ISL28023_15 Datasheet, PDF (53/55 Pages) Intersil Corporation – Precision Digital Power Monitor with Margining

TO SENSE CIRCUITRY
SENSE
RESISTOR
ISL28023
.
The length of the
trace between the
two sense lines
defines the sense
resistor value.
B _to  B _from
B _to  B _from
B _to  B _from
FIGURE 125. CLOSELY ROUTED TRACES THAT CONNECT TO THE SENSE
RESISTOR REDUCES THE MAGNETIC INTERFERENCE
SOURCED FROM THE CURRENT FLOWING THROUGH THE
TRACES
A Trace as a Sense Resistor
In previous sections, the resistance and the current carrying
capabilities of a trace were discussed. In high current sense
applications, a design may utilize the resistivity of a current
sense trace as the sense resistor. This section will discuss how to
design a sense resistor from a copper trace.
Suppose an application needs to measure current up to 200A.
The design requires the least amount of voltage drop for
maximum efficiency. The full-scale voltage range of 40mV is
chosen. From Ohms law, the sense resistor is calculated to be
200µΩ. The power rating of the resistor is calculated to be 8W.
Assume the PCB trace thickness of the board equals
2oz/2.8mils and the maximum temperature rise of the trace is
20°C. Using Equation 28, the calculated trace width is 2.192in.
The trace width, thickness and the desired sense resistor value is
known. Utilizing Equation 29, the trace length is calculated to be
1.832in.
Figure 126 illustrates a layout example of a current sense
resistor defined by a PCB trace. The serpentine pattern of the
resistor reduces current crowding as well as limiting the
magnetic interference caused by the current flowing through the
trace.
FIGURE 126. ILLUSTRATES A LAYOUT EXAMPLE OF A CURRENT
SENSE RESISTOR MADE FROM A PCB TRACE
For the example discussed, the width of the trace in Figure 126
illustration would equal 2.192in and the length between the
sense lines equals 1.832in.
The width of the resistor is long for some applications. A means
of shortening the trace width is to connect two traces in parallel.
For calculation ease, assume the resistive traces are routed on
the outside layers of a PCB. Using Equations 28 and 29, the
width of the trace is reduced from 2.192in to 1.096in.
When using multiple layers to create a trace resistor, use
multiple vias to keep the trace potentials between the two
conductors the same. Vias are highly resistive compared to a
copper trace. Multiple vias should be employed to lower the
voltage drop due to current flowing through resistive vias.
Figure 127 illustrates a layout technique for a multiple layered
trace sense resistor.
TRACE
TOP
VIA
PCB
VIA
TRACE _PCB
BOTTOM
VIA
TRACE
(A) Cross Section View
(B) Top View
FIGURE 127. LAYOUT EXAMPLE OF A MULTIPLE LAYER TRACE
RESISTOR
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FN8389.4
June 17, 2015