Datasheet

LTC4314
9
4314f
APPLICATIONS INFORMATION
The LTC4314 is a 1:4 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 LTC4314 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 allows users
to choose larger bus pull-up resistors, reducing power
consumption and improving logic low noise margins, to
design with bus capacitances outside of the I
2
C specifi ca-
tion, or 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 func-
tionality is shown in Table 1. Set ACC open or high when a
high noise margin is required, such as when the LTC4314
is used in a system having I
2
C devices with V
OL
> 0.4V.
The ACC pin has a resistive divider between V
CC
and
ground to set its voltage to 0.5•V
CC
if left open. In the
current source accelerator mode, the LTC4314 provides
a 3mA constant current source pull-up.
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
Hi-Z 3mA 0.4•V
MIN
0.33•V
MIN
High None N/A 0.33•V
MIN
In the strong mode, the LTC4314 sources pull-up current
to make the bus rise at 75V/μs (typical). The strong mode
current is therefore directly proportional to the bus capaci-
tance. The LTC4314 is capable of sourcing a maximum of
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 identical 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 volt-
age is also lower in the strong mode than in the current
source mode. For identical bus loading conditions, the
busses return high faster in Figure 1 compared to Figure 2
because of both the higher I
RTA
and the lower turn-on volt-
age 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
4314 F01
0V
0V
C
IN
= C
OUT
= 200pF
R
BUS
= 10kΩ
ACC = 0
V
CC
= V
CC
= 5V
SDAOUT1
SDAIN
500ns/DIV
2V/DIV
4314 F02
0V
0V
C
IN
= C
OUT
= 200pF
R
BUS
= 10kΩ
ACC = OPEN
V
CC
= V
CC2
= 5V
SDAOUT1
SDAIN