ISL28022_15 Datasheet, PDF (29/32 Pages) Intersil Corporation – Precision Digital Power Monitor

ISL28022
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 (PGA 00) 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 20 on
page 27, the calculated trace width is 2.192in. The trace width,
thickness and the desired sense resistor value is known. Utilizing
Equation 21 on page 27, the trace length is calculated to be
1.832in.
THE LENGTH OF THE
TRACE BETWEEN
THE TWO SENSE
LINES DEFINES THE
SENSE RESISTOR
VALUE.
FIGURE 48. ILLUSTRATES A LAYOUT EXAMPLE OF A CURRENT
SENSE RESISTOR MADE FROM A PCB TRACE
The width of the trace in Figure 48 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 20 and 21 on
page 27, 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 49 illustrates a layout technique for a multiple layered
trace sense resistor.
TOP
TRACE
VIA
PCB
VIA
TRACE _PCB
BOTTOM
VIA
TRACE
(A) CROSS SECTION VIEW
(B) TOPVIEW
FIGURE 49. ILLUSTRATES A LAYOUT EXAMPLE OF A MULTIPLE
LAYER TRACE RESISTOR
Figure 48 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.
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FN8386.7
October 2, 2015