7.5.7.1.2 Calibrating Current Measurements

The IOUT_CAL_GAIN and IOUT_CAL_OFFSET commands are available to allow the user to fine-tune current measurements. Setting the PHASE command to 80h also allows the total current measurement to be calibrated in a similar manner. The TI power stage devices supply current information to the controller device, using the CSPx and CSNx pins, with a scale of 5 mV/A. The IOUT_CAL_GAIN command may be used to fine-tune the scaling inside the controller to account for any gain mismatch. Likewise, the IOUT_CAL_OFFSET command may be used to apply an offset to the controller current measurements, to null offset errors.

Example: Calibrating Total Output Current Measurement

1. Select Channel A. Write PAGE to 00h
2. Select the total current. Write PHASE to 80h
3. First read back READ_IOUT under two known output currents:
   1. For this example, apply a load of 60 A
   2. Read back READ_IOUT, record the value. For this example, consider that READ_IOUT gives 59.0 A.
   3. For this example, apply a load of 120 A
   4. Read back READ_IOUT, record the value. For this example, consider that READ_IOUT gives 119.6 A
4. Calculate the gain error:
   1. The current reading increased (119.6 A - 59.0 A) = 60.6 A for a 60 A current step
   2. Hence, the current reading gain is (60.6 A / 60 A ) = 1.01.
   3. Ideally, the current reading gain is 1.00, so the readings show a +1% gain error.
5. Apply IOUT_CAL_GAIN to correct the gain error:
   1. The gain error is +1%, and the nominal current sense gain is 5 mΩ, so the current sense gain must be lowered by 1%.
   2. Hence, the IOUT_CAL_GAIN should be programmed to 5 mΩ × (1-1%) = 4.95 mΩ
   3. Referring to Table 21 the closest acceptable value is 4.953125 mΩ, or D13Dh.
   4. Write IOUT_CAL_GAIN to D13Dh.
6. Determine the offset error at a given point:
   1. Apply a known load. In this example, 90 A.
   2. Read back READ_IOUT. For this example, consider that READ_IOUT gives 89 A.
   3. Hence, the offset error is (89 A - 90A) = -1A.
7. Apply IOUT_CAL_OFFSET to correct the gain error
   1. The offset error is -1 A, so an offset of +1 A must be applied.
   2. Refer to Table 24, the closest available value of IOUT_CAL_OFFSET is +1.0 A or E808h
   3. Write IOUT_CAL_OFFSET to E808h.
8. Issue STORE_DEFAULT_ALL to commit the calibration values to NVM.