ISL28023 Datasheet, PDF (52/55 Pages) Intersil Corporation – Bidirectional current sensing

ISL28023
Orthogonal routing for high current flow traces will result in
current crowding, localized heating of the trace and a change in
trace resistance.
The utilization of arcs and 45° traces in routing large current flow
traces will maintain uniform current flow throughout the trace.
Figure 122 illustrates the routing technique.
CURRENT FLOW
CURRENT BEARING TRACE
LANDING PAD
SENSE TRACE
LANDING PAD
SENSE TRACE
FIGURE 122. USE ARCS AND 45 DEGREE TRACES TO SAFELY ROUTE
TRACES WITH LARGE CURRENT FLOWS
CONNECTING SENSE TRACES TO THE CURRENT SENSE
RESISTOR
Ideally, a 4-terminal current sense resistor would be used as the
sensing element. Four terminal sensor resistors can be hard to
find in specific values and in sizes. Often a two terminal sense
resistor is designed into the application.
Sense lines are high impedance by definition. The connection
point of a high impedance line reflects the voltage at the
intersection of a current bearing trace and a high impedance
trace.
The high impedance trace should connect at the intersection
where the sense resistor meets the landing pad on the PCB. The
best place to make current sense line connection is on the inner
side of the sense resistor footprint. The illustration of the
connection is shown in Figure 123. Most of the current flow is at
the outer edge of the footprint. The current ceases at the point
the sense resistor connects to the landing pad. Assume the
sense resistor connects at the middle of each landing pad, this
leaves the inner half of each landing pad with little current flow.
With little current flow, the inner half of each landing pad is
classified as high impedance and perfect for a sense connection.
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.
CURRENT BEARING TRACE
FIGURE 123. CONNECTING THE SENSE LINES TO A CURRENT SENSE
RESISTOR
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 124 illustrates
the direction the magnetic field flows versus current flow.
B
 _oI
2 r
FIGURE 124. THE CONDUCTOR ON THE LEFT SHOWS THE MAGNETIC
FIELD FLOWING IN A CLOCKWISE DIRECTION FOR A
CURRENT FLOWING INTO THE PAGE. A CURRENT FLOW
OUT OF THE PAGE HAS A COUNTERCLOCKWISE
MAGNETIC FLOW
The equation in Figure 124 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.
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 125 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
manner and the furthest layer from the current bearing trace.
The interference from the current bearing trace will be limited.
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FN8389.5
March 18, 2016