Newbie - 12V Regulator Question

[john2k]

Someone recommeded that I should use a 12V LM2940CT which is a low dropout regulator and specially designed for automotive use. Datasheet available here

I've calculated that with this 12V regulator the max power loss in watts is 0.84W (assuming max input voltage=14.4V, output voltage = 12V and current draw = 0.35A). So based on the datasheet, seems like this regulator has a higher operating temperate so a 0.84W power loss I am assuming will not need a heatsink.

Can anyone kindly confirm for me if this is the case? I am now more for the option of going for the 12V LM2940CT but would like opinions.

Thanks in advance

 

[CocaCola]

You still face the fact that it wants 12.5 volts to operate properly... How this effects function bellow 12.5 volts is a guess without testing... You have to decide your worst case and decide if that is acceptable... As for a heat sink, I would again test it real time and decide if you need a heat sink, just because perfect math suggest it will be fine without one doesn't mean it won't get too hot for your application...

 

[john2k]

You still face the fact that it wants 12.5 volts to operate properly... How this effects function bellow 12.5 volts is a guess without testing... You have to decide your worst case and decide if that is acceptable... As for a heat sink, I would again test it real time and decide if you need a heat sink, just because perfect math suggest it will be fine without one doesn't mean it won't get too hot for your application...

Thanks for your reply CocaCola, your a star. very much appreciate it. I will get myself one those low drop-outs that require 12.5 and see how it functions with a input below 12.5V. I will test and see but I am assuming lets say the car voltage is less than 12.5V and the regulator fails to operate, technically would the regulator not just let the full 12.5V through?

Regarding the heat sink. I did run a test with a 14V input via a 12V regular 2V dropout regulator via my driver and the heat after 30minutes or so is not high i can touch the back of the heatsink and keep my finger on it, its just a little warm like its been in the sun for a little while. I'll test it with the low dropout one aswell. What kind of testing would tell me if i need a heatsink or not?

Many thanks once again.

 

[CocaCola]

I am assuming lets say the car voltage is less than 12.5V and the regulator fails to operate, technically would the regulator not just let the full 12.5V through?

Maybe or it might shut down/block and not let anything through... It depends upon how the manufacture designed the chip...

What kind of testing would tell me if i need a heatsink or not?

Test it under the load you will be using, if that is just your constant current device and a few LEDs test it like that... It will get warm no doubt, but the temp will vary based on the output load of the device as well as the input voltage...

As for when you need a heat sink or not, there is math you can push around but I'm far to lazy in most cases, if it's too hot for me to touch when it use, I will implement a heat sink...

 

[john2k]

Thanks, I will get one and test what happens to the chip when the input voltage drops below 12.5V and do a test under load to see the heat generation.

Also quick question regarding the capacitors to use. In the data sheet for the LM2940CT it says that a 0.41uF capacitor is needed on the input if regulator is located far from the power supply and a 22uF is required on the output to maintain stability (may be increase without bound to maintain regulation during transients. The capacitor must be rated over the same operating range as the regulator and the ESR is critical). I am assuming this is referring to a 22uF and not a 0.22uF. When i search for a 22uF, i get so many different types of capacitors, what particular spec capacitor would you recommend on the output and input? I am not very familiar with capacitors. For the output I have found so many different caps when e.g. Radial Capacitor 50V 22uF 105°C. What is the ESR the data sheet is referring to?

Many Thanks

 

[CocaCola]

what particular spec capacitor would you recommend on the output and input?

Personally I would not worry too much, just pick up some generic ceramic caps and run with them... If you experience problems you can then tinker with the values and/or type(s) but from my experience this generally isn't necessary...

 

[john2k]

Thanks. I've just found a pack of 22uF Low ESR 105°C Electrolytic Capacitors and the supplier says its definately suitable for application with my regulator. They say that because the ESR is low the cap will charge/discharge much faster which apparently is important for a regulator

They are rather cheap so I guess no harm buying a few to test. What do you think?

 

[CocaCola]

Thanks. I've just found a pack of 22uF Low ESR 105°C Electrolytic Capacitors and the supplier says its definately suitable for application with my regulator. They say that because the ESR is low the cap will charge/discharge much faster which apparently is important for a regulator

They are rather cheap so I guess no harm buying a few to test. What do you think?

They should work well...

 

[john2k]

Thanks CocaCola