MIC2591B Datasheet, PDF (30/34 Pages) Micrel Semiconductor – Dual-Slot PCI Express Hot-Plug Controller

MIC2591B
Applications Information
Sense Resistor Selection
The 12V and the 3.3V supplies employ internal current sens-
ing 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_THILIMIT
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 re-
sistor (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 MIC2591B’s minimum current limit threshold voltage
is 45mV, the minimum hot swap load current is determined
where the sense resistor is 3% high:
45mV
43.7mV
I_LIMIT(MIN) = _{(1.03 × RSENSE(NOM))} = _RSENSE(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:
55mV
56.7mV
I_LIMIT(MAX) = _{(0.97 × RSENSE(NOM))} = _RSENSE(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
MIC2591B circuit must pass a minimum hot swap load current
of 1.5A without nuisance trips, R_SENSE should be set to:
45mV
R_SENSE(NOM) = _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:
56.7mV
I_LIMIT(MAX) = _30.1mΩ = 1.88A
With a knowledge of the application circuit’s maximum hot
swap load current, the power dissipation rating of the 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
Micrel
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 volt-
age 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 13 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 con-
nections 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 14 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 de-
termine if filtering is necessary and to select the appropriate
cutoff frequency for each specific application.
Other Layout Considerations
Figure 15 is a suggested PCB layout diagram for the MIC2591B
power traces, Kelvin sense connections, and capacitor com-
ponents. 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 wide enough to allow the current to flow
while the rise in temperature for a given copper plate (e.g.,
1oz. or 2oz.) is kept to a maximum of 10°C to 25°C. The
return (or power ground) trace should be the same width as
the positive voltage power traces (input/load) and isolated
from any ground and signal planes so that the controller’s
power is common mode. Also, these traces should be as
short as possible in order to minimize the IR drops between
the input and the load. As indicated in the Pin Description
section, an external connection must be made that ties to-
gether both channel inputs ((+) Kelvin sense) of each Main
power rail (i.e., 3VINA and 3VINB, 12VINA and 12VINB
must be externally connected). These connections should
be implemented directly at the chip. Insure that the voltage
drop between the two(+) Kelvin sense inputs for each rail
is no greater than 0.2mV by using a common power path
March 2005
30
M9999-033105