LMP8640/LMP8640HV
(4.1V) , the max Sense voltage across the shunt resistor is
evaluated according the following formula:
V
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 :
R
S
= (max V
SENSE
) / I_MAX
For each gain option the max shunt resistors are the follow-
ing : 20.5mΩ, 8.2mΩ, 4.1mΩ respectively.
One of the project constraints requires RS<10mΩ, it means
that the 20.5mΩ 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
CMRR calibrated ad mid
VCM range
PSRR calibrated at 5V
Total error (squared sum of
contribution)
Resolution
(Total error / R
S
)
Rs=4.1mΩ
77.9µV
8.9µV
78µV
19.2mA
Rs=8.1mΩ
77.9µV
8.9µV
78µV
9.6mA
Accuracy Calculation
ERROR SOURCE
Tc Vos
Nosie
Gain drift
Total error (squared sum of
contribution)
Accuracy
100*(Max_V
SENSE
/ Total
Error)
Rs=4.1mΩ
182µV
216µV
75.2µV
293µV
0.7%
Rs=8.1mΩ
182µV
216µV
151µV
320µV
0.4%
From the tables above is clear that the 8.2mΩ shunt resistor
allows the respect of the project's constraints. The power
burned on the Shunt is 820mW at 10A.
13
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