MIC2341 Datasheet, PDF (28/32 Pages) Micrel Semiconductor – Dual-Slot PCI Express Hot-Plug Controller

Micrel, Inc.
Applications Information
Sense Resistor Selection
The 12V and the 3.3V supplies employ internal current
sensing circuitry to detect overcurrent conditions that
may trip the circuit breaker. An external sense resistor is
used to monitor the current that passes through the
external MOSFET for each slot of the 12V and 3.3V
rails. The sense resistor is nominally valued at:
R_SENSE(NOM)
=
V_THLIMIT
I_LIMIT
where V_THILIMIT is the typical (or nominal) circuit breaker
threshold voltage (50mV) and I_LIMIT is the nominal inrush
load current level to trip the internal circuit breaker.
To accommodate worse-case tolerances in the sense
resistor (for a ±1% initial tolerance, allow ±3% tolerance
for variations over time and temperature) and circuit
breaker threshold voltages, a slightly more detailed
calculation must be used to determine the minimum and
maximum hot swap load currents.
As the MIC2341’s minimum current limit threshold
voltage is 45mV, the minimum hot swap load current is
determined where the sense resistor is 3% high:
I_LIMIT(MIN)
=
45mV
(1.03 × R_SENSE(NOM) )
=
43.7mV
R_SENSE(NOM)
Keep in mind that the minimum hot swap load current
should be greater than the application circuit’s upper
steady-state load current boundary. Once the lower
value of R_SENSE has been calculated, it is good practice
to check the maximum hot swap load current (I_LIMIT(MAX))
which the circuit may let pass in the case of tolerance
build-up in the opposite direction. Here, the worse-case
maximum is found using a V_{THILIMIT(MAX)} threshold of
55mV and a sense resistor 3% low in value:
I_LIMIT(MAX)
=
55mV
(0.97 × R_SENSE(NOM) )
=
56.7mV
R_SENSE(NOM)
In this case, the application circuits must be sturdy
enough to operate up to approximately 1.25x the steady-
state hot swap load currents. For example, if one of the
12V slots of the MIC2341 circuit must pass a minimum
hot swap load current of 1.5A without nuisance trips,
R_SENSE should be set to:
R_SENSE(NOM)
=
45mV
1.5A
= 30mΩ
where the nearest 1% standard value is 30.1mΩ. At the
other tolerance extremes, I_LIMIT(MAX) for the circuit in
question is then simply:
I_LIMIT(MAX)
=
56.7mV
30.1mΩ
= 1.88A
With a knowledge of the application circuit’s maximum
hot swap load current, the power dissipation rating of the
MIC2341/2341R
sense resistor can be determined using P = I²R. Here,
the current is I_LIMIT(MAX) = 1.88A and the resistance
R_SENSE(MAX) = (1.03)(R_SENSE(NOM)) = 31.00mΩ. Thus, the
sense resistor’s maximum power dissipation is:
P_MAX = (1.88A)² X (31.00mΩ) = 0.110W
A 0.25W sense resistor is a good choice in this
application.
PCB Layout Suggestions and Hints
4-Wire Kelvin Sensing
Because of the low value required for the sense resistor,
special care must be used to accurately measure the
voltage drop across it. Specifically, the measurement
technique across R_SENSE must employ 4-wire Kelvin
sensing. This is simply a means of ensuring that any
voltage drops in the power traces connected to the
resistors are not picked up by the signal conductors
measuring the voltages across the sense resistors.
Figure 12 illustrates how to implement 4-wire Kelvin
sensing. As the figure shows, all the high current in the
circuit (from V_IN through R_SENSE and then to the drain of
the N-channel power MOSFET) flows directly through
the power PCB traces and through R_SENSE. The voltage
drop across R_SENSE is sampled in such a way that the
high currents through the power traces will not introduce
significant parasitic voltage drops in the sense leads. It is
recommended to connect the hot swap controller’s
sense leads directly to the sense resistor’s metalized
contact pads. The Kelvin sense signal traces should be
symmetrical with equal length and width, kept as short
as possible, and isolated from any noisy signals and
planes.
Additionally, for designs that implement Kelvin sense
connections that exceed 1" in length and/or if the Kelvin
(signal) traces are vulnerable to noise possibly being
injected onto these signals, the example circuit shown in
Figure 13 can be implemented to combat noisy
environments. This circuit implements a 1.6 MHz low-
pass filter to attenuate higher frequency disturbances on
the current sensing circuitry. However, individual system
analysis should be used to determine if filtering is
necessary and to select the appropriate cutoff frequency
for each specific application.
Other Layout Considerations
Figure 14 is a suggested PCB layout diagram for the
MIC2341 power traces, Kelvin sense connections, and
capacitor components. In this illustration, only the 12V
Slot B is shown but a similar approach is suggested for
both slots of each Main power rail (12V and 3.3V). Many
hot swap applications will require load currents of
several amperes. Therefore, the power (12VIN and
Return, 3VIN and Return) trace widths (W) need to be
October 2007
28
M9999-102507-A
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