Datasheet

LTC4312
9
4312f
APPLICATIONS INFORMATION
The LTC4312 is a 1:2 pin selectable I
2
C multiplexer that
provides a high noise margin, capacitance buffering and
level translation capability on its clock and data pins. Rise
time accelerators accelerate rising edges to enable opera-
tion at high frequencies with heavy loads. These features
are illustrated in the following subsections.
Rise Time Accelerators and DC Hold-Off Voltage
Once the LTC4312 has exited UVLO and a connection has
been established between the SDA and SCL inputs and
outputs, the rise time accelerators on both the input and
output sides of the SDA and SCL busses are activated
based on the state of the ACC pin and the V
CC2
supply
voltage. During positive bus transitions of at least 0.2V/
μs, the rise time accelerators provide pull-up currents to
reduce rise time. Enabling the rise time accelerators al-
lows users to choose larger bus pull-up resistors, reduc-
ing power consumption and improving logic low noise
margins, to design with bus capacitances outside of the
I
2
C specifi cation or to switch at a higher clock frequency.
The ACC pin sets the turn-off threshold voltage for the
buffers, the turn-on voltage for the rise time accelerators,
and the rise time accelerator pull-up current strength. The
ACC functionality is shown in Table 1. Set ACC open or
high when a high noise margin is required such as when
the LTC4312 is used in a system having I
2
C devices with
V
OL
> 0.4V.
Table 1. ACC Control of the Rise Time Accelerator Current I
RTA
and Buffer Turn-Off Voltage V
IL,RISING
ACC I
RTA
V
RTA(TH)
V
IL,RISING
Low Strong 0.8V 0.6V
Open 3mA 0.4•V
MIN
0.33•V
MIN
High None N/A 0.33•V
MIN
The ACC pin has a resistive divider between V
CC
and GND
to set its voltage to 0.5•V
CC
if left open. In the current
source accelerator mode, the LTC4312 provides a 3mA
constant current source pull-up. In the strong mode, the
LTC4312 sources pull-up current to make the bus rise at
75V/μs (typical). The strong mode current is therefore
directly proportional to the bus capacitance. The LTC4312
is capable of sourcing up to 45mA of current in the strong
mode. The effect of the rise time accelerator strength is
shown in the SDA waveforms in Figures 1 and 2 for iden-
tical bus loads for a single enabled output channel. The
rise time accelerator supplies 3mA and 10mA of pull-up
current (I
RTA
) respectively in the current source and strong
modes for the bus conditions shown in Figures 1 and 2.
The rise time accelerator turn-on voltage in the strong
mode is also lower as compared to the current source
mode. For identical bus loading conditions, the bus returns
high faster in Figure 1 compared to Figure 2 because of
both the higher I
RTA
and the lower turn-on voltage of the
rise time accelerator. In each fi gure, note that the input
and output rising waveforms are nearly coincident due to
the input and output busses having nearly identical bus
current and capacitance.
Figure 1. Bus Rising Edge for the
Strong Accelerator Mode
Figure 2. Bus Rising Edge for the
Current Source Accelerator Mode
500ns/DIV
2V/DIV
4312 F01
0V
0V
C
IN
= C
OUT
= 200pF
R
BUS
= 10kΩ
ACC = 0
V
CC
= V
CC
= 5V
SDAOUT1
SDAIN
500ns/DIV
2V/DIV
4312 F02
0V
0V
C
IN
= C
OUT
= 200pF
R
BUS
= 10kΩ
ACC = OPEN
V
CC
= V
CC2
= 5V
SDAOUT1
SDAIN