommended way how to achieve good result (e.g. using big/supercapacitor and forcing PSU power down when MCU is sleeping, low/high temperature issue?, maybe less efficient because of cap self discharging?).

Optionally if somebody has another suitable chip please provide reference.

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  • \$\begingroup\$ MP1593DN-LF smaller footprint lesser parts cheaper on production \$\endgroup\$ – user2967920 May 28 '15 at 12:33
  • \$\begingroup\$ Seems it has 1mA supply current (ie. high Iq ?), no support for light load and it's rare in Europe. \$\endgroup\$ – TMa May 28 '15 at 12:41
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The LT1934 is pretty good: -

enter image description here

And available in SOT-23.

If you want to search a few of LT's devices they have a good search engine and here is the link I used. BTW the LTC3103 looks really good on low power (1.8uA quiescent).

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  • \$\begingroup\$ It looks pretty good. Suprisingly datasheet does not mention what is switching frequency. But the inductors are rather large, i.e.frequency is not too high. I'd need 500+kHz. \$\endgroup\$ – TMa May 28 '15 at 13:18
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    \$\begingroup\$ @TMa The LTC3103 is 1.2MHz switching frequency. BTW you didn't mention switching speed was a requirement in your question - maybe you can also state why it is important? \$\endgroup\$ – Andy aka May 28 '15 at 13:30
  • \$\begingroup\$ I know I forgot to mention. I've stated/added in original post. LTC3103 is promising too, but it is not so easily available in Europe and has 0.5mm pitch. \$\endgroup\$ – TMa May 28 '15 at 13:49
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The Microchip MCP1703 low-dropout linear regulator features 2.7-16v input, 250mA out, 0.4% accuracy, and 2uA active current in a hot-air-friendly SOT-223 package.

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  • \$\begingroup\$ But it's LDO chip. I believe max.possible eficiency is 3.3V/12V=28%. I'm not sure if it's ok for battery powered app. \$\endgroup\$ – TMa May 28 '15 at 13:59
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    \$\begingroup\$ Efficiency = Io * Vo * 100 / (Io + Iq) * Vi, so 100mA * 3.3v * 100 / (100mA + 3uA) * 12v = indeed 27.49%. However, it only wastes 2uA + 50uA when off, compared to 12uA + 50uA for the (very good) LT1934. Using 8 pieces LiFeS2 1.5v AA cells (2A/h, 0.05% discharge/mo.), for each week, "sleep" (50uA + regulator uA) and 5 minutes "run" (75mA + regulator uA), the batteries will last 3 years, 274 days with the MCP1703, and 3 years, 139 days with the LT1934. Google XLP Battery Life. If the device isn't used much, a switcher can actually be a worse decision, plus add cost and complexity. \$\endgroup\$ – rdtsc May 28 '15 at 22:02
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I am not going to offer you any ICs but a guidance about how could you select right parts using available options.
As you have already checked some TI chips let's stay at them. They have a very useful and practical part searching surface.

enter image description here

Here you could set your parametes, like: \$ V_{inMax}, V_{inMin}, I_{outMax}, package \$ and so on and list the matching components.
In the example above I have made a query with few parameters, with the main parameters. But many more are available through the Add/Hide parameters option. I have listed DIP, SOIC, SOT-23 packages, (these are easy to solder by hand) with your other parameters and as you can see there are 54 matching ICs.
Now you have a list of a lot componenet which match your basic requirements, you can choose the best from them shortly.

With later added parameter (switching frequency > 500kHz) and with special feature search (light load efficiency) 16 components found, list.

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  • \$\begingroup\$ Exactly I'm using this tool. With note to get real efficiency I need put in famous TI webench.ti.com. But I did not find suitable TI chip as I expressed in original post. \$\endgroup\$ – TMa May 28 '15 at 13:16
  • \$\begingroup\$ What about this, if you extend the search to components above 500kHz switching frequency the third match. Same efficiency like the ones you mentioned, SOT package, 300mA max current option, suitable input-output voltage range. \$\endgroup\$ – Bence Kaulics May 28 '15 at 13:24
  • \$\begingroup\$ I don't think it's designed for light load as the webench says 0% efficiency @ 50uA. \$\endgroup\$ – TMa May 28 '15 at 13:53
  • \$\begingroup\$ I did not checked everything for you, so it could be right. Here LMR14006, add parameter--->special feature----> select Light Load Efficiency, 5th match. There are 16 parts with above 500kHz frequency, light load efficiency, and so on. You definitely did not do a detailed search. \$\endgroup\$ – Bence Kaulics May 28 '15 at 14:05
  • \$\begingroup\$ Sorry but I did detailed search [ti.com/product/TPS62125/…, testing in Webench and I did not find any chip but TPS62xxx series with issues expressed in original post. Not every "Light load efficiency" chip when designed in Webench provides good efficiency at 50uA. But maybe the number is not correct as it's marginal current range. \$\endgroup\$ – TMa May 28 '15 at 14:22
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if you want you can prefer AOZ1280CI. its switching frequency is 1.5 MHz.Also your inductance value is decreasing and it is cheaper than other ICs.

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  • \$\begingroup\$ I don't it's low current IC as has high Iq 1mA. It's very rare but on the other side very cheap. \$\endgroup\$ – TMa May 28 '15 at 18:02

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