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I have a 4.8v battery pack and I made a simple power supply with a L7805CV voltage regulator, which has a 2v dropout. So I'm only getting like 3.5v out. Is it possible to throw an op-amp or something like that before the voltage regulator to make sure I get at least the 7v I need to achieve 5v on vout?

If not, what is best practice?

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Leon is correct here. Let me approach your question in more detail.

The issue here is simple, for an op-amp to output 7V it needs to have a rail that is at-least 7V, so you will still need another power source to power the op-amp. In most designing for op-amp they also do no pull much power from the first source(actually dissipating this power to ground) so this is not the method you want.

What you need is something that takes power from your first source and delivers it at an increased voltage. You could step up your voltage and then use your 2V drop Linear regulator to step it back down, which they have very good noise characteristics, so if you need low noise, you just have to cope.

Now if you get a buck/boost style switching regulator you can step it to 5V from the 5.4V input voltage, all the way down to 2V input with still getting 5V out. These are nasty to layout so you want to purchase a solution that has already been done.

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  • \$\begingroup\$ This thing should work then, right? sparkfun.com/products/317 \$\endgroup\$ – Matt Williamson Nov 19 '10 at 15:19
  • \$\begingroup\$ Yes, if you can get the layout fabricated properly. \$\endgroup\$ – Kortuk Nov 19 '10 at 15:49
  • \$\begingroup\$ I will note, since it is sparkfun it is probably very easy to get fabricated, just noting, buck/boosts depend on capacitance and inductance, so they will have issues if you add large capacitance or inductance with your layout. \$\endgroup\$ – Kortuk Nov 19 '10 at 15:50
  • \$\begingroup\$ I've got a video to help me out here eevblog.com/2010/09/10/… but I think I may just get a bigger battery and use my linear regulator, haha \$\endgroup\$ – Matt Williamson Nov 19 '10 at 15:50
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    \$\begingroup\$ I would suggest it, it is easier. \$\endgroup\$ – Kortuk Nov 19 '10 at 15:51
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A switching voltage regulator can produce 5V from a lower voltage and you will not need the L7805C at all. On the other hand, 4.8V (when the battery is fully charged) is very close to 5V. Your electronics may work fine without any power supply circuitry.

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  • \$\begingroup\$ Sometimes an option; some AVRs will run just fine from 1.7 to 5.5 V \$\endgroup\$ – Nick T Nov 19 '10 at 21:39
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Use a switching boost regulator. One of the National Semiconductor Simple Switchers would be ideal, you could use a 3V input and you won't need the 7805.

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No, you can't do that with an op amp.

Op-amps require power supplies. Many opamps have dropouts just like your 7805, so the output cannot quite reach the power rails. Others, called rail-to-rail opamps, can operate over the entire supply voltage. They're just drawn as triangles in some textbooks because it gets tedious to draw the power supplies every time (same with logic symbols).

Even if you could power an op-amp to beyond the rails, it's not advisable as a replacement for a 7805. Most opamps are low current (<20mA), and suitable only as precision voltage references.

Your options are, from easiest and cheapest to most difficult and expensive.:

  • Tolerate the 4.8V and associated droop as the battery discharges (4.8V is a nominal value, not exact/constant). Many simple circuits can run off of 4.8V just as well as 5V.
  • Use to get a higher voltage battery pack or put more battery packs in series to raise the supply voltage.
  • Use a low dropout regulator[LDO] to regulate to a slightly lower voltage (3.3V, for example). 78XX isn't the only series of regulators! Many LDOs are available with dropout voltages of less than 0.2V.
  • Use a boost regulator.

The final option uses switching of current through an inductor to generate voltages above the input. You will probably need a boost/buck regulator, since your input is so close to your output.

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Opamps can never output more voltage then what is given to them on their rails. Also, even if they could, opamps generally can't output much current, so they aren't great solutions for anything requiring power.

A switching boost regulator will do what you need it to do, but sometimes this can be a little difficult for DIYers to use depending on what you buy. You might want to look into a charge pump. In many ways they aren't as good as boost chips, but I think they are easier to use.

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