Datasheet

LMP8640
LMP8640HV
SNOSB28F –AUGUST 2010–REVISED APRIL 2013 www.ti.com
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 calibration of the Gain, Offset, CMRR, and PSRR, helps
in the selection of the shunt resistor. In the table below the contribution of each error source is calculated
considering the values of the Electrical Characteristics table at 5V supply.
Table 2. Resolution Calculation
ERROR SOURCE R
S
= 4.1mΩ R
S
= 8.1mΩ
CMRR calibrated ad mid VCM range 77.9µV 77.9µV
PSRR calibrated at 5V 8.9µV 8.9µV
Total error (squared sum of contribution) 78µV 78µV
Resolution (Total error / R
S
) 19.2mA 9.6mA
Table 3. Accuracy Calculation
ERROR SOURCE R
S
= 4.1mΩ R
S
= 8.1mΩ
Tc Vos 182µV 182µV
Nosie 216µV 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 8.2mΩ shunt resistor allows the respect of the project's constraints. The
power burned on the Shunt is 820mW at 10A.
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