MIC2590B Micrel Semiconductor, MIC2590B Datasheet - Page 18

• Manufacturer Part Number: MIC2590B
• Description: Dual-Slot PCI Hot Plug Controller
• Manufacturer: Micrel Semiconductor

MIC2590B
Application Information
Current Sensing
For the three power supplies switched with internal MOSFETs
(+12V, –12V, and V
sary 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
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/I
for both the sense resistor, (allow ±3% over time and tem-
perature 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
numbers through for the case where the value of the sense
resistor is 3% high, this yields:
Once the value of R
it is good practice to check the maximum I
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:
As an example, if an output must carry a continuous 4.4A
without nuisance trips occurring, R
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
I
67mV/7.5mΩ = 8.93A. Knowing this final datum, we can
determine the necessary wattage of the sense resistor, using
P = I
(0.97)(R
A 1.0W sense resistor would work well in this application.
MIC2590B
I
LOAD(CONT, MAX)
LOAD(CONT, MAX)
the
2
P
R
R. Here I will be I
MAX
LOAD(CONT)
SENSE
SENSE(NOM)
= (8.93A)
other
=
=
(
1.03 I
(0.97)(R
for the output in question is then simply
. To accommodate worst-case tolerances
AUX
SENSE
). These numbers yield the following:
35m
)
2
(
set
(7.28mΩ) = 0.581W
65mV
LOAD(CONT)
), the MIC2590B provides all neces-
IN
SENSE(NOM)
Ω
LOAD(CONT, MAX)
has been chosen in this manner,
connection to the IC from each
of
)
LOAD(CONT)
=
tolerance
)
I
LOAD(CONT)
=
SENSE
34m
R
SENSE(NOM)
67mV
LOAD(CONT)
Ω
, and R will be
for that output
. Carrying the
extremes,
which
18
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
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 R
output MOSFET) flows directly through the power PCB
traces and R
is sampled in such a way that the high currents through the
power traces will not introduce any extraneous IR drops.
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 straightforward tasks:
1. Choice of a MOSFET which meets the minimum
2. Determination of maximum permissible on-state
3. Selection of a device to handle the maximum continu-
4. Verification of the selected part’s ability to withstand
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
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.
The second breakdown voltage criteria which must be met is
a bit subtler than simple drain-source breakdown voltage, but
is not hard to meet. Low-voltage MOSFETs generally have
low breakdown voltage ratings from gate to source as well. In
MIC2590B applications, the gates of the external MOSFETs
are driven from the +12V input to the IC. That supply may well
be at 12V + (5% x 12V) = 12.6V. At the same time, if the output
of the MOSFET (its source) is suddenly shorted to ground,
the gate-source voltage will go to (12.6V – 0V) = 12.6V. This
Figure 9. Kelvin Sensing Connections for R
voltage requirements.
resistance [R
ous current (steady-state thermal issues).
current peaks (transient thermal issues).
Power Trace
From V
CC
SENSE
to MIC2590B V
Signal Trace
D-S(ON)
. The voltage drop resulting across R
SENSE
IN(MAX)
CC
].
and then to the drain of the +5VA
R
SENSE
for the slot in question. For
Signal Trace
to MIC2590B V
To MOSFET Drain
SENSE
Power Trace
August 2002
SENSE
SENSE
Micrel
SENSE
must