ISL28025 Datasheet, PDF (41/48 Pages) Intersil Corporation – Precision Digital Power Monitor with Real Time Alerts

ISL28025
0.5
-0.5
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-6.5
-7.5 ^{ExtDiv = 0; (FreqInt = FreqExtClk / 8)}
-8.5
0.01
0.1
1
10
100
ExtClk FREQUENCY (MHz)
FIGURE 80. EXTERNAL CLOCK BANDWIDTH vs MEASUREMENT
ACCURACY
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ExtClk FREQUENCY = 45MHz
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-8.5
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
ExtClkDiv BIT VALUE
FIGURE 81. EXTERNAL CLOCK vs EXTERNAL BIT VALUE
Figure 81 illustrates the effects of dividing the external clock
frequency on the V_SHUNT measurement accuracy.
Figures 80 and 81 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 ISL28025 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 (V_INP - V_INM) of the ISL28025 is 80mV. The
ISL28025 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 14.
R _sense
V _{shunt_range}
Imeas _Max
(EQ. 14)
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 15.
P _{res_rating} V _{shunt_range}Imeas _Max
(EQ. 15)
A general rule of thumb is to multiply the power rating calculated
in Equation 15 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 15), 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 dissipate 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 16
R _sense R _senseRsense _TCTemperature
(EQ. 16)
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.
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FN8388.4
February 19, 2016