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I have a 7.4V battery supply which will gradually discharge and reduce until at about 5V I won't have enough power for my device anymore.

Ideally, I need to maintain a 7V (+/- 0.5V) supply rail throughout this time so I've been looking at the MC34064A.

I've used this tool to generate the component values I need.

Proteus Schematic

Simulating the design in Proteus off the 7.4V supply gives me 6.8V with about 100mV of ripple. However, if I then try and run it off 5V this drops dramatically.

So either I've designed my schematic wrong or I'm misunderstanding the function of this IC. Either way could you please advise on how I can obtain a steady 7V rail from a supply that varies from 7.4V to 5V?

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  • \$\begingroup\$ You are looking for a buck-boost chip and not a boost or a buck solution. Neither a buck nor a boost circuit can do what you require. \$\endgroup\$ – Andy aka Jan 23 at 12:23
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    \$\begingroup\$ Is the value you've indicated you're using for your timing capacitor C6 correct or is that a typo? 100uF is orders of magnitude larger than what you should be using there. \$\endgroup\$ – brhans Jan 23 at 13:06
  • \$\begingroup\$ Or if possible use just a boost converter and run your boosted line at something common like 12V? That way you can get away with a cheaper and simpler boost only solution... Just my 2 cents. \$\endgroup\$ – MadHatter Jan 23 at 13:34
  • \$\begingroup\$ Whoa! 100uF for timing capacitor? o.O \$\endgroup\$ – Rohat Kılıç Jan 23 at 14:27
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I will not describe how a boost converter works, because it's not too difficult to reach to the required information.

But I have to say that the switch (inside the IC) will never turn on thus the device will never operate when the input voltage is higher than the desired output voltage (7.4V input, 7V desired output). Thats why you see 6.8V output (7.4V input minus 0.6V diode drop equals to 6.8V).

Besides, 100uF timing capacitor will never let the IC to operate. You should decrease it to something like a few nano- or even pico-Farads.

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The MC34063 that you have in your design (not the MC34064 that you state in your question) is a step up (boost) or step down (buck) converter. It is not capable of doing both. You need a buck-boost converter for your application.

You could use ICs such as: TPS55162-Q1, TPS55160-Q1, TPS55165-Q1 or TPS63070. The first 3 of those are available in packages that are easier to solder by hand. These ICs don't require too many external components either.

The search I did with parameters is here: http://www.ti.com/power-management/non-isolated-dc-dc-switching-regulators/buck-boost-inverting/buck-boost-inverting-split-rail-converters-integrated-switch/products.html#p238min=1.3;5&p238max=8;40&p634min=-25;7&p634max=7;40&p834typ=3.22;4.5&p1129=Buck-Boost;SEPIC&p212max=87.5;100

There may well be buck-boost DC-DC converters available in DIP packages, you should be able to find these with a search online.

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