SMH4042 Datasheet, PDF (15/28 Pages) Summit Microelectronics, Inc. – Hot Swap™ Controller

SMH4042
CompactPCI Applications Aid
Design Considerations for a CompactPCI Board
Figure 2 is a generic representation of a CompactPCI board and it illustrates how the SMH4042 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 SMH4042.
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#
SMH4042
Precharge
Circuit
V(I/O)
eP
Current
limiting
resistors
Capacitance
8.8 µf each
Figure 2. Block 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
2037 ILL23.1
Power Busses
It is important in the design of the board to insure 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 the illustration shown below, the early power
busses for +5V, +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 spec for that supply.
It should be noted that there is an inherent delay from VGATE turning on to VGATE3 turning on. The typical delay is
illustrated in Figure 4.
2037-07 9/23/99
15