MIC2587 Datasheet, PDF (13/16 Pages) Micrel Semiconductor – Single-Channel, Positive High-Voltage Hot Swap Controller

Micrel
the maximum hot swap load current (I_{HOT_SWAP(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_TRIP(MAX) threshold of 55mV and a sense
resistor 3% low in value:
( ) ^{IHOT_SWAP(MAX) =}
55mV
0.97 × R_SENSE(NOM)
= ^56.7mV
R _SENSE(NOM)
In this case, the application circuit must be sturdy enough
to operate over a ~1.5-to-1 range in hot swap load
currents. For example, if an MIC2587 circuit must pass a
minimum hot swap load current of 4A without nuisance
trips, R_SENSE should be set to:
R _SENSE(NOM)
=
39mV
4A
= 9.75mΩ
where the nearest 1% standard value is 9.76mΩ. At the
other tolerance extremes, I_{HOT_SWAP(MAX)} for the circuit in
question is then simply:
I_{HOT_SWAP(MAX)}
=
56.7mV
9.76mΩ
=
5.8A
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 = I² × R. Here,
The current is I_{HOT_SWAP(MAX)} = 5.8A and the resistance
R_SENSE(MIN) = (0.97)(R_SENSE(NOM)) = 9.47mΩ. Thus, the sense
resistor's maximum power dissipation is:
P_MAX = (5.8A)² × (9.47mΩ) = 0.319W
A 0.5W sense resistor is a good choice in this application.
When the MIC2587/MIC2587R’s foldback current limiting
circuit is engaged in the above example, the current limit
would nominally fold back to 1.23A when the output is
shorted to ground.
PCB Layout Recommendations
4-Wire Kelvin Sensing
Because of the low value typically 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 6 illustrates how to implement 4-wire Kelvin
sensing. As the figure shows, all the high current in the
circuit (from V_CC 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
MIC2587/MIC2587R
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 7 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 8 is a recommended PCB layout diagram for the
MIC2587-2BM. Many hot swap applications will require
load currents of several amperes. Therefore, the power
(V_CC 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. Also, these traces should be
as short as possible in order to minimize the IR drops
between the input and the load. The feedback network
resistor values in Figure 8 are selected for a +24V
application. The resistors for the feedback (FB) and ON
pin networks should be placed close to the controller and
the associated traces should be as short as possible to
improve the circuit’s noise immunity. The input “clamping
diode” (D1) is referenced in the “Typical Application Circuit”
on Page 1. If possible, use high-frequency PCB layout
techniques around the GATE circuitry (shown in the
“Typical Application Circuit”) and use a dummy resistor
(e.g., R3 = 0Ω) during the prototype phase. If R3 is
needed to eliminate high-frequency oscillations, common
values for R3 range between 4.7Ω to 20Ω for various
power MOSFETs. Finally, the use of plated-through vias
will be needed to make circuit connection to the power and
ground planes when utilizing multi-layer PCBs.
MOSFET and Sense Resistor Vendors
Device types, part numbers, and manufacturer contacts for
power MOSFETs and sense resistors are provided in
Table 1.
October 2004
13
M9999-102204
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