OAR1R01JP Datasheet, PDF (5/58 Pages) Token Electronics Industry Co., Ltd. – Chip Current Sense / Open-Air Current Shunts / Axial Current Sense Resistors

TOKEN Design & Select Current Sense Resistors
Design & Select
Current Sense
Resistors
Point 1 - Power Rating
Calculate the power dissipation under operating conditions
Equation: Pavg = IRMS2 × R; where Power (P), Current (I), Root Mean Square (RMS),
Resistance (R).Allowing for transient or fault conditions and high ambient temperature if applicable,
select the required power rating.For many current sense products, only the maximum temperature of
the solder joints limits the power rating.Power rating is thus a function of the PCB layout design as
well as of component selection (see point 4.).
Point 2 - Resistance Value
Determine the minimum suitable resistance value. This is the lowest value of peak sense voltage
consistent with an acceptable signal to noise ratio, divided by the peak current to be measured.
Point 3 - Temperature Coefficient of Resistance (TCR)
Establish the accuracy needed in terms of a tolerance on the value and of sensitivity to temperature.
The latter factor is quoted as Temperature Coefficient of Resistance (TCR), defined as the value
change in parts per million for a 1°C temperature rise. It is generally higher for low value resistors
because the metallic leads or terminations, which have a very high TCR, make up a significant part of
the total resistance value.
To achieve acceptable accuracy it is normally necessary to make four-terminal (Kelvin) connections
to the resistor. This means connecting the current carrying tracks and the voltage sense tracks directly
to the component pads. Even when this is done, there is still some pad area and solder in series with
the resistor, which may compromise the actual tolerance and TCR of the soldered part. For very high
accuracy or very low values, a four-terminal resistor type is the best choice.
Point 4 - PCB Layout
Care must be taken when laying out a PCB if the stated performance of a sense resistor is to be
achieved. The current carrying tracks should be as wide as possible, using multiple layers connected
by many vias near the component pad. This also improves the heatsinking of the joints.
The best way to make four-terminal connections to a two-terminal through-hole resistor is to use
different sides of the PCB for the current and voltage connections. Failing this, current and voltage
tracks should connect to opposite sides of the component pad.
Version 2010
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