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MIC2590B-2YTQ Datasheet(PDF) 18 Page - Micrel Semiconductor

Part No. MIC2590B-2YTQ
Description  Dual-Slot PCI Hot Plug Controller
Download  23 Pages
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Maker  MICREL [Micrel Semiconductor]
Homepage  http://www.micrel.com
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MIC2590B-2YTQ Datasheet(HTML) 18 Page - Micrel Semiconductor

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Micrel, Inc.
MIC2590B
September 2008
18
M9999-091808
Application Information
Current Sensing
For the three power supplies switched with internal
MOSFETs (+12V, –12V, and VAUX), the MIC2590B
provides all necessary current sensing functions to protect
the IC, the load, and the power supply. For the remaining
four supplies which the part is designed to control, the
high currents at which these supplies typically operate
makes sensing the current inside the MIC2590B
impractical. Therefore, each of these supplies (3VA, 5VA,
3VB, and 5VB) requires an external current sensing
resistor. The VIN connection to the IC from each supply
(e.g., 5VINA) is connected to the positive terminal of the
slot’s current sense amplifier, and the corresponding
SENSE input (in this case, 5VSENSEA) is connected to the
negative terminal of the current sense amplifier.
Sense Resistor Selection
The MIC2590B uses low-value sense resistors to
measure the current flowing through the MOSFET
switches to the loads. These sense resistors are
nominally valued at 50mΩ/ILOAD(CONT). To accommodate
worst-case tolerances for both the sense resistor, (allow
±3% over time and temperature for a resistor with ±1%
initial tolerance) and still supply the maximum required
steady-state load current, a slightly more detailed
calculation must be used.
The current limit threshold voltage (the “trip point”) for the
MIC2590B may be as low as 35mV, which would equate
to a sense resistor value of 35mΩ/ILOAD(CONT). Carrying the
numbers through for the case where the value of the
sense resistor is 3% high, this yields:
()
()
LOAD(CONT)
LOAD(CONT)
SENSE
I
34mΩ
I
1.03
35mΩ
R
=
=
Once the value of RSENSE has been chosen in this
manner, it is good practice to check the maximum
ILOAD(CONT) which the circuit may let through in the case of
tolerance build-up in the opposite direction. Here, the
worst-case maximum is found using a 65mV trip voltage
and a sense resistor which is 3% low in value. The
resulting current is:
()
()
SENSE(NOM)
SENSE(NOM)
MAX)
LOAD(CONT,
R
67mΩ
R
0.97
65mΩ
I
=
=
As an example, if an output must carry a continuous 4.4A
without nuisance trips occurring, RSENSE for that output
should be 34mΩ/4.4A = 7.73mΩ. The nearest standard
value is 7.5mΩ, so a 7.5mΩ ±1% resistor would be a
good choice. At the other set of tolerance extremes,
ILOAD(CONT, MAX) for the output in question is then simply
67mV/7.5mΩ = 8.93A. Knowing this final datum, we can
determine the necessary wattage of the sense resistor,
using P = I
2R. Here I will be I
LOAD(CONT, MAX), and R will be
(0.97)(RSENSE(NOM)). These numbers yield the following:
PMAX = (8.93A)
2 (7.28mΩ) = 0.581W
A 1.0W sense resistor would work well in this application.
Kelvin Sensing
Because of the low values of the sense resistors, special
care must be used to accurately measure the voltage
drop across them. Specifically, the voltage across each
RSENSE must employ Kelvin sensing. This is simply a
means of making sure that any voltage drops in the power
traces connecting to the resistors are not picked up in
addition to the voltages across the sense resistors
themselves. If accuracy must be paid for, it’s worth
keeping.
Figure 9 illustrates how Kelvin sensing is performed. As
can be seen, all the high current in the circuit (let us say,
from +5VINA through RSENSE and then to the drain of the
+5VA output MOSFET) flows directly through the power
PCB traces and RSENSE. The voltage drop resulting across
RSENSE is sampled in such a way that the high currents
through the power traces will not introduce any
extraneous IR drops.
RSENSE
Power Trace
From VCC
Power Trace
To MOSFET Drain
Signal Trace
to MIC2590B VCC
Signal Trace
to MIC2590B VSENSE
Figure 9. Kelvin Sensing Connections for RSENSE
MOSFET Selection
Selecting the proper MOSFET for use as current pass
and switching element for each of the 3V and 5V slots of
the MIC2590B involves four straight forward tasks:
1. Choice of a MOSFET which meets the minimum
voltage requirements.
2. Determination of maximum permissible on-state
resistance [RDS(ON)].
3. Selection of a device to handle the maximum
continuous current (steady-state thermal issues).
4. Verification of the selected part’s ability to
withstand current peaks (transient thermal
issues).
MOSFET Voltage Requirements
The first voltage requirement for each MOSFET is easily
stated: the drain-source breakdown voltage of the
MOSFET must be greater than VIN(MAX) for the slot in
question. For instance, the 5V input may reasonably be
expected to see high-frequency transients as high as
5.5V. Therefore, the drain-source breakdown voltage of
the MOSFET must be at least 6V.


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