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I'm currently designing a battery powered, 2xAA in this case, project. The problem that I have is very simple, I want to power a DS1307 (the only part in my circuit which requires 5V). I've looked into similar parts like the DS1337 and the MCP7940, which operate under 3V, but they don't have a battery backup pin like the DS1307, which is something I want.

So, the first thing that I thought was to build a step-up DC/DC converter circuit to power the RTC (that's the first thing I thought, so I'm open for suggestions). Since I never used a switchmode converter before I decided to watch Dave's video about the MC34063, but the problem with it is that its minimum input voltage is 3V. So I would like to know some tips on how to choose a suitable switchmode converter for this kind of project.

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  • \$\begingroup\$ You didn't come across the DS1338 in your search? \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 15 '13 at 1:49
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    \$\begingroup\$ @IgnacioVazquez-Abrams Yes, I came across it, but I don't want to solder a $3.22 part in my prototype boards (which may not work and I'll have wasted a expensive chip). That's why I prefer DIP for expensive ICs. \$\endgroup\$ – Nathan Campos Jul 15 '13 at 2:31
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If simplicity is a key goal, an integrated voltage doubler such as the ICL7660 or MAX1044 would work well:

ICL7660 Application

Two capacitors, 2 diodes, and it's done.

For the two diodes shown in the schematic, Schottky diodes with forward voltage of 0.35 Volts or lower would result in output voltages of:

  • For fresh batteries: 2 x (1.5 x 2) - 2 x 0.35 = 5.3 Volts
  • For depleted batteries: 2 x (1.3 x 2) - 2 x 0.35 = 4.5 Volts

Both are within the DS1307 supply range.

Examples of suitable Schottky diodes:

  • Vishay VS-95SQ015, through-hole, but overkill and expensive
  • NXP RB521CS30L, Leadless SMD (SOD882) but inexpensive, with great specifications
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You've already got the main concerns there.

  1. The converter needs to be able to run on your supply voltage.
  2. It needs to have sufficient output voltage and current.

From here on out, there are more options. You will generally want a boost converter for this, as it is the simplest circuit. DS1307 uses almost no power, so current isn't too important, most any converter will have more than enough.

The last thing left in picking a IC, is frequency. a higher frequency one will allow smaller passive components, making the circuit physically smaller. Being newer they tend to be more expensive though, and the higher frequency makes layout more critical. Some ICs will have greater efficiency than others as well, which can be important in a battery powered operation.

It would be easier to use a 3V RTC though, you are right. You could try the PCF8523 from NXP, it seems to fit your requirements. I don't believe there is a DIP version though, which may or may not be problematic for you.

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