LMP8640 datasheet(13/16 Pages) NSC

13 / 16 page

(4.1V) , the max Sense voltage across the shunt resistor is

evaluated according the following formula:

SENSE= (MAX Vin ADC) / Gain;

hence the max V

SENSE will be 205mV, 82mV, 41mV respec-

tively. The shunt resistor are then evaluated considering the

maximum monitored current :

S = (max VSENSE) / I_MAX

For each gain option the max shunt resistors are the follow-

ing : 20.5m

Ω, 8.2mΩ, 4.1mΩ respectively.

One of the project constraints requires RS<10m

Ω, it means

that the 20.5m

Ω will be discarded and hence the 50V/V and

100V/V gain options are still in play.

Third step – Shunt resistor selection

At this point an error budget calculation, considering the cal-

ibration of the Gain, Offset, CMRR, and PSRR, helps in the

selection of the shunt resistor. In the table below the contri-

bution of each error source is calculated considering the

values of the EC Table at 5V supply.

Resolution Calculation

ERROR SOURCE

Rs=4.1m

Ω

Rs=8.1m

Ω

CMRR calibrated ad mid

VCM range

77.9µV

PSRR calibrated at 5V

8.9µV

Total error (squared sum of

contribution)

78µV

Resolution

(Total error / R

19.2mA

9.6mA

Accuracy Calculation

ERROR SOURCE

Rs=4.1m

Ω

Rs=8.1m

Ω

Tc Vos

182µV

Nosie

216µV

Gain drift

75.2µV

151µV

Total error (squared sum of

contribution)

293µV

320µV

Accuracy

100*(Max_V

SENSE / Total

Error)

0.7%

0.4%

From the tables above is clear that the 8.2m

Ω shunt resistor

allows the respect of the project's constraints. The power

burned on the Shunt is 820mW at 10A.

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