LTC4307
8
4307f
moment of connection, therefore minimizing the amount
of disturbance caused by the I/O card.
Once the LTC4307 comes out of UVLO, it monitors both
the backplane and card sides for either a stop bit or bus idle
condition to indicate the completion of data transactions.
When both sides are idle or one side has a stop bit condi-
tion while the other is idle, the input-to-output connection
circuitry is activated, joining the SDA and SCL busses on
the I/O card with those on the backplane. In addition, the
precharge circuitry is deactivated and will not be reactivated
unless the V
CC
voltage falls below the UVLO threshold.
Connection Circuitry
Once the connection circuitry is activated, the functionality
of the SDAIN and SDAOUT pins is identical. A low forced
on either pin at any time results in both pin voltages be-
ing low. The LTC4307 is tolerant of I
2
C bus DC logic low
voltages up to the 0.3V
CC
V
IL
I
2
C specifi cation.
When the LTC4307 senses a rising edge on the bus, it
deactivates its pull-down devices for bus voltages as low
as 0.48V and activates its accelerators. This methodology
maximizes the effectiveness of the rise time accelerator
circuitry and maintains compatibility with the other devices
in the LTC4300 bus buffer family. Care must be taken to
ensure that devices participating in clock stretching or
arbitration force logic low voltages below 0.48V at the
LTC4307 inputs.
SDAIN and SDAOUT enter a logic high state only when
all devices on both SDAIN and SDAOUT release high.
The same is true for SCLIN and SCLOUT. This important
feature ensures that clock stretching, clock synchroniza-
tion, arbitration and the acknowledge protocol always
work, regardless of how the devices in the system are
tied to the LTC4307.
Another key feature of the connection circuitry is that it
provides bidirectional buffering, keeping the backplane
and card capacitances isolated. Because of this isolation,
the waveforms on the backplane busses look slightly
different than the corresponding card bus waveforms as
described here.
OPERATION
Input to Output Offset Voltage
When a logic low voltage, V
LOW1
, is driven on any of the
LTC4307’s data or clock pins, the LTC4307 regulates the
voltage on the opposite data or clock pins to a slightly
higher voltage, typically 60mV above V
LOW1
. This offset is
practically independent of pull-up current (see the Typical
Performance curves).
Propagation Delays
During a rising edge, the rise time on each side is de-
termined by the bus pull-up resistor and the equivalent
capacitance on the line. If the pull-up resistors are the
same, a difference in rise time occurs which is directly
proportional to the difference in capacitance between
the two sides. This effect is displayed in Figure 2 for
V
CC
= 5.5V and a 10k pull-up resistor on each side (50pF
on one side and 150pF on the other). Since the output
side has less capacitance than the input, it rises faster
and the effective propagation delay is negative.
There is a fi nite propagation delay through the connec-
tion circuitry for falling waveforms. Figure 3 shows the
falling edge waveforms for the same pull-up resistors and
equivalent capacitance conditions as used in Figure 2.
An external N-channel MOSFET device pulls down the
voltage on the side with 150pF capacitance; the LTC4307
pulls down the voltage on the opposite side with a delay
of 80ns. This delay is always positive and is a function
of supply voltage, temperature and the pull-up resistors
and equivalent bus capacitances on both sides of the bus.
The Typical Performance Characteristics section shows
propagation delay as a function of temperature and voltage
for 10k pull-up resistors and 50pF equivalent capacitance
on both sides of the part. Also, the t
PHL
vs C
OUT
curve for
V
CC
= 5.5V shows that increasing the capacitance from
50pF to 150pF results in a t
PHL
increase from 81ns to 91ns.
Larger output capacitances translate to longer delays (up
to 125ns). Users must quantify the difference in propaga-
tion times for a rising edge versus a falling edge in their
systems and adjust setup and hold times accordingly.
