Getting 3.8v from a 9v supply

[8ue]

Hi all,

I've got a 9v source and I'm wanting to get down to a 3.8v output. This is for powering a Telit gm862, so it will peak at 2A and needs to handle that.

I bought a Linear 1185CT regulator as that was recommended as doing the job, though I can't figure out how to get this to work.

It's got 5 pins and I've tried various combinations of resistors and can't get the voltage to drop at all.

Does anyone have any idea on the right way to hook one of these up?

Thanks

 

[alexan_e]

The datasheet has the explenation of how the device works
**broken link removed**



Alex

 

[8ue]

The datasheet has the explenation of how the device works
**broken link removed**

View attachment 53005 View attachment 53006

Alex

Thank you.

I managed to find this as R1: Arcol | Passives | Resistors | Through Hole, Fixed | Axial - 0.25W/0.5W |RCC050 4K3 J

I can't find anything for R2 though that I would need. I can only find 143K resistors. Do you have any idea where I could get one from or one that's close enough?


Thank you again

 

[alexan_e]

Why don't you use a value for R1 closer to the recommended,
you can also parallel 2 resistors to achieve a value that you can't find.

Alex

---------- Post added at 02:48 ---------- Previous post was at 02:35 ----------

8k2 in parallel with 3k3 = 2k35 for R1

then for R2 you need 1k42
you can use 3900 // 2200 =1k40

Alex

 

[Genomerics]

The LT1185 is a 'Low Drop Out Regulator' and will be driving your load with what is effectively a current source and therefore its loop gain will be very dependent on the load...

As per the link from Alexan_e,

**broken link removed**

Page 6) under Output Capacitor



Blah blah blah. Sorry if it is not helpful. Given you have the 'headroom', 9V versus 3.8V you do not really need a Low Dropout Regulator and something else and more 'solid' might suit. I can't think of one off the top of my head.. 317 something?

Otherwise it is possible that your 1185 is not seeing the correct 'AC' load and is unable to find the right answer.

Genome.