LM5163: LM5163 Switching Frequency - Not measuring expected value

Val Stratulat

Part Number: LM5163

The Switching frequency for LM5163 is based on Ron resistor:

Ron = Vout (2500) / fsw

We use 39K resistor (verified and measured) , and expect 320KHz switching frequency, with Vout = 5V . (Vout is measured to be 5V)

Would you please be able to provide any input on the unexpected Fsw? This is affecting the Vout pk-pk ripple.

Capture below is with a load on 5V , of approx 0.2A.

over 5 years ago

0 Ankit Gupta95 over 5 years ago

TI__Expert 4555 points

Hello Val,

Can you please provide the SW waveform as well. It should be pretty straight forward. I would like to see the SW waveform to understand this issue. Are you doing measurements to TI EVM or custom made EVM. Can you please also provide the Application schematic as well. Thanks!

Best Regards,

Ankit Gupta

Application Engineer

0 Val Stratulat over 5 years ago in reply to Ankit Gupta95

Prodigy 40 points

Hello Ankit,

Sounds good, please see below the SW waveform. From the SW waveform, it looks that the fsw is 1/6us, so approx 166KHz.

Below is the capture of the Schematic application. R201 is measured to be 33k (which should result in a fsw between 300 to 400kHz)

0 Ankit Gupta95 over 5 years ago in reply to Val Stratulat

TI__Expert 4555 points

Hello Val,

From your scope capture, it seems you are seeing double pulse on the SW waveform. Is it true?

Best Regards,

Ankit Gupta

0 Val Stratulat over 5 years ago in reply to Ankit Gupta95

Prodigy 40 points

That would be right Ankit. it seems that there are double pulse on the SW waveform.

I was searching through other forums with regards to SW and switching frequency issues on the TI forum, I came across this particular one where there's an instance of a double pulse at Fsw.

This still doesn't address the issue with the frequency, which is smaller than expected.

https://e2e.ti.com/support/power-management/f/196/p/718953/2652829?tisearch=e2e-sitesearch&keymatch=switching%2525252525252520frequency%2525252525252520fsw#2652829

0 Val Stratulat over 5 years ago in reply to Val Stratulat

Prodigy 40 points

Ankit,

I was able to fix the issue. I was only able to get the correct frequency in 'COT Type 2' mode of operation, with a 100pf capacitor across the first feedback resistor. (Cff capacitor) This also reduced my ripple Vpk-pk. In addition, reducing the Resr in 'COT Type 2' futher reduces the output ripple.

I am satisfied that my issue is resolved, though I am not certain TI should have 'COT Type 1' in the datasheet as an option, as I was unable to have it work based on bench testing with the outlined values in my initial schematic.

Power management

Power management forum

LM5163: LM5163 Switching Frequency - Not measuring expected value