From: Jason White <whitewaterssoftwareinfo@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Sun, 12 Sep 2021 08:16:10 -0700
Subject: Re: [EE] LM1084 regulator question
Thread-Topic: [EE] LM1084 regulator question
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Hi William,

As mad scientist notes, what you are trying to do probably won't work. I
would consider using a switching power supply module, they are more
efficient - most switching modules will be able to do the job without
getting very hot.

I would like to highlight a parameter in the datasheet called
"Junction-to-case (bottom) thermal resistance."

If the thermal resistance is roughly 3C/W and the power dissipation is 57W
then with a perfect heat sink the die of your regulator will be 3*57=3D171C
above the ambient temperature (usually room temperature). This is far too
hot, and no heat sink is perfect.

Jason White





On Sunday, September 12, 2021, William Couture <bcouture@gmail.com> wrote:
> Hi!
>
> I need about 3A of 5V, with a 24V supply.  I'm trying to use a LM1084 in =
a
> TO-220, which is
> rated at 5A output.
>
> However, when I try to pull more than about 250mA, the regulator becomes
> very hot and I
> start getting dropouts.  A heatsink does not help.
>
> I have an electrolytic capacitor on the output, not a tantalum like in th=
e
> datasheet, could that
> be the problem?
>
> I tried putting a 7805 in the circuit (with appropriate pinout changes)
and
> it's fine up to 1A,
> so the problem is the LM1084 itself.
>
> If the LM1084 will not work, can anyone suggest a low-cost circuit to get
> 3A or so of 5V
> from 24V?
>
> Thanks,
>    Bill
>
> --
> Psst...  Hey, you... Buddy...  Want a kitten?  straycatblues.petfinder.or=
g
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