SI8540 Datasheet, PDF (8/18 Pages) List of Unclassifed Manufacturers – HIGH-SIDE CURRENT SENSE AMPLIFIER

Si8540
3.1.1. Selecting R_SENSE
Selecting R_SENSE involves making the best trade-off between power efficiency and accuracy. Low R_SENSE
values dissipate less power while higher values maximize accuracy. In general, it is best to choose a relatively high
value for R_SENSE in applications where the measured current is small. For higher current applications, the sense
resistor should be able to dissipate the heat from its power loss; otherwise, its value may drift or it may fail open,
possibly causing a large differential voltage across RG1 and RG2 that may damage the device. In most
applications, R_SENSE should have low inductance to reduce the impact of any high-frequency components in the
current being measured (low inductance metal film resistors are recommended). Also, note that the Si8540
requires at least 3.5 V of voltage headroom between the voltage at pin RG1 and pin OUT. This voltage headroom
decreases as R_SENSE increases. A good guideline for determining the maximum value for R_SENSE is shown in the
following equation:
R_SENSEmax = (V_SOURCE – V_OUTmax – 3.5 V)/I_LOAD
Where:
V_SOURCE is the high-side voltage
V_OUTmax is the full-scale output voltage at the OUT pin
I_LOAD is the current passing through R_SENSE measured by the Si8540
3.1.2. Selecting RG1 and RG2
The values of resistors R_G1 and R_G2 determine the sense amp current-gain. These two resistors must have the
same value, and resulting current gain is equal to R_SENSE / R_G (where R_G = R_G1 = R_G2).
The minimum value of R_G is determined by the maximum current at the OUT pin (1.5 mA) and by the resistance
between the internal current sense amp input and the sense resistor (approximately 0.2 ). As the value of R_G is
reduced, the input resistance becomes a larger portion of the total gain-setting resistance. This gain error can be
compensated by trimming R_G or R_OUT. A good guideline for determining the maximum value for R_G is shown in the
following equation:
R_Gmax = (V_SENSEmax / 1.5 mA)
Where:
R_Gmax is the largest value for RG1 and RG2
V_SENSEmax is the value of V_SENSE at maximum I_LOAD
Note that for a given value of V_SENSE, a decrease of the R_G resistor values causes a corresponding increase in
current at the OUT pin. This causes additional power to be dissipated in R_OUT rather than in the load, which can
reduce efficiency. Note also that mismatches in the currents passing R_G1 and R_G2 (IOS) together with R_G affect the
full scale error.
This error can be reduced by lowering the values of R_G1, R_G2 and/or lowering their tolerances. This error can also
be reduced by increasing the value of R_SENSE.
3.1.3. Choosing R_OUT
R_OUT must be chosen to generate the required full-scale output voltage at the full scale I_OUT, which, in turn, is
determined by R_G1, R_G2, and R_SENSE. The upper limit of R_OUT is determined by the input impedance of the device
that it drives. This input impedance should be much larger than R_OUT; otherwise, measurement accuracy will be
degraded. A good guideline for choosing the value of R_OUT is shown in the following equation:
(V_OUTfullscale x R_G) / (I_LOAD x R_SENSE)
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Rev. 1.2