ISL28023 Datasheet, PDF (51/55 Pages) Intersil Corporation – Bidirectional current sensing

ISL28023
The relationship between the inductor load current (I_L) and the
voltage across the capacitor simplifies if the following
component selection holds true;
L
R _dcr
C _senR _sen
(EQ. 26)
If Equation 26 holds true, the numerator and denominator of the
fraction in Equation 25 cancels reducing the voltage across the
capacitor to the equation represented in Equation 27.
V _c R _dcri _L
(EQ. 27)
Most inductor datasheets will specify the average value of the
R_dcr for the inductor. R_dcr values are usually sub 1mΩ with a
tolerance averaging 8%. Common chip capacitor tolerances
average to 10%.
Inductors are constructed out of metal. Metal has a high
temperature coefficient. The temperature drift of the inductor
value could cause the DCR circuit to be untuned. An untuned
circuit results in inaccurate current measurements along with a
chop signal bleeding into the measurement. To counter the
temperature variance, a temperature sensor may be incorporated
into the design to track the change in component values.
A DCR circuit is good for gross current measurements. As
discussed, inductors and capacitors have high tolerances and are
temperature dependent which will result in less than accurate
current measurements.
In Figure 119, there is a resistor in series with the ISL28023
negative shunt terminal, VINM, with the value of R_sen + R_dcr. The
resistor’s purpose is to counter the effects of the bias current
from creating a voltage offset at the input of the ADC.
Layout
The layout of a current measuring system is equally important as
choosing the correct sense resistor and the correct analog
converter. Poor layout techniques can result in severed traces,
signal path oscillations, magnetic contamination, which all
contribute to poor system performance.
TRACE WIDTH
Matching the current carrying density of a copper trace with the
maximum current that will pass through is critical in the
performance of the system. Neglecting the current carrying
capability of a trace will result in a large temperature rise in the
trace, and the loss in system efficiency due to the increase in
resistance of the copper trace. In extreme cases, the copper
trace could be severed because the trace could not pass the
current. The current carrying capability of a trace is calculated
using Equation 28.
1
Trace _width

Imax
0.725



kT^0.44


Trace _Thickness
(EQ. 28)
I_max is the largest current expected to pass through the trace. T
is the allowable temperature rise in Celsius when the maximum
current passes through the trace. Trace_Thickness is the thickness
of the trace specified to the PCB fabricator in mils. A typical
thickness for general current carrying applications (<100mA) is
0.5oz copper or 0.7mils. For larger currents, the trace thickness
should be greater than 1.0oz or 1.4mils. A balance between
thickness, width and cost needs to be achieved for each design.
The coefficient k in Equation 28 changes depending on the trace
location. For external traces, the value of k equals 0.048 while
for internal traces the value of k reduces to 0.024. The k values
and Equation 28 are stated per the ANSI IPC-2221(A) standards.
TRACE ROUTING
It is always advised to make the distance between voltage
source, sense resistor and load as close as possible. The longer
the trace length between components will result in voltage drops
between components. The additional resistance will reduce the
efficiency of a system.
The bulk resistance, , of copper is 0.67µΩ/in or 1.7µΩ/cm at
+25°C. The resistance of trace can be calculated from
Equation 29.
R _trace
Trace _length

Trace _widthTrace _thickness
(EQ. 29)
Figure 120 illustrates each dimension of a trace.
TRACE
THICKNESS
TRACE
WIDTH
TLREANCGETH
FIGURE 120. ILLUSTRATION OF THE TRACE DIMENSIONS FOR A STRIP
LINE TRACE
For example, assume a trace has 2oz of copper or 2.8mil
thickness, a width of 100mil and a length of 0.5in. Using
Equation 29, the resistance of the trace is approximately 2mΩ.
Assume 1A of current is passing through the trace. A 2mV
voltage drop would result from trace routing.
Current flowing through a conductor will take the path of least
resistance. When routing a trace, avoid orthogonal connections
for current bearing traces.
FIGURE 121. AVOID ROUTING ORTHOGONAL CONNECTIONS FOR
TRACES THAT HAVE HIGH CURRENT FLOWS
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FN8389.5
March 18, 2016