# I am looking for a -2.5 V negative output regulator

Discussion in 'The Projects Forum' started by simo_x, Jan 24, 2015.

1. ### simo_x Thread Starter Member

Dec 23, 2010
200
6
Hi all,

I need a stable -2.5 V on a negative power supply rail so I am looking for a fixed -2.5 V negative regulator (same as 7905 for -5 V ).
I found the LM2937-2.5 for positive 2.5 V, but I would need for a negative voltage output.

Can you suggest me if there is a LM2937-2.5 negative equivalent ?

Last edited: Jan 24, 2015

Oct 2, 2009
5,451
1,068
3. ### ScottWang Moderator

Aug 23, 2012
5,056
784
Try LM337 and adjust the output to 2.5V .

4. ### simo_x Thread Starter Member

Dec 23, 2010
200
6
I know the LM337 but I would like to avoid to adjust the output with the external resistors and having problems with tolerances etc.
There is not a 'negative equivalent' for the LM2937 ?

5. ### ScottWang Moderator

Aug 23, 2012
5,056
784
You just using the pot to try and then using the fixed resistors to replace them.

6. ### simo_x Thread Starter Member

Dec 23, 2010
200
6
I already mounted a trimmer on my breadboard as well, where I am doing some testing before making the PCB. It's not so trivial to set it to -2.5 V..
Maybe I have to try with 1 % resistors..

7. ### ScottWang Moderator

Aug 23, 2012
5,056
784
Sometimes you need to using 1% and different value of resistors in series with them.

8. ### simo_x Thread Starter Member

Dec 23, 2010
200
6
That's why I would like to avoid putting resistors in series etc..

Actually, from datasheet, with R1 equals R2:

$-1.25 \left (2 \right ) + (120\,\Omega \times 100\,\mu\text{A}) \approx -2.5\,\text{V}$

I will try with 1 % resistors, I will buy these during the week. : )

9. ### #12 Expert

Nov 30, 2010
16,852
7,500
There are dozens of 2.5 volt regulators. If you would say what you need instead of what you want (an exact complement to an LM2937), you might some good results.

Do you need -2.5 volts at 500 ma?
Do you need 1/2% accuracy?
Do you need it in a 3 pin package?
Do you need it to survive 60 volt surges?
What is the supply voltage?
What is the actual current you need?

10. ### simo_x Thread Starter Member

Dec 23, 2010
200
6
I need a -2.5 V regulator with -5 V input from a 7905.

The load current is not a problem, it will be very low, no more than 10 or 20 mA because I need it for reference voltage and for an op amp supply with high load to its output. TO220 package for the regulator is good enough.

11. ### crutschow Expert

Mar 14, 2008
13,865
3,506
The LT3015-2.5 has a -2.5V fixed output.

#12, atferrari, simo_x and 1 other person like this.
12. ### simo_x Thread Starter Member

Dec 23, 2010
200
6
Thank you crutschow !

13. ### simo_x Thread Starter Member

Dec 23, 2010
200
6
Finally I found at home 2 resistor of 100 Ohm 1 % tolerance. I tested the LM337 with them instead of using the trimmer and it works like a charm.

Thank you all guys.

Last edited: Jan 25, 2015
14. ### crutschow Expert

Mar 14, 2008
13,865
3,506
1% resistors will give a worst-case voltage error of ±1% plus the ±2 tolerance of the LM337 internal reference (at 25°C) or ±3% total output voltage error from nominal.