WEBENCH® Tools/TPS40400: Vout and Vin Calibration Gain

liwei qi 

Linear Measurement systems have two primary factors:  A Gain factor and an Offset Factor.

With an offset only trim, the trim is only accurate at a single point if the gain factor is not correctly matched.

To trim with a gain and offset trim, it is possible to take 2 or 3 measurements points and align the measurement system to multiple points.

For Input Voltage Trim:

Force a known voltage on the input and READ the input voltage from the digital measurement system

Force a second known voltage on the input and READ the input voltage from the digital measurement system.

To correct the gain, divided the actual difference (Vin2 - Vin1) by the measured difference (READ_VIN2 - READ_VIN1) that ratio, minus 1 is the calibration factor that needs to be programmed into the device.  Multiply by 100 to convert into percent.

VIN_CAL_GAIN = 100 x [ (Vin2 - Vin1) / (Read-Vin2 - Read-Vin1) - 1 ]

After you have adjusted the gain, read VIN again to calculate the offset adjustment:

VIN_CAL_OFFSET = Vin3 - Read-Vin3

It is always best to re-measure the offset after adjusting the gain as changing the gain can affect the measured offset value.

Repeat for READ VOUT.  It can be useful to use the VOUT_COMMAND function to help force output voltage values, but for the best possible accuracy, the actual output voltage should be measured.

liwei qi 

My apologies for that error.  You are correct, the TPS40400 does not support the VOUT_COMMAND function to change the output voltage, you would need to use the VOUT_TRIM function to adjust the output voltage to a few different points in order to measure the gain and offset errors.

For Output Current, the gain is adjusted by setting the IOUT_CAL_GAIN function.  The IOUT_CAL_GAIN units are mV/A (mOhms) and should be matched to the current sense element used, either a shunt resistor or the inductor DCR.  If you wish to further trim the IOUT_CAL_GAIN value based on measurements, you could use the following formula:

IOUT_CAL_GAIN(trimmed) = IOUT_CAL_GAIN(untrimed) x [ (Iout2 - Iout1) / (Read_Iout2 - Read_Iout1)

For the offset trim, I would not recommend trimming the offset at 0A load current as this could add some non-linear errors to the measurement.  Typically a mid, or "typical" load is used, though if heavy load current or OCP levels are most critical, those can be measured as well.  The READ_IOUT function will be most accurate at the level that the offset trim was performed at and will degrade by additional gain error as the load moves away from that level.