Datasheet

LTC4315
14
4315f
APPLICATIONS INFORMATION
TELECOMMUNICATIONS SYSTEMS
The LTC4315 has several features that make it an excel-
lent choice for use in telecommunications systems such
as ATCA. Referring to Figures 8 and 9, buffers are used
on the edges of the field replaceable units (FRUs) and
shelf managers to shield devices on these cards from the
large backplane capacitance. The input capacitance of the
LTC4315 is less than the 10pF maximum specification for
buffers used in bussed ATCA applications. The LTC4315
buffers can drive capacitances >1nF, which is greater than
the maximum backplane capacitance of 690pF in bused
ATCA systems. The precharge feature, the low input
capacitance of the LTC4315 and the high impedance of
the SDA and SCL pins of the LTC4315 when it is unpow-
ered, minimize disturbances to the bus when cards are
being hot swapped. In Figure 8, the RTA of the LTC4315
on the shelf manager supplies 2.5mA of pull-up current,
allowing the 1µs rise time requirement to be met on the
heavily loaded backplane for loads well beyond the 690pF
maximum specification. The 0.33 • V
MIN
turn-off voltage
of the LTC4315’s buffers provides a large logic low noise
margin in these systems.
In the bused ATCA application shown in Figure 8, the
LTC4315s located on the shelf managers #1 and #2 and on
the FRUs, drive the large backplane capacitance while the
microcontrollers on the shelf managers and the I
2
C slave
devices on the FRUs drive the small input capacitance of the
LTC4315. The LTC4315 on only one of the shelf managers
is enabled at any given time. The hot insertion logic on
the LTC4315 allows the FRUs to be plugged or unplugged
Figure 8. LTC4315s Used in a Bused ATCA Application. Only the Clock Path Is Shown for Simplicity.
LTC4315
µP
V
CC
3.3V
V
CC2
SCLOUTSCLIN
ENABLE
ACC
R1
10k
R2
2.7k
4315 F08
BACKPLANE
IPMB-A
SCL
IPMB-B
SCL
SHELF MANAGER #1
SHELF MANAGER #2
IDENTICAL TO SHELF MANAGER #1
IPMB-A
IPMB-B
IPMB-B
IPMB-B DETAILS (NOT SHOWN) ARE IDENTICAL TO IPMB-A
FRU #1
LTC4315
I
2
C
DEVICE
3.3V
3.3V
3.3V
SCLOUT
V
CC2
V
CC
SCLIN
ACC
R3
10k
R4
10k
LTC4315
3.3V
SCLOUT
V
CC2
V
CC
SCLIN
ACC
FRU #N
LTC4315
I
2
C
DEVICE
3.3V
3.3V
3.3V
SCLOUT
V
CC2
V
CC
SCLIN
ACC
R5
10k
R6
10k
LTC4315
3.3V
SCLOUT
V
CC2
V
CC
SCLIN
ACC
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