MIC2580A Datasheet, PDF (20/21 Pages) Micrel Semiconductor – Hot-Swap PCI Power Controller

MIC2580A
CompactPCI™ BD_SEL# Pin Tied to Ground
For applications that use system backplanes with the
BD_SEL# pin tied to ground, the MIC2580A /ON pin is edge
sensitive. Therefore, the /ON pin requires a delay circuit for
proper start-up when the board has already been inserted
into the backplane and the supplies are switched off, then
back on. The circuit, shown in the figure below, allows the
MIC2580A /ON pin to transition from high to low which is
necessary for start-up. The delay time may be increased or
decreased by changing the RC time constant in the circuit,
but the delay time must exceed the ramp time of all system
backplane supplies. The same circuit is functional for hot
swap insertion.
Micrel, Inc.
PCB Layout Considerations
To achieve accurate current sensing Kelvin connections are
recommended between the supply pin and the respective
sense resistor as shown in Figure 8. PCB trace length should
be kept at a minimum. 0.02 inches per ampere is a minimum
width for 1 oz. copper to prevent damage to traces carrying
high current. Keep these high-current traces as short as
possible.
short-length, high-current
(wide) copper traces
sense resistor
from
to
supply
load
MIC2580A
2k
/FAULT
2k
/RESET
/ON
V_IO
(3.3V or 5V)
Kelvin
connections
to
to
VIN SENSE
pin
pin
Figure 8. Layout Recommendation
BD_SEL#
50k
1N914
1.2k
2N3904
10k
3.3µF
On PCB
Figure 7. /ON Pin Assertion Delay Circuit
MIC2580A
20
February 2005