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# TPSM8A28: Rtrip calculation

Part Number: TPSM8A28
Other Parts Discussed in Thread: TPS548A28

Datasheet Equation 8 calculates Ilimvalley, and Equation 9 is supposed to use Ilimvalley, but Equation 9 just uses Iout instead? Looks like Equations 10 & 11 just use Iout as well.

What method does WEBENCH use to calculate Rtrip? Because its Rtrip does not agree with Equation 9.

• I can answer the WEBENCH part myself: Looks like Iocp=Iout*1.25, then Rtrip=60000/Iocp

• Rtrip adjusts the valley current the device trips.    Iout= ILvalley - 0.5*ILripple.    The equations in webench maybe considering inductor value variation.

• Can you please review Equations 9, 10 & 11 in the datasheet and let us know if Ilimvalley or Iout is supposed to be used?

• The equation has a minus where a + should be (markup). I will get the equation fixed in the datasheet.

If the meantime, the tps548a28 datasheet shows the equation correctly www.ti.com/.../tps548a28.pdf

• Here's Equation 10. It shows Ilimvalley but plugs in 15A instead of Ilimvalley value from Equation 8?

• The 4kOhm resistor is used which gives a ~15A ILIMVALLEY.

If ILIMVALLEY of 13.75A is used, RTRIP will be a greater value than the 4kohm.

Since 4kOhm is used for Rtrip, 15A is used in the equations.

Rtrip should set a value greater than calculated ILIMVALLEY so that the Iout can be reached.

• I guess the question starts with Equation 9 for Rtrip. It shows Ilimvalley but plugs in 15A instead of Ilimvalley value from Equation 8? Based on your previous response, Equation 9 should be corrected to remove Ilimvalley?

• The RTRIP set the valley current that trips overcurrent.    The eq 8 calculates the minimum valley current set point to get an output current of 15A assuming the nominal inductance of 600nH.

The ILIMVALLEY value in equation 9 should be greater than the eq8.  Otherwise, the device may trip at a lower output current.