icrel, Inc.
MIC2586/MIC2586R
ecember 2012
18
M9999-122012
In solving for V
OUT(GOOD)
, substitute V
FBH
for V
FBL
in
Equation 16.
Sense Resistor Selection
The sense resistor is nominally valued at:
OM)
HOT_SWAP(N
TRIP(TYP)
SENSE
I
V
R
=
Eq. 19
where V
TRIP(TYP)
is the 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 worse-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.
The MIC2586/MIC2586R has a minimum current limit
threshold voltage of 39mV, thus 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>
=
Eq. 20
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
worse-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>
=
Eq. 21
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 MIC2586 circuit must
pass a minimum hot swap load current of 4A without
nuisance trips, R
SENSE
should be set to:
9.75m?/DIV>
4A
39mV
R
SENSE(NOM)
=
=
Eq. 22
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?/DIV>
56.7mV
AX)
HOT_SWAP(M
=
=
Eq. 23
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:
(    )   (
)   0.319W
9.47m?/DIV>
2
5.8A
P
MAX
=
?/DIV>
=
Eq. 24
A 0.5W sense resistor is a good choice in this
application.
When the MIC2586/MIC2586R's 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.
PCB Layout Considerations
4-Wire Kelvin Sensing
Because of the low value typically required for the sense
resistor, special care must be used to accurately
measure the voltage drop across it. Specifically, the
measurement technique across R
SENSE
must employ 4-
wire Kelvin sensing. This is simply a means of ensuring
that any voltage drops in the power traces connected to
the resistors are not picked up by the signal conductors
measuring the voltages across the sense resistors.
Figure 8 illustrates how to implement 4-wire Kelvin
sensing. As the figure shows, all the high current in the
circuit (from V
CC
through R
SENSE
and then to the drain of
the N-channel power MOSFET) flows directly through
the power PCB traces and through R
SENSE
.
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MIC2587-1BM TR |
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MIC2587-1YM |
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