MIC2593
Micrel
Kelvin Sensing
Application Information
Because of the low values of the sense resistors, special
attentiontothelayoutmustbeusedinorderfortheMIC2593's
circuit breaker function to operate properly. Specifically, the
use of a 4-wire Kelvin connection to measure the voltage
Current Sensing
For the three power supplies switched with internal MOS-
FETs (+12V, –12V, and V
), the MIC2593 provides all
AUX
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
across R
is highly recommended. Kelvin sensing is
SENSE
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.
current sensing resistor. The V connection to the IC from
IN
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
Figure 12 illustrates how Kelvin sensing is performed. All the
highcurrentinthecircuit(fromthe5VsupplythroughR
SENSE
50mV/I
. To accommodate worst-case tolerances
andthentothedrainofthe5V(SlotA)outputMOSFET)flows
LOAD(CONT)
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.
directly through the power PCB traces and R
. The
SENSE
voltage drop resulting across R
is sampled in such a
SENSE
way that the high currents through the power traces will not
introduce any extraneous IR drops.
The current limit threshold voltage (i.e., the “trip point”) for the
MIC2593 may be as low as 35mV, which would equate to a
Power Trace
From 5VIN
Power Trace
To MOSFET Drain
RSENSE
sense resistor value of 35mV/I
. Carrying the
LOAD(CONT)
numbers through for the case where the value of the sense
resistor is 3% high yields this:
Signal Trace
to MIC2593 VIN
22pF
Signal Trace
to MIC2593 VSENSE
35mV
34mV
RSENSE
=
=
ILOAD(CONT)
1.03 I
(
)(
)
LOAD(CONT)
Once the value of R
has been chosen in this manner,
SENSE
it is good practice to check the maximum I
which
LOAD(CONT)
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:
MIC2593
Figure 12. Kelvin Sense Connections for R
(Applicable to 5V[A/B] and 3V[A/B])
SENSE
65mV
67mV
ILOAD(CONT, MAX)
=
=
MOSFET Selection
(0.97)(RSENSE(NOM)
)
RSENSE(NOM)
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:
As an example, if an output must carry a continuous 4.5A
without nuisance trips occurring, R for that output
shouldbe34mΩ/4.5A=7.55mΩ. Theneareststandardvalue
is 7.5mΩ, so a 7.5mΩ ±1% resistor would be a good choice.
At the other set of tolerance extremes for the output in
SENSE
1. Choice of a MOSFET which meets the minimum voltage
requirements.
2. Selection of a device to handle the maximum continuous
current (steady-state thermal issues).
question, I
= 67mV/7.5mΩ = 8.93A. Knowing
LOAD(CONT,max)
this final datum, we can determine the necessary wattage of
the sense resistor, using P = I R, where I is I
and R is (0.97)(R
following:
2
,
LOAD(CONT, MAX)
3. Verification that the selected part can withstand any
current peaks (transient thermal issues).
). These numbers yield the
SENSE(NOM)
MOSFET Voltage Requirements
2
P
= (8.93A) (7.28mΩ) = 0.581W
MAX
The first voltage requirement for each MOSFET is easily
stated: the drain-source breakdown voltage of the MOSFET
A 1W sense resistor would work well in this application.
April 2004
21
M9999-042204
STMICROELECTRONICS [ ST ]
