ucc3956 operating frequency

suriyanarayanan santhanam

Intellectual 280 points

Hi,

i am working on li ion battery charger.

while testing the ic (ucc3956) i gave 160pf in cosc (12th pin) and 162kohm in rset (13th pin) as given in datasheet.

but instead of the required 100kHz i m getting 66kHz. i dont know what the problem is?

i also dont know how the equation for the frequency (3.475/(cosc+20Pf)*rset) was derived.

can anyone help me on this?

Regards,

suriya

over 14 years ago

0 Chris Glaser over 14 years ago

TI__Guru**** 150760 points

Actually, for 100kHz, it recommends Cosc be 180 pF. You need to use an accurate COG or NPO dielectric cap that does not change value over temp, etc.

That equation is a simplified and easy to use equation based on internal IC parameters that affect the oscillator.

What type of battery are you charging and why did you select this IC? We have much newer, easy to use ICs that would likely be better for you.

0 suriyanarayanan santhanam over 14 years ago in reply to Chris Glaser

Intellectual 280 points

Actually, i used 180pF only. while typing i typed it wrong as 160.

i m using it to charge li ion battery of 48V (2 series connected 24V battery).

i wanted a switched mode control so i chose this. if u can suggest any simpler ICs for this sort of application it would be helpful

i m also not sure why am i not getting the required frequency. Is there any way of troubleshooting this?

-suriya

0 Chris Glaser over 14 years ago in reply to suriyanarayanan santhanam

TI__Guru**** 150760 points

This app note discusses how to use our newer bq246xx family to charge higher voltage packs. http://focus.ti.com/lit/an/slua580a/slua580a.pdf

There is also an EVM available for that configuration.

If that's the performance that you get, you just need to target for a higher frequency operation and pick new component values to get where you wanted to be.

0 suriyanarayanan santhanam over 14 years ago in reply to Chris Glaser

Intellectual 280 points

isn't ucc3956 suitable for this application? from the datasheet it is seen there was no constraint regarding the limit on voltage?

0 Chris Glaser over 14 years ago in reply to suriyanarayanan santhanam

TI__Guru**** 150760 points

Yes, it is suitable. But newer parts will be easier to work with, have better applications information, and give better performance.

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ucc3956 operating frequency