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ZHCSN24A August 2019 – May 2021 TPS53676

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7.5.3.2 Input current calibration (measured)

Use MFR_CALIBRATION_CONFIG command to adjust the gain and offset of the input current sensor. First, set analog front-end gain such to keep the signal at the ADC to be less than 800 mV. Then set the digital gain to fine-tune the total gain based on the selected input current shunt. Finally adjust the input current offset based on lab measurements. A detailed example of input current sensor calibration is shown in Pin Functions. .

The equation for input current sense measurements is shown in Equation 8.

Equation 8.

READ_IIN = I_{IN} \times R_{SENSE} \times G_{INSHUNT} \times (\frac{G_{IINMAX}}{800 mV}) + IIN_OFS

where

I_IN is the true input current in amperes
R_SENSE is the effective sense element gain in ohms
G_INSHUNT is the analog front-end gain
G_IINMAX is a digital-domain gain factor used for fine tuning
IIN_OFS is an offset factor applied to the resulting value in amperes

Estimate the maximum input current for the design using Equation 9.

Equation 9.

I_{IN(MAX)} = (\frac{V_{OUT(A)} \times I_{PEAK(A)}}{V_{IN} \times η_{IPEAK(A)}} + \frac{V_{OUT(B)} \times I_{IPEAK(B)}}{V_{IN} \times η_{IPEAK(B)}}) \times K_{MARGIN}

where

V_OUT(A) and V_OUT(B) are the output voltage for channels A and B respectively
I_PEAK(A) and I_PEAK(B) are the peak design currents for channels A and B respectively
V_IN is the input voltage for the design
ɳ_IPEAK(A) and ɳ_IPEAK(B) are the full-load conversion efficiency for channels A and B respectively
K_MARGIN is a factor of safety used for design margin

Select the analog front-end gain, G_IINSHUNT, to maximize the signal level at the ADC whie remaining within its full scale range of 800 mV. Select the closest available value less than the result of Equation 10.

Equation 10.

G_{IINSHUNT} \leq \frac{800 mV}{I_{IN(MAX)} \times R_{SENSE}}

Finally select the digital gain factor, G_IINMAX, with a resolution of 0.5 per LSB, to fine-tune the current sense gain using Equation 11.

Equation 11.

G_{IINMAX} = \frac{800 mV}{G_{IINSHUNT} \times R_{SENSE}}

Example: 12V to 1.0 V 4+0 design at 100 A, R_SENSE = 1.0 mΩ

Channel B is not used in this design. Estimate the maximum input current, according to the calculation below.

Equation 12.

I_{IN(MAX)} = (\frac{1.0 V \times 100 A}{12 V \times 90 %}) \times 1.25 = 11.6 A

Select the analog front-end gain, and digital gain factors as shown below. Set the IIN_OFS to 0.0 A, and tune based on design characterization measurements.

Equation 13.

G_{IINSHUNT} \leq \frac{800 mV}{11.6 A \times 1.0 mΩ} \to G_{IINSHUNT} = 60.0

Equation 14.

G_{IINMAX} = \frac{800 mV}{60.0 \times 1.0 mΩ} \approx 13.5

Finally, the calibrated input current measurement is verified to be calibrated properly.

Equation 15.

READ_IIN = I_{IN} \times 1.0 mΩ \times 60 \times (\frac{13.5}{800 mV}) \approx 1.0 \times I_{IN}

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7.5.3.2 Input current calibration (measured)