Micrel, Inc.
MIC2085/2086
May 2006
22
M9999-050406
(408) 955-1690
Figure 10. A 50W InfiniBand" Application
Sense Resistor Selection
The MIC2085 and MIC2086 use a low-value sense
resistor to measure the current flowing through the
MOSFET switch (and therefore the load). This sense
resistor is nominally valued at 48mV/I
LOAD(CONT)
. To
accommodate worst-case tolerances for both the sense
resistor (allow ?% over time and temperature for a
resistor with ?% initial tolerance) and still supply the
maximum required steady-state load current, a slightly
more detailed calculation must be used. The current limit
threshold voltage (the trip point ) for theMIC2085/86
may be as low as 40mV, which would equate to a sense
resistor value of 40mV/I
LOAD(CONT)
. Carrying the numbers
through for the case where the value of the sense
resistor is 3% high yields:
( )(
)
LOAD(CONT)
LOAD(CONT)
SENSE(MAX)
I
38.8mV
I
1.03
40mV
R
=
=
(11)
Once the value of R
SENSE
has been chosen in this
manner, it is good practice to check the maximum
I
LOAD(CONT)
which the circuit may let through in the case of
tolerance build-up in the opposite direction. Here, the
worst-case maximum cur-rent is found using a 55mV trip
voltage and a sense resistor that is 3% low in value. The
resulting equation is:
( )(
)
SENSE(NOM)
SENSE(NOM)
MAX)
LOAD(CONT,
R
56.7mV
R
0.97
55mV
I
=
=
(12)
As an example, if an output must carry a continuous
6Awithout nuisance trips occurring, Equation 11 yields:
6.5m&
6A
38.8mV
R
SENSE(MAX)
=
=
The next lowest standard value is 6.0mW. At the other
set of tolerance extremes for the output in question:
9.45A,
6.0m&
56.7mV
I
MAX)
LOAD(CONT,
=
=
almost 10A. Knowing this final datum, we can determine
the necessary wattage of the sense resistor, using
P = I
2
R, where I will be I
LOAD(CONT, MAX)
, and R will be
(0.97)(R
SENSE(NOM)
). These numbers yield the following:
PMAX = (10A)
2
(5.82m&) =0.582W.
In this example, a 1W sense resistor is sufficient.
MOSFET Selection
Selecting the proper external MOSFET for use with
theMIC2085/86 involves three straightforward tasks:
" Choice of a MOSFET which meets minimum
voltage requirements.
" Selection of a device to handle the maximum
continuous current (steady-state thermal
issues).
" Verify the selected parts ability to withstand any
peak currents (transient thermal issues).
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