voltage requlator question

[sal_park]

Hi All,

I'm building my first voltage regulator circuit and have run up against a problem you guys might have some clue about. I'm using an LM2576 to power an arduino and some other bits (LCD display) which all together runs at around 100mA @ 5v. While developing I've been using between 1 and 3 PP3 9v batteries in series as the source with no problem what so ever.

However I've now hooked up the circuit to my final power supply which is a 42v 10Ah battery and  as soon as I plugged it got a popping sound and all the magic smoke escaped....
(btw the board is fused with a 1A fuse and this did blow - good job too!)

After some investigation I can see that the arduino board is fried - no visual damage but when I now power it up it takes ~150mA (9v PP3 battery power again so that is probably the limit). The big clue that there is a problem on the arduino board is the LED's light and the go out after 1/4 of a second or so.  (even with the ATmega 328 removed)

After I'd blow the Arduino I check the LM2576 and it still seems to be working (steady 5v output) when connected to a single 9v PP3 battery , but I've not check what it outputs when connected to the 42v battery.

I suspect that I'm having an issue with inrush current, although my knowledge here gets a bit sketchy!

The LM2576 circuit is just the basic one as suggested in the data sheet with a relatively big cap (2200uF from memory) since my current draw is so low (~100mA). I've connected the ON/OFF pin straight to ground but have read last night that it might be worth using this to delay switch-on for the output to give the VR some time to settle.

All comments welcome


sal_park

[shebu18]

Do you have the HV version? The normal version has a max of 40V input, so your input is a bit over the max.

[sal_park]

Hi,

no it's not the HV version

Looking at the datasheet (http://www.onsemi.com/pub_link/Collateral/LM2576-D.PDF) page 2 gives a maximum voltage of 45V, but on the next page it lists the supply voltage as 40V.

Am I expecting too much by using 42V ?

Perhaps the easiest way forward is just to go the the high voltage version ?


sp

[alm]

I've never used the LM2576 with high input voltages, but according to the datasheet, 45 V is the maximum voltage that the LM2576 can withstand without letting out the magic smoke, and 40 V is the max input voltage under which it will maintain regulation. So outputting 40 V with 42 V input would be perfectly legal according to the datasheet. I would lower the input voltage or use a regulator designed for the higher input voltage. Maybe you can drop the input voltage with a few series diodes?

[dr_p]

Pay very close attention to the difference between MAXIMUM (absolute) RATINGS and OPERATING RATINGS.

So 45V is the maximum voltage the device can withstand without damage, but it only functions properly under 40V.

edit: oh, I took too much time to write this, someone got there faster.

[dr_p]

I would lower the input voltage or use a regulator designed for the higher input voltage. Maybe you can drop the input voltage with a few series diodes?

I found myself in the same situation when I wanted to use a 48V transformer:  most regulators I had were maxed out at 40V. My solution was an LM317 used to drop 48V down to 39V, since the LM317 doesn't care what the absolute input voltage is, it just wants that the input-output differential voltage is less than 40V. So you can input 170V and output 130V. Mind you, you have to make sure there will be no short circuits on the output, no over current or over temperature. That's because all of there imply lowering the output voltage and that makes the differential voltage (much) more than 40V.

I know, I know...I'm using too big of a transformer and then I'm pissing away the surplus voltage. It's impractical and inefficient, but I wanted to use what parts I had.

[sal_park]

thanks for the responses guys.

I think I'll try an HV version and see how I get on with that