LTC4245
15
4245fa
The typical LTC4245 application is in a high availability sys-
tem where boards using multiple supplies are hot plugged.
The device enables the system to periodically monitor board
power consumption and fault status over the I
2
C interface.
Boards in CompactPCI and PCI Express systems typically
utilize three to four supplies. Figure 1 shows the LTC4245
being used in a CompactPCI application.
The following sections describe the turn on, turn off and
fault response behavior of the LTC4245. The ADC and I
2
C
interface are discussed next. External component selection
is discussed in detail in the Design Example section.
CPCI Connection Pin Sequence
The staggered lengths of the CPCI male connector pins on
the backplane ensures that all power supplies are physi-
cally connected to the LTC4245 before back-end power
is allowed to ramp up (BD_SEL# asserted low). The long
pins, which include 5V, 3.3V, V(I/O) and GND, mate rst.
The short BD_SEL# pin mates last. At least one long 3.3V
power pin must be connected to the LTC4245 in order for
the PRECHARGE pin voltage to be available before the
CPCI bus pins mate.
The following is a typical hot plug sequence.
  1. ESD clips make contact.
  2. Long power and ground pins make contact and Early
     Power is established. The 1V precharge voltage
     becomes valid at this stage. Power is also applied
     to the pull-up resistors connected to the HEALTHY#
     and BD_SEL# signals. LOCAL_PCI_RST# is held
     in reset. All power switches are held off at this stage
     of insertion.
  3. Medium length pins make contact. The 12V and 12V
     connector pins make contact at this stage. The
     internal low voltage supply of the LTC4245 (INTV
CC
)
     powers up from the 12V supply. An internal 10糀
     pull-up from INTV
CC
to BD_SEL# turns on. Other
     connector pins that mate are HEALTHY#, PCI_RST#
     and the bus I/O pins (which are precharged to 1V).
  4. Short pins make contact. If the BD_SEL# signal is
     grounded on the backplane, the plug-in board
     power-up cycle may begin immediately. If the ON pin
     is tied high then turn-on is automatic, else the LTC4245
     waits for a serial bus turn-on command. System
     backplanes that do not ground the BD_SEL# signal
     will instead have circuitry that detects when
  
BD_SEL# makes contact with the plug-in board. The
     system logic can then control the power up process
     by pulling BD_SEL# low. The precharge potential may
     be optionally disconnected from the CPCI bus signals
     at this stage.
Turn-On
The back-end power planes are isolated from the input
power planes by external N-channel pass transistors Q1
through Q4. Sense resistors R1 to R4 provide current
fault detection. Resistors R5 to R8 prevent high frequency
oscillations in MOSFETs Q1 to Q4 respectively.
The following conditions must be satis ed for a duration of
100ms before the external switches can be turned on.
  1. All input supplies and the internally generated supply,
     INTV
CC
, must exceed their undervoltage lockout
     thresholds. The V
EE
undervoltage lockout can be
     disabled by not tying the CFG pin low.
  2. No undervoltage, overcurrent or PGI fault bits must
     be set unless the corresponding auto-retry is enabled.
     When 12V
IN
powers up for the rst time, INTV
CC
     rises above its undervoltage threshold which
     generates a 60約 to 120約 internal power-on-reset
     pulse. During reset, the fault registers are cleared
     and the control bits are initialized. If INTV
CC
is already
     up, then the I
2
C interface can be used to clear the
     fault bits or set the auto-retry bits.
  3. The BD_SEL# pin must be pulled low.
When these initial conditions are satis ed, the ON pin is
checked. If it is high, the four FET On control bits (D0 to
D3) are set either simultaneously (the default state) or
in a 12V, 5V, 3.3V, 12V sequence (register bit C6 set).
If ON is low, the external switches turn on when the ON
pin is brought high or if a serial bus turn-on command
is received. Figure 2 shows all supplies turning on after
BD_SEL# goes low.
APPLICATIO  S I FOR  ATIO
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