WEBENCH® Tools/TPS53119: Iocp and Rtrip calculations

Hi Vishwanath,

In step 1, you have found a datasheet error.  While the title says Figure 4 is the TRIP Pin Current vs. Temperature, this is incorrect and the chart is just a copy of Figure 3 with the Title changed.  In multiple place, including Electrical Characteristics on Page 6 and in the description of Overcurrent Protection on Page 15, the datasheet says there is slope versus temperature of 4700 ppm/C.  Also please note Figure 4 Y-axis is labeled as OVP/UVP Threshold.

However, my question is actually a little different.  Webench calculates a value for Rtrip but does not offer any information on Iocp.  When I use the equation in Step 3 to calculate Iocp based on the value of Rtrip provided by Webench, Iocp is very large. 

The first thing I did was check my calculations versus the Typical Application in section 8.2.2.  Using equation 7 with the values from 8.2.2 of Rtrip=20k, Itrip=10uA @ 25C, Rdson=1.1mOhm from CSD86350 datasheet,  Fsw=500kHz, L=0.44uH, Vin=12V, Vout=1.2V, and then calculating Iocp=25.18A.  The table in 8.2.2.1 shows that Iover=25A which matches the calculated value of Iocp=25.18A.  This confirmed that my calculations in a spreadsheet do in fact match the datasheet.  This also confirms that my selection of the Rdson value from the CSD86350 is matches the values used in the Typical Application.

Next to find Iocp in my application because Webench does not output the overcurrent value.  Using equation 7 with the values from Webench of Rtrip=25.5K, Itrip=10uA, Rdson=0.7mOhm from CSD86360 datasheet, Fsw=650kHz, L=1.5uH, Vin=12V, Vout=5V and then calculating Iocp=47A.  Note, I selected the same Rdson value which the datasheet application note in 8.2.2 selected.  The problem is my application only has a maximum current of 15A DC and the simulations from Webench show the current waveforms of the inductor never get over 17A.  Calculated Iocp=47A versus typical application of 17A peak current means the overcurrent protection is basically useless.

My questions are multiple,

1.  Why does Webench set such a high value for Iocp? 

2. What value for the Rdson of the CSD86360 does Webench use?

3. Why does the datasheet set Iocp=25A for an 8A application?  

Hi Robbie,

I've raised the request to modify the data sheet image. Thanks for noticing it.

1. Wenech sets Iocp = 1.3*Iout. but limits it to minimum of 1A.

2. Rds is a function of temperature and driver voltage. 0.7 mOhms is the typical ZDS value for the condition given in CSD86360 data sheet. We need M2 RDS for Rtrip calculation.

3. We are consulting with the applications team for this device to find more information regarding Rtrip calculation and Iocp setting.

Please let us know if you have any further queries.

Regards,

Vishwanath

Robbie,

The TRIP voltage needs to be greater than 0.2V and less than 3V.

Rtrip x Itrip   20kx10uA is 0.2V   With the low on resistance fets, current limit is higher. 

Hi David,

Sounds like all my calculations are correct but this opens a different line of questions. 

As Rdson of the Sync Fet has a direct and in this case negative impact on the actual current limit threshold if the Vocl=0.2V, are there any other methods to lower the actual current limit?

Design is maximum 15A design but with the design from Webench the Iocp calculates to 37A.

Thank you, Robbie

Hi Robbie,

    We will add the Iocp calculated value to be displayed in the opval section of the WEBENCH. At this point, I am not sure if there is a way to lower the current limit apart from increasing the Rdson of the LS FET. Anyway, will check and get back to you shortly.

Regards,

Gerold

Hi Robbie,

    At the moment, increasing Rdson of the LSFET is the known way to get the current limit lower.

Even with Vtrip of 0.2V, the accuracy can have a +/-32% variation as per the datasheet.

Do you want us to give you alternate part number recommendations?

regards,

Gerold

Hi Robbie,

     Just wanted to check if you need further assistance with this issue.

regards,

Gerold