Micrel
MIC2589/MIC2595
December 2005
21
M9999-120505
(408) 955-1690
1
14
13
5
4
3
2
/PWRGD1
CFILTER
CNLD
VEE
/PWRGD3
PGTIMER
GATE
/PWRGD2
SENSE
VDD
-48V
OUT
-48V RTN
C
TIMER
R2
11.8k
1%
C
FILTER
2.2礔
C
NLD
0.068礔
*C2
0.1礔
R
SENSE
0.01
5%
R4
10
M1
SUM110N10-09
C1
0.47礔
*D1
SMAT70A
-48V RTN
(Long Pin)
-48V RTN
(Long Pin)
-48V RTN
(Short Pin)
System
Reset
UV
MIC2589-2BM
DRAIN
OV
8
7
6
12
10
11
9
N/C
R1
689k
1%
*C3
0.1礔
*M2
*D2
ZMM5266B
C4
0.1礔
C5
100礔
R5
47k
R3
12.4k
1%
*Optional components (See Functional Description and Applications Information for more details)
M2 is an SOT-323, B8S138W or equivalent
D2 is a 68V, 500mW Zener diode
Figure 7. Optional Components for Added Performance/Protection
The circuit in Figure 7 consisting of M2, R5, and a
digital control signal, can be used to reset the
controller after the GATE (and output) turns off. Once
the output has been latched off, applying a low-high-
low pulse on the GATE of M2 via the System Enable
control can toggle the UV pin. System Enable is a
user-defined signal referenced to V
EE
.
Sense Resistor Selection
The sense resistor is nominally valued at:
om)
HOT_SWAP(n
TRIP(typ)
SENSE(nom)
I
V
R
=
where V
TRIP(TYP)
is the typical (or nominal) circuit
breaker threshold voltage (50mV) and I
HOT_SWAP(NOM)
is
the nominal load current level necessary 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.
As the MIC2589s minimum current limit threshold
voltage is 40mV, the minimum hot swap load current
is determined where the sense resistor is 3% high:
(
)
SENSE(nom)
SENSE(nom)
in)
HOT_SWAP(m
R
38.8mV
R
1.03
40mV
I
=
?/DIV>
=
Keep in mind that the minimum hot swap load current
should be greater than the application circuits 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)
) that 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 60mV and a sense resistor 3% low in
value:
(
)
SENSE(nom)
SENSE(nom)
ax)
HOT_SWAP(m
R
61.9mV
R
0.97
60mV
I
=
?/DIV>
=
In this case, the application circuit must be sturdy
enough to operate up to approximately 1.5x the
steady-state hot swap load current. For example, if an
MIC2589 circuit must pass a minimum hot swap load
current of 4A without nuisance trips, R
SENSE
should be
set to:
10m&
4A
40mV
R
SENSE(nom)
=
=
where the nearest 1% standard value is 10.0m&. At
the other tolerance extremes, I
HOT_SWAP(MAX)
for the
circuit in question is then simply:
6.19A
10m&
61.9mV
I
ax)
HOT_SWAP(m
=
=
With a knowledge of the application circuits 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)
= 6.19A and the
resistance
R
SENSE(max)
= (1.03)(R
SENSE(nom)
) = 10.3m&.
Thus, the sense resistors maximum power dissipation
is:
P
MAX
= (6.19A)
2
?(10.3m&) = 0.395W
发布紧急采购，3分钟左右您将得到回复。
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