MIC2085 Datasheet, PDF (26/29 Pages) Micrel Semiconductor – Single Channel Hot Swap Controllers

Micrel, Inc.
PCB Layout Considerations
Because of the low values of the sense resistors used
with theMIC2085/86 controllers, special attention to the
layout must be used in order for the device’s circuit
breaker function to operate properly. Specifically, the
use of a 4-wire Kelvin connection to measure the voltage
across R_SENSE is highly recommended. Kelvin sensing is
simply a means of making sure that any voltage drops in
the power traces connecting to the resistors does not get
picked up by the traces themselves. Additionally, these
Kelvin connections should be isolated from all other
signal traces to avoid introducing noise onto these
sensitive nodes. Figure 13 illustrates a recommended,
multi-layer layout for the R_SENSE, Power MOSFET,
timer(s), overvoltage and feedback network connections.
The feed-back and overvoltage resistive networks are
selected for a12V application (from Figure 5). Many hot
MIC2085/2086
swap applications will require load currents of several
amperes. Therefore, the power (VCC 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., 1 oz. or 2 oz.) is kept to a maximum of 10°C
~ 25°C. Also, these traces should be as short as
possible in order to minimize the IR drops between the
input and the load. For a starting point, there are many
trace width calculation tools available on the web such
as the following link:
http://www.aracnet.com/cgi-usr/gpatrick/trace.pl
Finally, plated-through vias are utilized to make circuit
connections to the power and ground planes. The trace
connections with indicated vias should follow the
example shown for the GND pin connection in Figure 13.
May 2006
Figure 13. Recommended PCB Layout for Sense Resistor, Power MOSFET,
and Feedback/Overvoltage Network
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M9999-050406
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