# Should I use a DC-DC converter or an LDO?

In my project, I am designing a board which is supplied by a battery. The battery is a LiPo, 3.7 V, 1000 mAh, so its voltage range is 3 - 4.2 V.

All the components on the board need a fixed voltage of 3.3 V. In some configuration the board can draw 1 A, but most of the time it draws about 50 mA.

I need to convert the battery voltage to 3.3 V. I was thinking of 3 options:

• Option 1. Using a buck-boost converter
• Option 2. Using a buck converter and electronically cut the power supply when it is approaching 3.3 V (let's say 3.6 V)
• Option 3. Same as option 2 but with an LDO

Do you see better options?

My conclusions are as follows:

• Option 1 is the most expensive option, and price does matter.
• All the 3.3 V buck converters I have found only accepts input voltages starting at 4 V. Is it a normal behavior that bucks need a voltage gap to work?
• Option 3 seems to be the best option. But when the board draws 1 A, I would need to dissipate maximum (4.2 V - 3.3 V) * 1 A = 0.9 W, which is too much because the board is very small so it would badly dissipate heat, and heat does matter...

Which options would you use in such a case? Do you have advice?

• If you need 3.3 V out with only 3.0 V in, then buck-boost is the only one of your options that can possibly do the job. Commented May 16, 2022 at 16:05
• You should also consider the efficiency required (that is -battery life?) and the fact that a switching regulator is producing a noisy output comparing to LDO, and needs certain design considerations to reduce the effects. Given all the points, you have all of the information to decide yourself. Commented May 16, 2022 at 16:05
• How often does the board draw 1A? For how long? What are the discharge curves for your battery? Most Lipos stay > 3.5V until they're almost fully discharged so a LDO might work.
– vir
Commented May 16, 2022 at 16:07
• It is okay if I do not use the battery all the way down to 3 V. Cutting it at 3.6 V is acceptable. It can draw 1 A 25 % of the time, sometimes during dozens of minutes. I do not have the discharge curve of the battery but I think most LiPo 3.7 V battery discharge curves are more or less the same, am I right? Commented May 16, 2022 at 16:10
• Related/similar: electronics.stackexchange.com/q/330922/2028 Commented May 16, 2022 at 16:22

It looks like LiPo batteries stay above 3.3V in normal charge levels, and drop to 3.0V when they're below ~80% capacity.

Based on this, you can stick with an LDO since it's the cheapest and easiest way to step down a voltage, assuming you don't mind the efficiency losses when your battery is >4.0V. LDOs will dissipate any unused power as heat; the greater the difference between Vin and Vout, the lower the efficiency. A benefit when compared with switching topologies is a much smaller ripple on Vout, which is good for sensitive electronics.

Consider using a boost converter chip in a SEPIC configuration. This can act as step up and step down in one circuit.