12V to 5V, 600mA LDO required!

HelpMe2020

Joined Feb 24, 2021
22
Hello All,

I need inputs and help on this task. I know LDO is quite poor when it comes to efficiency with high voltage drop.
However, I need to pick LDO which can drop voltage from 12v to 5V and can work fine with 600mA load on 5V output.

I understand there is large 7V (12V-5V) drop and 7V*0.6A = 4.2W that would be dissipated via LDO at max. That's huge if we have RthJa = (50C to 70C) meaning increase by > 200C temp without heatsink. I know max Ta is generally = 125C.

I have never used LDO in such a situation. Please suggest if LDO L7805ABD2T might work or any other recommendation?

AlbertHall

Joined Jun 4, 2014
11,505
If you have 7V headroom for the regulator then you don't need a low dropout regulator (LDO), any old 5V regulator that can handle 600mA will do the job. The 7805 would be first choice.
If you want to avoid the heatsink then you might use a switching regulator but that needs more components but would be much more efficient.

crutschow

Joined Mar 14, 2008
27,734
As AH noted, you only need LDO when you actually need low-drop-out for the regulator.
That's when the input to output voltage across the regulator is typically less than about 2.5-3V.

Whether it's LDO or not has no effect on its dissipation which is always the current through it times the voltage drop across it.

schmitt trigger

Joined Jul 12, 2010
463
You can find switchmode modules with a 5V output on Ebay. If you don’t trust Ebay, both Digikey and Mouser stock them too.
Some modules are direct footprint replacements for the common 7805.

BobaMosfet

Joined Jul 1, 2009
1,850
Hello All,

I need inputs and help on this task. I know LDO is quite poor when it comes to efficiency with high voltage drop.
However, I need to pick LDO which can drop voltage from 12v to 5V and can work fine with 600mA load on 5V output.

I understand there is large 7V (12V-5V) drop and 7V*0.6A = 4.2W that would be dissipated via LDO at max. That's huge if we have RthJa = (50C to 70C) meaning increase by > 200C temp without heatsink. I know max Ta is generally = 125C.

I have never used LDO in such a situation. Please suggest if LDO L7805ABD2T might work or any other recommendation?

@HelpMe2020 An LDO is not a bad idea, but not for how you're thinking. If you drop 12V across a 5V regulator, you are killing your output current ability by making the regulator deal with more voltage in than it's putting out. 7V in .v. 5V out.

How much current a regulator puts out is function of thermal junction temperature. 100C is considered 'normal' by industry, but you'll burn your finger if you touch it. If you want the regulator to run at cooler temp for safety (even with a heatsink), that limits the amount of current you can output safely.

As an exercise, let's look at the humble LM7805. We'll set the top temp of the regulator with a heatsink to 101F (38.8C), which is only slightly warmer than you are- you can hold your finger on it. Using your inputs, you'll only get 378mA out of it.

But if you make your input 9V, you get this:

You _always_ have to account for energy in and out and understand that junction temperature is your enemy. Manage that, and you can get what you want.

You can easily get a TO-220-3 heatsink (here's what I use);

Code:
+---------+-----------------+--------------+-----------+-----------+----------------------+--------+--------+-------+-----+----------+--------------------+----------+--------+-------+------+------+-----------------+-----------+---------------+---------------+
| Code    | Name            | Rating       | Typical_V | Typical_A | Description          | Loc    | Stored | Stock | Obs | Class    | SubClass           | Comp_Pkg | Wt     | Scale | Seal | Bake | OEM             | Vendor    | O_Part        | V_Part        |
+---------+-----------------+--------------+-----------+-----------+----------------------+--------+--------+-------+-----+----------+--------------------+----------+--------+-------+------+------+-----------------+-----------+---------------+---------------+
| G000152 | Heatsink, Small | 24.000 R(th) |      NULL |      NULL | TO-220-3 Heatsink; S | AD-2-3 | Loose  |     1 |   0 | Heatsink | Thermal Dissipator | Radial   | 1.9000 | Gram  | NULL | NULL | Aavid Thermallo | Arrow NAC | 507302B00000G | 507302B00000G |
+---------+-----------------+--------------+-----------+-----------+----------------------+--------+--------+-------+-----+----------+--------------------+----------+--------+-------+------+------+-----------------+-----------+---------------+---------------+
Nylon screws and nuts can also be obtained from various vendors for mounting these to the regulator.

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