Micrel
MIC2589/MIC2595
the power (VEE 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°C to 25°C. The
return (or power ground) trace should be the same
width as the positive voltage power traces (input/load)
and isolated from any ground and signal planes so
that the controller’s power is common mode. Also,
these traces should be as short as possible in order to
minimize the IR drops between the input and the load.
PCB Layout Considerations
4-Wire Kelvin Sensing
Because of the low value typically required for the
sense resistor, special care must be used to measure
accurately the voltage drop across it. Specifically, the
measurement technique across each RSENSE must
employ 4-wire Kelvin sensing. This is simply a means
of making sure that any voltage drops in the power
traces connecting to the resistors are not picked up by
the signal conductors measuring the voltages across
the sense resistors.
Finally, the use of plated-through vias will be
necessary to make circuit connections to the power,
ground and signal planes of multi-layer PCBs.
Figure 9 illustrates how to implement 4-wire Kelvin
sensing. As the figure shows, all the high current in
the circuit (from VEE through RSENSE, and then to the
source of the output MOSFET) flows directly through
the power PCB traces and RSENSE. The voltage drop
resulting across RSENSE is sampled in such a way that
the high currents through the power traces will not
introduce any 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.
RSENSE metalized
contact pads
Power Trace
From VEE
Power Trace
To MOSFET Source
RSENSE
PCB Track Width:
0.03" per Ampere
using 1oz Cu
Signal Trace
to MIC2589/MIC2595 VEE Pin
Signal Trace
to MIC2589/MIC2595 SENSE Pin
Note: Each SENSE lead trace shall be
balanced for best performance with equal
length/equal aspect ratio.
Other Layout Considerations
Figure 9. 4-Wire Kelvin Sense Connections for
RSENSE
Figure 10 is a suggested PCB layout diagram for the
MIC2589/MIC2595. Many hot swap applications will
require load currents of several amperes. Therefore,
25
M9999-120505
(408) 955-1690
December 2005
MICREL [ MICREL SEMICONDUCTOR ]
