Micrel
MIC2589/MIC2595
-48V RTN
(Long Pin)
-48V RTN
R5
47k
MIC2589-2BM
*D1
SMAT70A
1
2
3
4
5
6
7
14
13
12
11
10
9
/PWRGD1
VDD
/PWRGD3
/PWRGD2
DRAIN
C5
100µF
C4
0.1µF
R1
689k
1%
R2
11.8k
1%
CTIMER
PGTIMER
UV
-48V RTN
(Short Pin)
OV
CFILTER
2.2µF
C1
0.47µF
CFILTER
CNLD
VEE
GATE
*C3
0.1µF
System
Reset
*M2
SENSE
N/C
CNLD
0.068µF
R3
12.4k
1%
*C2
0.1µF
8
*D2
ZMM5266B
R4
10
RSENSE
0.01
5%
-48V RTN
(Long Pin)
-48VOUT
M1
SUM110N10-09
*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 VEE.
value of RSENSE has been calculated, it is good
practice to check the maximum hot swap load current
(IHOT_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 VTRIP(MAX)
threshold of 60mV and a sense resistor 3% low in
value:
60mV
61.9mV
Sense Resistor Selection
IHOT_SWAP(m ax)
=
=
(
0.97 × RSENSE(nom)
)
RSENSE(nom)
The sense resistor is nominally valued at:
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, RSENSE should be
set to:
V
TRIP(typ)
HOT_SWAP(nom)
R
=
SENSE(nom)
I
where VTRIP(TYP) is the typical (or nominal) circuit
breaker threshold voltage (50mV) and IHOT_SWAP(NOM) is
the nominal load current level necessary to trip the
internal circuit breaker.
40mV
R
=
= 10mꢀ
SENSE(nom)
4A
To accommodate worse-case tolerances in the sense
resistor (for a ±1% initial tolerance, allow ±3%
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.
where the nearest 1% standard value is 10.0mꢀ. At
the other tolerance extremes, IHOT_SWAP(MAX) for the
circuit in question is then simply:
61.9mV
IHOT_SWAP(m ax)
=
= 6.19A
10mꢀ
As the MIC2589’s minimum current limit threshold
voltage is 40mV, the minimum hot swap load current
is determined where the sense resistor is 3% high:
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 = I2 × R.
Here, the current is IHOT_SWAP(max) = 6.19A and the
resistance
40mV
38.8mV
IHOT_SWAP(min)
=
=
(
1.03 × RSENSE(nom)
)
RSENSE(nom)
R
SENSE(max) = (1.03)(RSENSE(nom)) = 10.3mꢀ.
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
Thus, the sense resistor’s maximum power dissipation
is:
P
MAX = (6.19A)2 × (10.3mꢀ) = 0.395W
21
M9999-120505
(408) 955-1690
December 2005
MICREL [ MICREL SEMICONDUCTOR ]
