ISL28023 Datasheet, PDF (49/55 Pages) Intersil Corporation – Bidirectional current sensing

ISL28023
The external clock pin can accept signal frequencies above
25MHz by programming the system clock frequency such that
the internal clock frequency is below 25MHz.
0.5
-0.5
-1.5
-2.5
-3.5
-4.5
-5.5
-6.5
ExtClk FREQUENCY = 45MHz
-7.5
-8.5
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
ExtClkDiv BIT VALUE
FIGURE 116. EXTERNAL CLOCK vs EXTERNAL BIT VALUE
Figure 116 illustrates the effects of dividing the external clock
frequency on the V_SHUNT measurement accuracy.
Figures 115 and 116 were generated by applying a DC voltage to
the V_SHUNT input and measuring the signal by way of an ADC
conversion.
Overranging
It is not recommended to operate the ISL28023 outside the set
voltage range. In the event of measuring a shunt voltage beyond
the maximum set range (80mV) and lower than the clamp
voltage of the protection diode (1V), the measured output
reading may be within the accepted range but will be incorrect.
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 maximum measurable range
for the V_SHUNT input (VINP-VINM) of the ISL28023 is 80mV. The
ISL28023 allows the user to define a unique range other than
±80mV.
Once the voltage range for the input is chosen and the maximum
measurable current is known, the sense resistor value is
calculated using Equation 19.
R _sense
V _{shunt_range}
Imeas _Max
(EQ. 19)
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 in Equation 20.
P _{res_rating} V _{shunt_range}Imeas _Max
(EQ. 20)
A general rule of thumb is to multiply the power rating calculated
in Equation 20 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 20), 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 21.
R _sense R _senseRsense _TCTemperature
(EQ. 21)
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 50 is a shunt resistor look up table for select full-scale
current measurement ranges (Imeas_Max). Table 50 also provides
the minimum rating for each shunt resistor.
TABLE 50. SHUNT RESISTOR VALUES AND POWER RATINGS FOR
SELECT MEASURABLE CURRENT RANGES
R_SENSE/P_RATING
Imeas_Max
100µA
VSHUNT 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
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FN8389.5
March 18, 2016