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LM1084 shuts on high loads

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rompelstilchen

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Hello,

i use a 12V 9000mAh Ion/Lithium battery to power an ODROID board wich loads from 0.5A to 4A

an lm1084 regulates the 12V the battery to 5V to power my board

the problem is that on high load the lm1084 shuts down

i have tried many heat sinks, even a big one, that has a thermal disipation factor < 2°C/W (like 3mm thick)

the circuit is the simplest possible and it is an non adjustable lm1084

i dont get it, this thing is supposed to take up to 5 Amps, and here is the picture of 2 heatsinks i use

DSC_0038.jpg

any idea what i could do ?

any alternative ic i could use ?

thx
 

u7karsh

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A wierd problem.. try using decoupling capacitors or possibly 2 regulators in parallel.. im unsure if this will work but still, you can give it a try..
 

rompelstilchen

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A wierd problem.. try using decoupling capacitors or possibly 2 regulators in parallel.. im unsure if this will work but still, you can give it a try..

not sure i am following you :)

i am not a pro electronician so what values should i use for the capacitors ?
between vcc and gnd ? how will it help, i dont really get it
 

u7karsh

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image.png

Using this configuration, you can adjust the value of R2 to get ur desired output voltage..
 

rompelstilchen

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i have the non adjustable package, besides i dont have a Vout issue :)
more of a shutdown issue
i'll try with a LM2596 , someone told me he could go up to 3.5A with a small heatsink

maybe my 1084 is corrupted somehow, i dont know
 

u7karsh

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I use 7805 and they are really good and very cheap.. the limitation they have is that their current capacity is 1.5A. So when i have no options, i do use parallel combinations of 7805 to achieve my goal ;)
If you solve this problem with LM2596, do let me know on this..
 

rompelstilchen

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yeah i need from 0.5 to 4A

i was hoping someone could give me advices for the LM1084
 

u7karsh

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With LM1084, you can do the stuff.. u just have to add a few cap and resistors to make decoupled output.. The exact diagram is given above in my post.. There is no adjustment to be done.. just replace the resistor R2 with 363ohm resistor and you are done..
 

rompelstilchen

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lm1084 comes in various packages types, including fixed Vout like the one i have (LM1084 IT-5.0 => Vout=5V cfr Electrical Characteristics in datasheet)

so the above picture does not come in to play otherwise i would have added the res and caps to adjust vout :)

that's why i am wondering if my lm1084 is working correctly
the heatsink is getting hot but i can put my hand on it, so i dont know why it shuts down.

i tried with my multimeter to measure the load but the lm1084 resets before even reaching 2Amps
 

u7karsh

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Oh!.. then in order to decouple both the inputs and outputs, use this config..
hope now ur system works..

image.png
 

rompelstilchen

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ok i was wondering if i could just do the caps trick, but can you tell me why these values ? or how these can make it work (just curious :) )
also why tantalum caps ?

i'll give these a try
 

u7karsh

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basically these caps are used to decouple the input and output of the regulator.. Thus any noise induced is shuntedd through the capacitor.. u can even consider this as a filter to filter out ripples.. so basically what this capacitor does is that it acts as a low pass filter filtering out all the high frequency ripples or noise that might lead to unwanted outputs.. thus using all this information and the frequency of operation, one might calculate the values.. typically people pick some values and many a times, values are recommended in the datasheet of such devices.. for more help, you might want to check the datasheet of this device..

Tantalum capacitors are used as they have lower equivalent series resistance (ESR), lower leakage, and higher operating temperature than other electrolytic capacitors.
 

rompelstilchen

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ok

but i had the feeling that the issue was more about crossing a particular load barrier
like above 2 or 2.5 Amps, the lm1084 shuts down

can i use regular caps instead of tantalum ?
 

FvM

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The datasheet instructs you to use 50 µF aluminium electrolyte or 10 µF tantal.

Without any capacitors, as apparently shown in your photo, there's a certain chance that the regulator starts to oscillate above a certain load current, which might explain the problems.
 

kripacharya

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With an Vin of 12v and at 4A, your regulator needs to dissipate upto 28watts.

You should try inserting a resistor 1.5ohm/ 30W between battery & your LDO to help with this. In addition as suggested by FvM, also use the caps at both input and output points.
 

rompelstilchen

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thx a lot everyone for your wonderful help :)

- - - Updated - - -

yes, i also calculated from the datasheet Pd=28W

so I was also wondering if it was not better to use another battery(5V or 7V) cos all this heat is also a big waste

but I need to power 2 motors(12v)
I also use an arduino wich requires 7-12V supply
and the rest of circuits IC's need 5V supply
+ that freaking odroid board (5V-5A)

so maybe a step up (5V->12V) device/system would dissipate less heat

or 7V->12V and I would use a 5V regulator from the 7V, i dont know, it is all about try and fail, but in the end it costs a lot of money

any advice ? anyone had the same problem ? I am not really experimented in power supplies apart from usinf lm78xx regs
 

kripacharya

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You could use a standard PC's power supply unit (SMPS) which are very cheap, but provide a hefty output. You'll get 12v, 5v, 3.3v with lots of current capability.

This runs from the AC mains of course. So if you HAVE to use the 12v battery (for portability?) then this is a no-go.

Or you can build your own SMPS to convert 12v -> 5v (buck converter). But thats not a beginners project.

Or buy one.. something like this.
 
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