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LTC4312

4312f

APPLICATIONS INFORMATION

The LTC4312 is a 1:2 pin selectable I2C 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 VCC2 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

I2C speciﬁcation 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 I2C devices with

VOL > 0.4V.

Table 1. ACC Control of the Rise Time Accelerator Current IRTA

and Buffer Turn-Off Voltage VIL,RISING

ACC

IRTA

VRTA(TH)

VIL,RISING

Low

Strong

0.8V

0.6V

Open

3mA

0.4•VMIN

0.33•VMIN

High

None

N/A

0.33•VMIN

The ACC pin has a resistive divider between VCC and GND

to set its voltage to 0.5•VCC 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 (IRTA) 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 IRTA and the lower turn-on voltage of the

rise time accelerator. In each ﬁgure, 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

4312 F01

CIN = COUT = 200pF

RBUS = 10kΩ

ACC = 0

VCC = VCC = 5V

SDAOUT1

SDAIN

500ns/DIV

4312 F02

CIN = COUT = 200pF

RBUS = 10kΩ

ACC = OPEN

VCC = VCC2 = 5V

SDAOUT1

SDAIN

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