ISL28022_15 Datasheet, PDF (28/32 Pages) Intersil Corporation – Precision Digital Power Monitor

ISL28022
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 45. 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 the each landing pad, this leaves
the inner half of the 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 BEARING TRACE
LANDING PAD
SENSE TRACE
B ^{ oI}
2r
FIGURE 46. THE CONDUCTOR ON THE LEFT SHOWS THE MAGNETIC
FIELD FLOWING IN A CLOCKWISE DIRECTION FOR
CURRENTS FLOWING INTO THE PAGE. A CURRENT
FLOW OUT OF THE PAGE HAS A COUNTER CLOCKWISE
MAGNETIC FLOW
The equation in Figure 46 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
4*10^-7
conductor,
H/m.
r.
The
permeability
of
air,
µ_o,
is
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 47 illustrates a layout that is less
sensitive to magnetic field interference.
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
interference from the current bearing trace will be limited.
TO SENSE CIRCUITRY
LANDING PAD
SENSE TRACE
SENSE
RESISTOR
CURRENT BEARING TRACE
FIGURE 45. 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 46 illustrates the
direction the magnetic field flows versus current flow.
B _to  B _from
B _to  B _from
B _to  B _from
FIGURE 47. CLOSELY ROUTED TRACES THAT CONNECT TO THE
SENSE RESISTOR REDUCES THE MAGNETIC
INTERFERENCE SOURCED FROM THE CURRENT
FLOWING THROUGH THE TRACES
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FN8386.7
October 2, 2015