Help me to produce -Vin out of +Vin using the MAX660 IC as a negative supply

[wood_girl]

hi!! ill appreciatte your help..im trying to produce -Vin out of +Vin, using the MAX660 IC as a negative supply for 4 OPA843. I apply +5V to their input but i cant get an output voltage of -5V .. the higher voltage i csan get its 3.4V, im using 10uF tantalum capacitors on SMD case, i just dont know how can i get the -5V at least 4V...

in the datasheets there is something about low ESR, but i cant get this type of capacitor here in col. nobody sell this type.

thanks!!!

 

[dick_freebird]

MAX660 PROBLEM

Any cap with a reversing potential, needs to be nonpolar.
Tantalums are not. Tamtalums are OK for bulk storage
but poor for high frequency and efficiency. You might
want to substitute a decent multilayer ceramic capacitor,
or N of them. These will have a superior ESR and you can
go to the manufacturers' web pages, who do sell into your
home market, and compare ESR values.

 

[keith1200rs]

MAX660 PROBLEM

Actually, you can use polarised capacitors with the MAX660. Make sure you have them the correct way round and also that they are rated at a sufficient voltage.

It can operate at different frequencies - try the highest (80kHz).

Try it with no load and see if it works.

Tantalums should be fine provided you choose them carefully - thereare recommendations in the data sheet. You should be able to find out the ESR of the ones you have from the manufacturers data sheet. If you post the exact part number of the capacitors we can help.

Keith.

 

[wood_girl]

Re: MAX660 PROBLEM

hi thanks for your help, someone knows why i get a warning?, what i do wrong??

otherwise,the problem is that i recycled this tantalum capacitor from other board (PS1, i think), and i dont really know what reference it is, but im going to chekout without any load the MAX660 and see what happend, ill be around here telling my results...


pd: sorry for my bad english!

Added after 5 minutes:

"Do not post multiple copies of the same topic. Cross postings not allowed. Read announcements before posting. Your identical post is already been replied here. "

this is why i get a warning, it was an accident,horrible network at work!!, and i refreshed the explorer. so the message was sent over again.. i guess.. so im going to do the test for my max 660, see u soon

 

[keith1200rs]

Re: MAX660 PROBLEM

hi thanks for your help, someone knows why i get a warning?, what i do wrong??

Could be the short subject. However, I think that simply counting characters is probably a bit harsh in this case - I understood you had a problem with the MAX660! Subjects like "I need help!!!!" are common and not helpful though.

Keith.

I see you have found the real cause of the warning while I was typing my reply.

Keith

 

[wood_girl]

Re: MAX660 PROBLEM

hi!, i have been tested the max660 whitout any load and with the FC pin tied to VCC, and it works perfect!, but why with 4 OPA843 the max660 voltage falls, if every OPA843 need only 23mA...... any idea??

im going to tested again with only 2 OPA843 and see what happend

 

[keith1200rs]

MAX660 PROBLEM

I think you have answered your own question! 23mA x4 = 92mA. That's a lot of current. At that current the MAX660 will be down to less than 4.4V (see graph on page 3 of datasheet) with 150uF capacitors. With 10uF it will really struggle.

Keith.

 

[wood_girl]

Re: MAX660 PROBLEM

hi keith1200rs, now i connected just 1 OPA843 and the max 660 works fine then i connected another OPA843 and the voltage drop at 3.4V. with 1 OPA843, Vout is just a few mV under +VCC (200mV i think), but i found this in the data sheet:

Note 4: In the test circuit, capacitors C1 and C2 are 0.2W maximum ESR capacitors. Capacitors with higher ESR will increase output resistance, reduce output voltage and efficiency.

i saw a configuration of 2 max660 paralleled to reduce the output resistance. so.. do I need 4 max660 for obtain negative voltage for my 4 OPA843? :s

maybe with low ESR capacitors, voltage can rise a little bit at least for supplying 2 OPA843 instead of 1.... i dont know.....

 

[keith1200rs]

MAX660 PROBLEM

Are you still using 10uF? If so, I think you need a lot more capacitance. It would also be useful to look at the -5V supply under load - I think you will find it has a lot of ripple.

Keith.

 

[wood_girl]

Re: MAX660 PROBLEM

sorry Keith but, i dont understand very well this

It would also be useful to look at the -5V supply under load - I think you will find it has a lot of ripple.

i dont get english sometimes , what did you mean?? im going to change the caapcitance and watch what happend

thanks a lot

 

[keith1200rs]

MAX660 PROBLEM

I am sure your capacitance is too small. If you look at the -5V with an oscilloscope under load it will not be very good DC. You need to use 150uF for the capacitors if you want to take 100mA from it. Even then, the voltage will dorp to 4.2V according to the datasheet.

Keith.