ISL28025 Datasheet, PDF (44/47 Pages) Intersil Corporation – Precision Digital Power Monitor with Real Time Alerts

ISL28025
CURRENT BEARING TRACE
LANDING PAD
SENSE TRACE
When routing high current traces, avoid routing high impedance
traces in parallel with high current bearing traces. A means of
limiting the magnetic interference from high current traces is to
closely route the paths connected to and from the sense resistor.
The magnetic fields will cancel outside the two traces and add
between the two traces. Figure 89 illustrates a magnetic field
insensitive layout.
If possible, do not cross traces with high current. If a trace
crossing cannot be avoided, cross the trace in an orthogonal
manor and the furthest layer from the current bearing trace. The
inference from the current bearing trace will be limited.
LANDING PAD
SENSE TRACE
CURRENT BEARING TRACE
FIGURE 87. CONNECTING THE SENSE LINES TO A CURRENT SENSE
RESISTOR
Current sense resistors are often smaller than the width of the
traces that connect to the footprint. The trace connecting to the
footprint is tapered at a 45° angle to control the uniformity of the
current flow.
MAGNETIC INTERFERENCE
The magnetic field generated from a trace is directly proportional
to the current passing through the trace and the distance from
the trace the field is being measured at. Figure 88 illustrates the
direction the magnetic field flows versus current flow.
B ^{ oI}
2r
FIGURE 88. THE CONDUCTOR ON THE LEFT SHOWS THE MAGNETIC
FIELD FLOWING IN A CLOCKWISE DIRECTION FOR
CURRENTS FLOWING INTO THE PAGE. CURRENT FLOW
OUT OF THE PAGE HAS A COUNTER CLOCKWISE
MAGNETIC FLOW
The equation in Figure 88 determines the magnetic field, B, the
trace generates in relation to the current passing through the
trace, I, and the distance the magnetic field is being measured
from the conductor, r. The permeability of air, µ_o, is 4 *10^-7
H/m.
TO ^{THREESSIESNTOSER}
TO SENSE
CIRCUITRY
SENSE
RESISTOR
B _to  B _from
B _to  B _from
B _to  B _from
FIGURE 89. 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 18 on page 42, the calculated trace
width is 2.192in. The trace width, thickness and the desired
sense resistor value is known. Utilizing Equation 19 on page 43,
the trace length is calculated to be 1.832in.
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FN8388.3
June 17, 2015