Technical

LDO or Buck?: Choosing the Right Power Supply

Recently during a brief overview for the team, I led a discussion about different power supply topologies. One of the questions that came up was: “How would you choose between an LDO versus a buck regulator?”

With an LDO design, voltage is lowered by turning excess power into heat. So if you need 1A out at 3V, you will need to input >1A at whatever your Vin might be.

For example, if Vin is 5V, you will be looking at an efficiency of <60%. You’ll have to put in over 5W of power to get out 3W, losing >2W to heat.

LDO’s are going to be preferred for small power requirements or when Vin is very close to Vout, e.g. 3.3V -> 3V. They also tend to be physically smaller and less complex with fewer components.

While a well-designed buck power supply can meet any electrical noise requirement you may have, an LDO can usually achieve a lower noise floor with fewer components. Also, inductors used in switching power supplies (and rarely also capacitors) have a chance of producing audible noise.

Finally, an LDO is likely to be cheaper.

So if efficiency is unimportant, heat is no object, Vin is only slightly higher than Vout, or the current needed is very small, go with an LDO.

With a buck regulator, power is stored in the inductor. You will still lose power to the various switching components and their parasitic impedances. However, you might achieve much higher efficiencies.

The highest efficiencies can be over 95%, but 80% should be easy to achieve in most cases. At 80% efficiency you will only see 0.6W lost to heat. As a result, a buck regulator will be a better choice if Vin >> Vout. Certainly, if you need to go from 12V to 1.8V, you will want to use a buck regulator. You will probably want to choose a buck if you need a lot of power or if you are worried about thermal performance.

Some more things to consider when choosing your power supply:

  • Power rating (will it produce enough power for your application?)
  • Vin range (does it work at the voltage you have?)
  • Vout either fixed or range (is it going to be able to source the voltage you need?)
  • Efficiency (do you have a limited power budget or are you concerned about thermal?)
  • Size (remember to include the IC package as well as any external components – inductors, caps, etc.)
  • PWM/PFM modes (a PFM, pulse skipping, or other variable frequency mode will increase efficiency at light loads)
  • Frequency (faster frequency = smaller external components required and better transient response but lower efficiency)