Micrel, Inc. MIC2593
September 2008
21
M9999-092208
Application Information
Current Sensing
For the three power supplies switched with internal MOS-
FETs (+12V, –12V, and VAUX), the MIC2593 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 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 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 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
LOAD(CONT)
. To accommodate worst-case tolerances
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
LOAD(CONT)
.
Carrying the numbers through for the case where the
value of the sense resistor is 3% high yields this:
()
()
LOAD(CONT)LOAD(CONT)
SENSE
I
34mV
I1.03
35mV
R ==
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 current 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
67mV
R0.97
65mV
I ==
As an example, if an output must carry a continuous 4.5A
without nuisance trips occurring, R
SENSE
for that output
should be 34m/4.5A = 7.55m. The nearest standard
value is 7.5m, so a 7.5m ±1% resistor would be a
good choice. At the other set of tolerance extremes for the
output in question, I
LOAD(CONT,max)
= 67mV/7.5m = 8.93A.
Knowing this final datum, we can determine the
necessary wattage of the sense resistor, using P = I
2
R,
where I is I
LOAD(CONT, MAX)
, and R is (0.97)(R
SENSE(NOM)
).
These numbers yield the following:
P
MAX
= (8.93A)
2
(7.28m) = 0.581W
A 1W sense resistor would work well in this application.
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
SENSE
is highly
recommended. Kelvin sensing is 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-
frequency noise filter across the sense inputs is highly
recommended 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 experimentally 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
SENSE
and then to the drain of the 5V (Slot A) output
MOSFET) flows directly through the power PCB traces
and R
SENSE
. The voltage drop resulting across R
SENSE
is
sampled in such away that the high currents through the
power traces will not introduce any extraneous IR drops.
R
SENSE
Powe
Trace
From 5V
IN
Powe
Trace
To MOSFET Drain
Signal Trace
to MIC2593 V
IN
Signal Trace
to MIC2593 V
SENSE
5VINA
5VSENSEA
MIC2593
22pF
Figure 12. Kelvin Sensing Connections for R
SENSE
(Applicable to 5V[A/B] and 3V[A/B])
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 requirements.
2. Selection of a device to handle the maximum
continuous current (steady-state thermal issues).
3. Verification that the selected part can withstand
any current peaks (transient thermal issues).
