icrel, Inc.
MIC2587/MIC2587R
anuary 24, 2013
18
Revision 2.0
Sense Resistor Selection
The sense resistor is nominally valued at:
OM)
HOT_SWAP(N
TRIP(TYP)
SENSE(NOM)
I
V
=
Eq. 11
where:
V
TRIP(TYP)
is the typical (or nominal) circuit breaker
threshold   voltage   (47mV)   and   I
HOT_SWAP(NOM)
is   the
nominal inrush load current level to trip the internal circuit
breaker.
To accommodate worst-case tolerances in the sense
resistor (for a ?% initial tolerance, allow ?% tolerance
for variations over time and temperature) and circuit
breaker threshold voltages, a slightly more detailed
calculation must be used to determine the minimum and
maximum hot swap load currents.
As   the   MIC2587/MIC2587R's   minimum   current-limit
threshold voltage is 39mV, the minimum hot swap load
current is determined where the sense resistor is 3%
high:
(
)
SENSE(NOM)
SENSE(NOM)
IN)
HOT_SWAP(M
R
37.9mV
R
1.03
39mV
=
?/DIV>
=
Keep in mind that the minimum hot swap load current
should be greater than the application circuit's upper
steady-state load current boundary. Once the lower value
of R
SENSE
has been calculated, it is good practice to check
the maximum hot swap load current (I
HOT_SWAP(MAX)
) which
the circuit may let pass in the case of tolerance build-up
in the opposite direction. Here, the worst-case maximum
is found using a V
TRIP(MAX)
threshold of 55mV and a sense
resistor 3% low in value:
(
)
SENSE NOM
SENSE NOM
AX)
HOT_SWAP(M
R
56.7mV
R
0.97
55mV
=
?/DIV>
=
In this case, the application circuit must be sturdy enough
to operate over a ~1.5-to-1 range in hot swap load
currents. For example, if an MIC2587 circuit must pass a
minimum hot swap load current of 4A without nuisance
trips, R
SENSE
should be set to:
&
=
=
9.75m
4A
39mV
SENSE(NOM)
where the nearest 1% standard value is 9.76m&. At the
other tolerance extremes, I
HOT_SWAP(MAX)
for the circuit in
question is then simply:
5.8A
.7 m
56.7mV
AX)
HOT_SWAP(M
=
&
=
With a knowledge of the application circuit's maximum
hot swap load current, the power dissipation rating of the
sense resistor can be determined using P = I
2
?R. Here,
The current is I
HOT_SWAP(MAX)
= 5.8A and the resistance
R
SENSE(MIN)
= (0.97)(R
SENSE(NOM)
) = 9.47m&. Thus, the
sense resistor's maximum power dissipation is:
   P
MAX
= (5.8A)
2
?(9.47m&) = 0.319W
A 0.5W sense resistor is a good choice in this application.
When the MIC2587/MIC2587Rs foldback current limiting
circuit is engaged in the above example, the current limit
would nominally fold back to 1.23A when the output is
shorted to ground.
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