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TPS40400: TPS40400 output current reported value

Part Number: TPS40400

Hi Ti member,

I have one question about the output current report problem (not accurate) with the TPS40400.

MY evaluated board is TPS40400EVM-351, therefore the current sense element is the inductor's DCR.

The inductor value is 0.47uH, and its DCR=0.85mohm.

According the application condition and inductor's parameters, I filled it in “Fusion Digital Power Designer” program.

However, the output current report value is not accuracy.

Please refer the test result as below table.     

Condition: Vin=12V, Vout=5V, Iout Cal Gain: 0.85mohm
Real Iout (A) 2 4 6 8 10 12 14 16 18 20
Iout_read (A) 5.38 7.69 9.81 11.81 14 16 18.38 20.56 22.81 25.25

**Note: (1). “Real Iout” is the real output current (sink from E-loader)

                (2). “Iout_read” is the value read from Fusion.


Would you help to give me some suggestions that would get more accurate output current through PMbus?


Best Regards,


  • Lance,

    I will look into this. In the meanwhile can you check the output current with an current meter and not rely on readout of electronic load. Just want to double check the real Iout.



  • Lance,
    Please read the two points below. You need to change the IOUT_CAL_GAIN to 0.85mohms via the Fusion GUI. You can also use IOUT_CAL_OFFSET after this step to fine for best accuracy. The default of Iout cal gain is 3mohms and you are using 0.85mohms inductor.

    Point 1: Output Current Reading
    The READ_IOUT command reports the average output current for the converter. The READ_IOUT command can only report positive output current, that is current sourced from the converter. If the converter is sinking current, this command results in a reading of 0 A. Another consideration is the amount of ripple voltage applied to the ISNS pins when the DC voltage level is low (low or no output current). The TPS40400 device averages the ripple voltage measurements when reporting the output current using the READ_IOUT command. Excessive negative ripple voltage (VISNS+ – VISNS– < 0) at the ISNSx pins causes an error in the reported output current. To ensure accurate readings, the differential voltage at these pins should not be allowed to exceed –45 mV.
    Point 2:
    7.6.17 IOUT_CAL_GAIN (38h)
    The IOUT_CAL_GAIN is the ratio of the voltage at the current sense element to the sensed current. The units are Ohms (Ω). The effective current sense element can be the DC resistance of the inductor or a separate current sense resistor. The default setting is 3 mΩ, and the resolution is 30.5 µΩ. The range is 0 to 15.6 mΩ. The contents of this register can be stored to non-volatile memory using the STORE_DEFAULT_ALL or