LM317 And The Magic Smoke

[Voidloop]

I'm designing a project which involves a microcontroller sending data over SPI to two SN74HC595N shift registers which, then control some level shifters to turn on and off some 12v end devices. (The MC and the register circuit are on two separate boards) I want to power the shift registers and the rest of that circuit on 4v using an LM317 with an input voltage of 12v.

I'm a beginner in electronic engineering, there is so much I still must learn. Please bestow upon me the knowledge of the masters, for I am but a young padawan.

The heat dissipation on the datasheet for the LM317 is about 39 degrees C/W. I really don't want to stick a heatsink on this PCB because, well, I'm lazy. I am trying to determine the resistance in my circuit so that I can calculate the heat that will be on this regulator. This is where my combat level in electronics becomes apparent. Looking at the datasheet of my 595's, I can't work out where it tells me either the resistance across the IC, or the maximum current would be. On the datasheet from TI, it's figure 6.1 (I think). Basically what I'm trying to do here is determine the total resistance in my circuit so that I can use Ohms Law to give me wattage so that I can apply that value to the thermal dissipation on the LM317. In a simple circuit with easy components, I can do that. I don't know how to work out more advanced circuits' resistance with ICs and the like. How do I find out my resistance in a circuit with ICs in it? Where can I find that information?

Any help would be appreciated. I know, I'm a noob, but I want to learn. Can't blame a guy for learning.

 

[danadak]

Can you post a schematic ?

 

[Voidloop]

Here you go. Some things don't have correct values, or don't have values at all. It's still a work in progress. Don't burn me at the stake just yet.

Thanks a lot.

 

[augustinetez]

All you need is an educated guess at the total current draw.

Hard to tell, but looks like only the 74HC595 are running from the LM317.

Taking a worst case scenario of 80mA (max the two '595's can handle plus a bit extra), power dissipation in the '317 is ~600mW and I would suspect a more realistic value will be well below half that.

That does of course depend on what else is hanging off the 4V supply - and why 4V?

 

[Diver300]

The power dissipation of any CMOS or HCMOS IC is only due to the output loads and the switching capacitance. If ICs like those are not changing state and are not driving loads, they essentially consume no current at all.

To work out the consumption, you need to know how many outputs will be turned on, because the 74HC595s are driving the 10kOhm resistors. You also need to know how often the outputs are switching, both for the internal consumption of the 74HC595s and for the current taken to charge the gate capacitances of the MOSFETS.

Unless the switching speeds are above about 100 kHz, I would guess that the consumption is minimal.

I hope this shows that the concept of resistance simply does not work for something as complicated as a 74HC595.

 

[rjenkinsgb]

The steady-state supply current for the HC595 ICs themselves is trivial (max 2uA); then plus any output loads on them.

The worst-case actual current will effectively be the same as all the gate-to-ground resistors on the low side FETs in parallel across the regulated supply.

The print is just too small to be clear.. If they were all 10K, 16 in parallel is 625 Ohms.

That puts the total circuit operating current at under 10mA.

 

[schmitt trigger]

If you ever find yourself in a situation where your LM317 definitely requires heat sinking, but you don’t want to add an external heat sink, consider the D2PAK version. Soldered to a 1” square board copper area provides a very effective heat sink.