UC1825-SP: setting pwm frequency

Prashant Surana

Part Number: UC1825-SP
Other Parts Discussed in Thread: UC1825, UC3825

I am using UC1825 pwm controller for synchronous buck converter operation. I am facing issue in setting desired pwm frequency using Ct and Rt.

according to a graph in datasheet, if we select : Ct = 22nF and Rt=2k, we are not getting 30kHz frequency. Kindly let us know to use the formulae. Thanks

over 7 years ago

0 Ramesh Khanna over 7 years ago

TI__Expert 7445 points

Prashant,

Acknowledge receipt of your post. Will respond to your question shortly.

0 Ramesh Khanna over 7 years ago in reply to Ramesh Khanna

TI__Expert 7445 points

Hi Prashant,

1. Are you measuring different oscillator frequency than predicted by the graph?
2. Curve is non-linear - an equating for the curve is not available.

0 Prashant Surana over 7 years ago in reply to Ramesh Khanna

Intellectual 840 points

HI Ramesh
Thanks for quick reply. I forgot to mention that I am not verifying it on hardware. I am simulating it on OrCAD. I doubt that 3rd Party spice model is not accurate. We are thinking to go for UC1825 but we are unable to find its spice model. Please let us know if there is any way to get spice model of UC1825. Thanks in advance.

0 Ramesh Khanna over 7 years ago in reply to Prashant Surana

TI__Expert 7445 points

Prashant,

There is a TINA model on TI website under UC3825. See link below.
www.ti.com/.../toolssoftware

Where are your getting the Pspice model that you are using on OrCAD?
A third party aesystem also provides pspice models for vairous UC devices.
https://www.aeng.com/spice.htm

0 Prashant Surana over 7 years ago in reply to Ramesh Khanna

Intellectual 840 points

Ramesh
there is link only to download TINA software, not uc1825 spice model.

0 Prashant Surana over 7 years ago in reply to Prashant Surana

Intellectual 840 points

Hi Ramesh
is there any update. kindly let me know. Thanks

0 Ramesh Khanna over 7 years ago in reply to Prashant Surana

TI__Expert 7445 points

Hi Prahant,

TI does not have a spice model for UC1825. You would have to get one from the 3rd party as indicated previously or you can build a test board to verify the oscillator frequency.

0 Prashant Surana over 7 years ago in reply to Ramesh Khanna

Intellectual 840 points

Thanks a lot ramesh.

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UC1825-SP: setting pwm frequency