LM317 heat dissipation (linear voltage reg)

[Cobalt60]

I may indeed be able to get away with just a variable switching regulator. Thanks again, I really feel I have all of the info I need and there is nothing left to do but start designing!

-Chris P

 

[dknguyen]

BTW, it sounds like you just need to step down a higher voltage to a lower voltage. If that is the case, a buck converter is what you should be looking for which is dedicated to this task and can't do anything else. As a result it is the simplest and most efficient at what it does, has the best noise, and fewest parts.

Other types step-up voltage, invert voltage or step-up and down (input voltage can be either above or below the output voltage for the same converter). These alternatives either have the wrong functionality, or exchange performance for increased functionality (ie. not as good as stepping down voltage as the buck converter in exchange for the ability to also step-up the voltage when required).

EDIT: I miswrote the converter name in earlier versions of this post. The type you need is the BUCK converter.

 

[Sceadwian]

Isn't a higher to a lower voltage switch mode called a buck converter? Boost is for step up. Buck-boost is for both ways, and Sepic is another topology for abuck-boost, I think there are more complex ones but only for those that really want to look into it.

 

[Cobalt60]

Yes I would be using a buck converter. BTW, any recommendations on one (for variable output)? One thing that attracted me to the 317 is everyone knows what they are, they are readily available, and I can easily find calculators, or at least the equations, for setting their voltages.

So whats a good variable buck converter, with a current capability similar to the 317? Ive been lookin through digikey, havent found much yet, found a Sharp PQ2CF1, but no equation to set its voltage.

Thanks

 

[dknguyen]

National seems to be one of the better ones for switching converter ICs.

Take a look at the LM5085/LM25085. IMHO it uses a better control scheme than most and is a PFET controller (ie. capable of 100% duty cycle) so it can have very low dropout voltage.

Most other buck switching controllers use NFETs so they have higher efficiency but can't do 100% duty cycle to achieve a very low dropout voltage (the NFET is on the high side so it needs to shut off periodically to allow the bootstrap capacitor to recharge. The bootstrap capacitor is responsible for providing the floating gate-source voltage required to switch a high-side NFET.

Note: A switching controller is just that- it just controls and requires an external power transistor, as well as the other resistive, capacitive and inductive components. A switching regulator IC has the power transistor integrated into it, but still needs the other RCL components. Which is better? Depends on what you need...size and cost vs efficiency, flexibility, and customization.

External regulators almost always tend to be designed for MOSFETs due to lower losses, dropout voltage, and ease of gate drive. But switching regulators with internal power transistors might use BJTs as well. You can tell either by the a "feature" on the datasheet that it uses a MOSFET, or just look at the internal block diagram of the IC to see if the power transistor is a BJT or FET. I definately prefer the MOSFET power transistor over the BJT though in this application.