MIC2085 Datasheet, PDF (22/29 Pages) Micrel Semiconductor – Single Channel Hot Swap Controllers

Micrel, Inc.
MIC2085/2086
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 ±3% over time and temperature for a
resistor with ±1% 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:
( )( ) ^{RSENSE(MAX) =}
40mV
1.03 I_LOAD(CONT)
= ^38.8mV
I_LOAD(CONT)
(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:
( )( ) ^{ILOAD(CONT,MAX) =}
55mV
0.97 R _SENSE(NOM)
= ^56.7mV (12)
R _SENSE(NOM)
As an example, if an output must carry a continuous
6Awithout nuisance trips occurring, Equation 11 yields:
R _SENSE(MAX)
=
38.8mV
6A
= 6.5mΩ
The next lowest standard value is 6.0mW. At the other
set of tolerance extremes for the output in question:
I_{LOAD(CONT,MAX)}
=
56.7mV
6.0mΩ
= 9.45A,
almost 10A. Knowing this final datum, we can determine
the necessary wattage of the sense resistor, using
P = I²R, 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 (5.82mΩ) =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 part’s ability to withstand any
peak currents (transient thermal issues).
May 2006
22
M9999-050406
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