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LTC4314

4314f

APPLICATIONS INFORMATION

The LTC4314 is a 1:4 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 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 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 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 I2C speciﬁca-

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 I2C devices with VOL > 0.4V.

The ACC pin has a resistive divider between VCC and

ground to set its voltage to 0.5•VCC 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 IRTA

and Buffer Turn-Off Voltage VIL,RISING

ACC

IRTA

VRTA(TH)

VIL, RISING

Low

Strong

0.8V

0.6V

Hi-Z

3mA

0.4•VMIN

0.33•VMIN

High

None

N/A

0.33•VMIN

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 (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 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 IRTA and the lower turn-on volt-

age 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

4314 F01

CIN = COUT = 200pF

RBUS = 10kΩ

ACC = 0

VCC = VCC = 5V

SDAOUT1

SDAIN

500ns/DIV

4314 F02

CIN = COUT = 200pF

RBUS = 10kΩ

ACC = OPEN

VCC = VCC2 = 5V

SDAOUT1

SDAIN

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