MIC2593 Datasheet, PDF (21/26 Pages) STMicroelectronics – Dual-Slot PCI Hot Plug Controller

MIC2593
Application Information
Current Sensing
For the three power supplies switched with internal MOS-
FETs (+12V, –12V, and V_AUX), 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 V_IN connection to the IC from
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_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:
( )( ) ^{RSENSE =}
35mV
1.03 I_LOAD(CONT)
= ^34mV
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 cur-
rent is found using a 65mV trip voltage and a sense resistor
which is 3% low in value. The resulting current is:
I_{LOAD(CONT, MAX)}
=
65mV
(0.97)(R_SENSE(NOM) )
=
67mV
R_SENSE(NOM)
As an example, if an output must carry a continuous 4.5A
without nuisance trips occurring, R_SENSE for that output
should be 34mΩ/4.5A = 7.55mΩ. 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 for the output in
question, I_{LOAD(CONT,max)} = 67mV/7.5mΩ = 8.93A. Knowing
this final datum, we can determine the necessary wattage of
the sense resistor, using P = I²R, 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(7.28mΩ) = 0.581W
A 1W sense resistor would work well in this application.
Micrel
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-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_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 a
way that the high currents through the power traces will not
introduce any extraneous IR drops.
Power Trace
From 5V_IN
R_SENSE
Power Trace
To MOSFET Drain
Signal Trace
to MIC2593 V_IN
22pF Signal Trace
to MIC2593 V_SENSE
MIC2593
Figure 12. Kelvin Sense 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).
MOSFET Voltage Requirements
The first voltage requirement for each MOSFET is easily
stated: the drain-source breakdown voltage of the MOSFET
April 2004
21
M9999-042204