ISL28025 Datasheet, PDF (42/48 Pages) Intersil Corporation – Precision Digital Power Monitor with Real Time Alerts

ISL28025
Table 46 is a shunt resistor look-up table for select full-scale
current measurement ranges (Imeas_Max). The table also
provides the minimum rating for each shunt resistor.
TABLE 46. SHUNT RESISTOR VALUES AND POWER RATINGS FOR
SELECT MEASURABLE CURRENT RANGES
R_SENSE/P_RATING
Imeas_Max
100µA
V_SHUNT RANGE (PGA SETTING)
80mV
800Ω/8µW
1mA
80Ω/80µW
10mA
8Ω/800µW
100mA
800mΩ/8mW
500mA
160mΩ/40mW
1A
80mΩ/80mW
5A
16mΩ/400mW
10A
8mΩ/800mW
50A
1.6mΩ/4W
100A
0.8mΩ/8W
500A
0.16mΩ/40W
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 100A with a maximum voltage drop across
the shunt resistor of 80mV. From Table 46, this requires a sense
resistor of 0.8mΩ, 8W resistor. Assume the power ratings and
the shunt resistor values to chose from are 1mΩ4W, 2mΩ/4W
and 4mΩ/4W.
Let’s use a 1mΩ and a 4mΩ resistor in parallel to create the
shunt resistor value of 0.8mΩ. Figure 82 shows an illustration of
the shunt resistors in parallel.
0.004
0.001
FIGURE 82. SIMPLIFIED SCHEMATIC ILLUSTRATING THE USE OF
TWO SHUNT RESISTORS TO CREATE A DESIRED SHUNT
VALUE
The power to each shunt resistor should be calculated before
calling a solution complete. The power to each shunt resistor is
calculated using Equation 17.
P _shuntRes
V
2
shunt_range
R _sense
(EQ. 17)
The power dissipated by the 1mΩ resistor is 6.4W. 1,6W is
dissipated by the 4mΩ resistor. 1.6W exceeds the rating limit of
1W for the 1mΩ sense resistor. Another approach would be to
use three shunt resistors in parallel as illustrated in Figure 83.
0.004
0.002
0.002
FIGURE 83. INCREASING THE NUMBER OF SHUNT RESISTORS IN
PARALLEL TO CREATE A SHUNT RESISTOR VALUE
REDUCES THE POWER DISSIPATED BY EACH SHUNT
RESISTOR
Using Equation 17, the power dissipated to each shunt resistor
yields 3.2W for the 2mΩ shunt resistors and 1.6W for the 4mΩ
shunt resistor. All shunt resistor are within the specified power
ratings.
Layout
The layout of a current measuring system is equally important as
choosing the correct sense resistor and the correct analog
converter. Poor layout techniques can result in severed traces,
signal path oscillations, magnetic contamination, which all
contribute to poor system performance.
TRACE WIDTH
Matching the current carrying density of a copper trace with the
maximum current that will pass through is critical in the
performance of the system. Neglecting the current carrying
capability of a trace will result in a large temperature rise in the
trace, and the loss in system efficiency due to the increase in
resistance of the copper trace. In extreme cases, the copper
trace could be severed because the trace could not pass the
current. The current carrying capability of a trace is calculated
using Equation 18.
1
Trace _width

Imax
0.725



kT^0.44


Trace _Thickness
(EQ. 18)
I_max is the largest current expected to pass through the trace. T
is the allowable temperature rise in Celsius when the maximum
current passes through the trace. Trace_Thickness is the thickness
of the trace specified to the PCB fabricator in mils. A typical
thickness for general current carrying applications (<100mA) is
0.5oz. copper or 0.7mils. For larger currents, the trace thickness
should be greater than 1.0oz. or 1.4mils. A balance between
thickness, width and cost needs to be achieved for each design.
The coefficient k in Equation 18 changes depending on the trace
location. For external traces, the value of k equals 0.048 while
for internal traces the value of k reduces to 0.024. The k values
and Equation 18 are stated per the ANSI IPC-2221(A) standards.
Submit Document Feedback ₄₂
FN8388.4
February 19, 2016