MIC2593 Micrel Semiconductor, MIC2593 Datasheet - Page 21

• Manufacturer Part Number: MIC2593
• Description: Dual-Slot PCI Hot Plug Controller
• Manufacturer: Micrel Semiconductor

Application Information
Current Sensing
For the three power supplies switched with internal MOS-
FETs (+12V, –12V, and V
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 make sensing the
current inside the MIC2593 impractical. Therefore, each of
these supplies, 3V[A/B] and 5V[A/B], requires an external
current sensing resistor. The V
each supply (e.g., 5VINA) is connected to the positive termi-
nal of the slot’s current sense amplifier, and the correspond-
ing SENSE input (in this case, 5VSENSEA) is connected to
the negative terminal of the current sense amplifier.
Sense Resistor Selection
The MIC2593 uses low-value sense resistors to measure the
current flowing through the MOSFET switches to the loads.
These sense resistors are nominally valued at
50mV/I
for 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 (i.e., the “trip point”) for the
MIC2593 may be as low as 35mV, which would equate to a
sense resistor value of 35mV/I
numbers through for the case where the value of the sense
resistor is 3% high yields this:
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 cur-
rent 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 4.5A
without nuisance trips occurring, R
should be 34m /4.5A = 7.55m . The nearest standard value
is 7.5m , so a 7.5m
At the other set of tolerance extremes for the output in
question, I
this final datum, we can determine the necessary wattage of
the sense resistor, using P = I
and R is (0.97)(R
following:
A 1W sense resistor would work well in this application.
April 2004
I
MIC2593
LOAD(CONT, MAX)
P
R
LOAD(CONT)
MAX
SENSE
LOAD(CONT,max)
= (8.93A)
1 03 I
(0.97)(R
. To accommodate worst-case tolerances
SENSE(NOM)
SENSE
35mV
2
LOAD CONT
(7.28m ) = 0.581W
65mV
1% resistor would be a good choice.
SENSE(NOM)
= 67mV/7.5m = 8.93A. Knowing
has been chosen in this manner,
AUX
2
R, where I is I
). These numbers yield the
), the MIC2593 provides all
IN
LOAD(CONT)
connection to the IC from
I
)
LOAD CONT
SENSE
34mV
R
SENSE(NOM)
67mV
LOAD(CONT)
LOAD(CONT, MAX)
for that output
. Carrying the
which
,
21
Kelvin Sensing
Because of the low values of the sense resistors, special
attention to the layout must be used in order for the MIC2593's
circuit breaker function to operate properly. Specifically, the
use of a 4-wire Kelvin connection to measure the voltage
across R
simply a means of making sure that any voltage drops in the
power traces connecting to the resistors does not get picked
up by the traces themselves. The Kelvin connections should
be isolated from all other signal traces to avoid introducing
noise onto these sensitive nodes. Additionally, a high-fre-
quency noise filter across the sense inputs is highly recom-
mended to avoid nuisance tripping of the (overcurrent) circuit
breaker on the opposite slot to the slot that incurred an
overcurrent event. Due to the variation of each system's
susceptibility to noise, the exact value of this filter is experi-
mentally determined. A value between 10pF to 100pF is a
good starting point.
Figure 12 illustrates how Kelvin sensing is performed. All the
high current in the circuit (from the 5V supply through R
and then to the drain of the 5V (Slot A) output MOSFET) flows
directly through the power PCB traces and R
voltage drop resulting across R
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 a current pass and
switching element for each of the 3V and 5V slots of the
MIC2593 primarily involves three straightforward tasks:
1. Choice of a MOSFET which meets the minimum voltage
2. Selection of a device to handle the maximum continuous
3. Verification that the selected part can withstand any
MOSFET Voltage Requirements
The first voltage requirement for each MOSFET is easily
stated: the drain-source breakdown voltage of the MOSFET
Figure 12. Kelvin Sense Connections for R
requirements.
current (steady-state thermal issues).
current peaks (transient thermal issues).
Power Trace
From 5V
SENSE
(Applicable to 5V[A/B] and 3V[A/B])
IN
to MIC2593 V
Signal Trace
is highly recommended. Kelvin sensing is
IN
MIC2593
R
22pF
SENSE
SENSE
Signal Trace
to MIC2593 V
is sampled in such a
To MOSFET Drain
SENSE
Power Trace
M9999-042204
SENSE
SENSE
SENSE
Micrel
. The