icrel, Inc.
MIC2587/MIC2587R
anuary 24, 2013
19
Revision 2.0
PCB Layout Recommendations
4-Wire Kelvin Sensing
Because of the low value typically required for the sense
resistor, special care must be used to accurately
measure the voltage drop across it. Specifically, the
measurement technique across R
SENSE
must employ 4-
wire Kelvin sensing. This is simply a means of ensuring
that any voltage drops in the power traces connected to
the resistors are not picked up by the signal conductors
measuring the voltages across the sense resistors.
Figure 6 illustrates how to implement 4-wire Kelvin
sensing. As the figure shows, all the high current in the
circuit (from V
CC
through R
SENSE
and then to the drain of
the N-channel power MOSFET) flows directly through the
power PCB traces and through R
SENSE
.
Figure 6. 4-Wire Kelvin Sense Connections for R
SENSE
The voltage drop across R
SENSE
is sampled in such a way
that the high currents through the power traces will not
introduce significant parasitic voltage drops in the sense
leads. It is recommended to connect the hot swap
controller's sense leads directly to the sense resistor's
metalized contact pads. The Kelvin sense signal traces
should be symmetrical with equal length and width, kept
as short as possible and isolated from any noisy signals
and planes.
In most applications, the use of a capacitor from the
TIMER pin to ground will effectively eliminate nuisance
tripping due to noise and/or transient overcurrent spikes.
If the circuit breaker trips regularly due to a system
environment that is vulnerable to noise being injected
onto the Kelvin sense connections, the example circuit
shown in Figure 7 can be implemented to combat such
noisy environments. This circuit implements a 1.6 MHz
low-pass filter to attenuate higher frequency disturbances
on the current sensing circuitry. However, individual
system analysis should be used to determine if filtering is
necessary and to select the appropriate cutoff frequency
for each specific application.
Other Layout Considerations
Figure 8 is a recommended PCB layout diagram for the
MIC2587-2YM. Many hot swap applications will require
load currents of several amperes. Therefore, the power
(V
CC
and Return) trace widths (W) need to be wide
enough to allow the current to flow while the rise in
temperature for a given copper plate (e.g., 1oz. or 2oz.) is
kept to a maximum of 10癈 to 25癈. Also, these traces
should be as short as possible in order to minimize the IR
drops between the input and the load. The feedback
network resistor values in Figure 8 are selected for a
+24V application. The resistors for the feedback (FB) and
ON pin networks should be placed close to the controller
and the associated traces should be as short as possible
to improve the circuits noise immunity. The input
clamping diode (D1) is referenced in the Typical
Application Circuit on Page 1. If possible, use high-
frequency PCB layout techniques around the GATE
circuitry (shown in the typical application circuit) and use
a dummy resistor (e.g., R3 = 0&) during the prototype
phase. If R3 is needed to eliminate high-frequency
oscillations, common values for R3 range between 4.7&
to 20& for various power MOSFETs. Finally, the use of
plated-through vias will be needed to make circuit
connection to the power and ground planes when utilizing
multi-layer PCBs.
Figure 7. Current-Limit Sense Filter for Noisy Systems
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