MIC2590B_08 Datasheet, PDF (18/23 Pages) Micrel Semiconductor – Dual-Slot PCI Hot Plug Controller

Micrel, Inc.
Application Information
Current Sensing
For the three power supplies switched with internal
MOSFETs (+12V, –12V, and V_AUX), 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 V_IN connection to the IC from each supply
(e.g., 5V_INA) is connected to the positive terminal of the
slot’s current sense amplifier, and the corresponding
SENSE input (in this case, 5V_SENSEA) 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/I_LOAD(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/I_LOAD(CONT). Carrying the
numbers through for the case where the value of the
sense resistor is 3% high, this yields:
( )( ) ^RSENSE
=
35mΩ
1.03 I_LOAD(CONT)
= ^34mΩ
I_LOAD(CONT)
Once the value of R_SENSE has been chosen in this
manner, it is good practice to check the maximum
I_LOAD(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:
( )( ) ^{ILOAD(CONT,MAX) =}
65mΩ
0.97 R _SENSE(NOM)
= ^67mΩ
R _SENSE(NOM)
As an example, if an output must carry a continuous 4.4A
without nuisance trips occurring, R_SENSE 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,
I_{LOAD(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 = I²R. Here I will be I_{LOAD(CONT, MAX)}, and R will be
(0.97)(R_SENSE(NOM)). These numbers yield the following:
September 2008
MIC2590B
P_MAX = (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
R_SENSE 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 +5V_INA through R_SENSE and then to the drain of the
+5VA output MOSFET) flows directly through the power
PCB traces and R_SENSE. The voltage drop resulting across
R_SENSE is sampled in such a way that the high currents
through the power traces will not introduce any
extraneous IR drops.
Power Trace
From V_CC
R_SENSE
Power Trace
To MOSFET Drain
Signal Trace
to MIC2590B V_CC
Signal Trace
to MIC2590B V_SENSE
Figure 9. Kelvin Sensing Connections for R_SENSE
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 [R_DS(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 V_IN(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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M9999-091808