TPS22917: Model simulation and parameters calculation

Hi Amir,

Thank you for your question, We do recommend using the EVM to test device functionality for specific loading conditions.

I will take a look at this question by the end of the week.

Best Regards,
Nicholas

Hi Amir,

Looking at the datasheet it seems that the example image in the data sheet, the calculations from the data sheet, and your PSpice simulation all have different operating conditions resulting in the differences you see in rise time and inrush current. 

If you would like precise data for inrush current using an EVM would be the best option to test for your specific loading conditions.

Best Regards,

Nicholas

Hi Nicholas,

Thanks for your answer.

It's understandable that the result from the simulation may deviate from the real life measurement depending on the model accuracy.

However regardless of the simulation result, I assume there should be a correlation between the calculation outcomes based on the formula and equations  and, the examples results in the datasheet.

I read the datasheet again, especially section 10.2.2, I could not find that the operating conditions of image 10.2 and image 10.3 differ apart from CT. Would you please let me know if I have looked at the wrong section?

Considering your elaboration, Does not it mean that I can not rely on the equations and formulas in the datasheet for finding an appropriate CT to achieve a desired inrush current and Slew rate?

Does it imply that I need to use an EVM to find an appropriate CT?

If it is Considered that the operating conditions of images 10-2 and 10-3 are similar apart from the CT, then:

With CT=1000pF, SRon=1900 and using equation (1) in section 9-3-2 returns Slew Rate=1.9 mV/us

With Slew Rate= 1.9 mV/us and CL=47uF and using equation (5) the inrush current should be 89.3mA.

So my question is Why the measured inrush current(142mA) in image 10-3 is almost 60% more than the calculated value(89.3mA)?

Best regards,

Amir

Hi Amir,

All numbers and formulas given in the datasheet are correct and can be used to find the correct slew rate, inrush current, and CT that you need.

The main reason you see a difference between the 89.3 mA you calculated on the graph is that the CL and operating conditions of the graphs are not given and it may be different from the 47uF you are using for your calculations resulting in the 60% difference you are seeing.

However I have noticed some inaccuracies with the PSpice model and I have reported those to my team I would recommend basing your schematic on the datasheet calculations.

If you would like to further verify your implementation you can use an EVM to accurately test your operating conditions.

Best Regards,
Nicholas

Hi Nicholas,

Thanks for the elaboration.

So it would be helpful and more clear if it is mentioned in the datasheet that the operating condition and CL of each graph are not identical.

Having said that, I have obtained an EVM of TPS22917 and performed some measurements with different CL, please see the following results:

The operating conditions of each test apart from CL are identical:

CT=1000pF (Based on the TPS22917 EVM user guide ) Vin= 3.6V and RL= 1K

CL=47uF

CL=22uF

CL=15uF

You can find the summer here:

As you can see above the measurement result for 47uF load is almost in concur with  the measurement result shown in graph 10-3 in the datasheet.

I would appreciate it if you could let me know why the measured inrush current for each load is higher than the calculated one and the rise time is lower?

Best regards,

Amir

Hi Amir,

I am going to verify your results and get back to you with an answer later this week.

Thanks,
Nicholas

Hi Amir,

Thanks for helping us work through the problem, your tests really helped us determine where the problem could be.

We will make sure to include the variation of RL in the next revision of the datasheet to add further clarity on how to find CT.

Best Regards,
Nicholas