ISL28022_15 Datasheet, PDF (25/32 Pages) Intersil Corporation – Precision Digital Power Monitor

ISL28022
Shunt Resistor Selection
In choosing a sense resistor, the following resistor parameters
need to be considered: the resistor value, resistor temperature
coefficient and resistor power rating.
The sense resistor value is a function of the full-scale voltage
drop across the shunt resistor and the maximum current
measured for the application. The ISL28022 has 4 voltage
ranges that are controlled by programming the PGA bits within
the configuration register. The PGA bits control the voltage range
for the V_SHUNT input (VINP-VINM) of the ISL28022. Once the
voltage range for the input is chosen and the maximum
measurable current is known, the sense resistor value is
calculated using Equation 11:
R _sense
V _{shunt_range}
Imeas _Max
(EQ. 11)
In choosing a sense resistor, the sense resistor power rating
should be taken into consideration. The physical size of a sense
resistor is proportional to the power rating of the resistor. The
maximum power rating for the measurement system is
calculated as the V_shunt__range multiplied by the maximum
measurable current expected. The power rating equation is
represented by Equation 12:
P _{res_rating} V _{shunt_range}Imeas _Max
(EQ. 12)
A general rule of thumb is to multiply the power rating calculated
in Equation 12 by 2. This allows the sense resistor to survive an
event when the current passing through the shunt resistor is
greater than the measurable maximum current. The higher the
ratio between the power rating of the chosen sense resistor and
the calculated power rating of the system (Equation 12), the less
the resistor will heat up in high-current applications.
The temperature coefficient (TC) of the sense resistor directly
degrades the current measurement accuracy. The surrounding
temperature of the sense resistor and the power dissipated by
the resistor will cause the sense resistor value to change. The
change in resistor temperature with respect to the amount of
current that flows through the resistor, is directly proportional to
the ratio of the power rating of the resistor versus the power
being dissipated. A change in sense resistor temperature results
in a change in sense resistor value. Overall, the change in sense
resistor value contributes to the measurement accuracy for the
system. The change in a resistor value due to a temperature rise
can be calculated using Equation 13:
R _sense R _senseRsense _TCTemperature
(EQ. 13)
Temperature is the change in temperature in Celsius. Rsense_TC
is the temperature coefficient rating for a sense resistor. R_sense
is the resistance value of the sense resistor at the initial
temperature.
Table 23 is a shunt resistor reference table for select full-scale
current measurement ranges (Imeas_Max). The table also
provides the minimum rating for each shunt resistor.
TABLE 23. SHUNT RESISTOR VALUES AND POWER RATINGS FOR
SELECT MEASURABLE CURRENT RANGES
R_sense/
P_rating
Imeas_Max
(PGA 00)
40mV
100µA 400Ω/4µW
V_SHUNT RANGE (PGA SETTING)
(PGA 01)
80mV
(PGA 10)
160mV
800Ω/8µW 1.6kΩ/16µW
(PGA 11)
320mV
3.2kΩ/
32µW
1mA 40Ω/40µW 80Ω/80µW
160Ω/160µW 320Ω/
320µW
10mA 4Ω/400µW 8Ω/800µW
16Ω/1.6mW 32Ω/
3.2mW
100mA 400mΩ/4mW 800mΩ/8mW 1.6Ω/16mW 3.2Ω/
30mW
500mA 80mΩ/20mW 160mΩ/40mW 320mΩ/
80mW
640mΩ/
160mW
1A 40mΩ/40mW 80mΩ/80mW 160mΩ/
160mW
320mΩ/
320mW
5A 8mΩ/200mW 16mΩ/400mW 32mΩ/
800mW
64mΩ/
1.6W
10A 4mΩ/400mW 8mΩ/800mW 16mΩ/1.6W 32mΩ/
3.2W
50A 0.8mΩ/2W 1.6mΩ/4W
3.2mΩ/8W
6.4mΩ/
16W
100A 0.4mΩ/4W 0.8mΩ/8W
1.6mΩ/16W 3.2mΩ/
32W
500A 0.08mΩ/20W 0.16mΩ/40W 0.32mΩ/80W 0.64mΩ/
160W
It is often hard to readily purchase shunt resistor values for a desired
measurable current range. Either the value of the shunt resistor
does not exist or the power rating of the shunt resistor is too low. A
means of circumventing the problem is to use two or more shunt
resistors in parallel to set the desired current measurement range.
For example, an application requires a full-scale current of 50A with
a maximum voltage drop across the shunt resistor of 40mV.
Table 23 shows this requires a sense resistor of 0.8mΩ, 2W resistor.
Assume the power ratings and the shunt resistor values to choose
from are 1mΩ 1W, 2mΩ/1W, and 4mΩ/1W.
Let’s use a 1mΩ and a 4mΩ resistor in parallel to create the
shunt resistor value of 0.8mΩ. Figure 39 shows an illustration of
the shunt resistors in parallel.
0.004
0.001
FIGURE 39. A SIMPLIFIED SCHEMATIC ILLUSTRATING THE USE OF
TWO SHUNT RESISTORS TO CREATE A DESIRED SHUNT
VALUE
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FN8386.7
October 2, 2015