MIC2589_05 Datasheet, PDF (21/29 Pages) Micrel Semiconductor – Single-Channel, Negative High-Voltage Hot Swap Power Controller/Sequencer

Micrel
MIC2589/MIC2595
-48V RTN
(Long Pin)
*D1
SMAT70A
-48V RTN
(Short Pin)
R1
689k
1%
R2
11.8k
1%
R5
47k
C_TIMER
MIC2589-2BM
1 _/PWRGD1
VDD ¹⁴
2 _{PGTIMER /PWRGD3} 13
3 _UV
/PWRGD2 ¹²
C5
100µF
System
Reset
-48V RTN
(Long Pin)
C1
0.47µF
*C3
0.1µF
*M2
R3
12.4k
1%
C_FILTER
2.2µF
C_NLD
0.068µF
4 _OV
5 _CFILTER
6 _CNLD
7 _VEE
DRAIN ¹¹
GATE ¹⁰
SENSE ⁹
N/C ⁸
*D2
ZMM5266B
R_SENSE
0.01
5%
*C2
0.1µF
R4
10
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
C4
0.1µF
-48V RTN
-48V_OUT
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:
R _SENSE(nom)
=
V_TRIP(typ)
I_{HOT_SWAP(n om)}
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 ±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.
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:
( ) ^{IHOT_SWAP(m in)}
=
40mV
1.03 × R_SENSE(nom)
= ^38.8mV
R _SENSE(nom)
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
December 2005
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:
( ) ^{IHOT_SWAP(m ax)}
=
60mV
0.97 × R_SENSE(nom)
= ^61.9mV
R _SENSE(nom)
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:
R _SENSE(nom)
=
40mV
4A
= 10mΩ
where the nearest 1% standard value is 10.0mΩ. At
the other tolerance extremes, I_{HOT_SWAP(MAX)} for the
circuit in question is then simply:
I_{HOT_SWAP(m ax)}
=
61.9mV
10mΩ
= 6.19A
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 I_{HOT_SWAP(max)} = 6.19A and the
resistance
R_SENSE(max) = (1.03)(R_SENSE(nom)) = 10.3mΩ.
Thus, the sense resistor’s maximum power dissipation
is:
P_MAX = (6.19A)² × (10.3mΩ) = 0.395W
21
M9999-120505
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