SMH4042A Datasheet, PDF (16/28 Pages) Summit Microelectronics, Inc. – Distributed Power Hot-Swap Controller for CompactPCI

APPLICATIONS
DESIGN CONSIDERATIONS FOR A COMPACTPCI
BOARD
Figure 11 is a generic representation of a CompactPCI
board and it illustrates how the SMH4042A is the key
component in the board insertion/removal process. The
illustrations that follow show in more detail how the
various blocks interface to the SMH4042A.
SMH4042A
Backend Power Plane
and Logic
Backend Power
SwitchingCircuits
Current
sense
resistors
RESET
SGNL_VLD
CBI_3
CBI_5
1Vref
VCC5
CARD_3V_MON ^HST_3V_MON
CARD_5V_MON
V(I/O)
V(I/O) ^eP
VGATE3
VGATE5
BD_SEL2#
GND
V(I/O)
LOCAL_PCI_RST#
PCI_RST#
BD_SEL1#
HEALTHY#
SMH4042A
Precharge
Circuit
V(I/O)
eP
Current
limiting
resistors
Capacitance
4.7µf each
Figure 11. Diagram of Typical CompactPCI Board
P2
ENUM#
V(I/O)
5V
3.3V
GND
GND
BD_SEL#
P1
GND
GND
GND
3.3V
GND, PCI_RST#
HEALTHY#
5V
5V
+12V, -12V
2070 Fig11
Power Busses
It is important in the design of the board to ensure the
backend logic is isolated from the power control circuits
and other early power circuits such as FPGAs and the I/
O interface circuits. In Figure 12 the early power busses
for 5V and 3V have series current limiting resistors.
These values should be calculated so as to limit the in-
rush current that will initially charge the capacitive load
of the early power circuits. As the card is inserted further,
the medium length pins engage and short out the current
limiting resistors. Note the placement of the sense (shunt)
resistors. They are in series with the power FETs and no
voltage drop will be detected across the resistor until
VGATE is applied to the power FETs. The sense resistor
values are determined by dividing 50mV by the current
specification for that supply.
It should be noted that there is an inherent delay from
VGATE5 turning on to VGATE3 turning on. The typical
delay is illustrated in Figure 13.
16
2070 9.1 5/27/03
SUMMIT MICROELECTRONICS, Inc.