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LT3960

Rev. B

For more information www.analog.com

APPLICATIONS INFORMATION

Multidrop Applications

The LT3960 can be used in a multidrop setup, employ-

ing multiple slave-mode LT3960’s to generate multiple

local I2C buses on multiple PCBs along the length of the

I2CAN bus lines. No additional termination is required in

a multidrop system, but some care must be taken in the

design of such systems. The stub length, or the distance

from twisted pairs to any additional LT3960, should be

less than 0.3m. Stub lengths to the CANSDA and CANSCL

buses should be as close as possible in length to avoid

adding unequal transmission delays to the clock and

data signals.

Figure 13.

LT3960

I2C SLAVE

LT3960

I2C SLAVE

LT3960

I2C SLAVE

120

3960 F14

Multidrop Setup

Maximum Data Transmission Rate

Successful communication in any I2C application is

dependent on slave I2C devices’ timely acknowledgment

(or ACK) upon receiving a byte of data. Specifically, I2C

slaves must assert the SDA line after the eighth clock

pulse leaving enough setup time before the ninth rising

edge of SCL to guarantee that the ACK will be received by

the I2C master. This requirement is straightforward when

all I2C devices share the same I2C bus, but in LT3960

applications where master and slave I2C buses are sepa-

rated by various propagation delays, extra care must be

taken to ensure that ACKs from slave I2C devices will be

received by the I2C master at the desired transmission

rate. In LT3960 applications, the SCL low period between

successive clock pulses (tLOW)must be less than the sum

of the propagation delays (tPI2CBD and tPBI2CD), slave ACK

time (tVD;ACK), and master data setup time (tSU;ACK). This

requirement is shown explicitly in Equation 1.

tLOW >2 tPI2CBD,max + tCABLE + tPBI2CD,max

(

)

+tVD;ACK,max + tSU;ACK

(1)

A conservative estimate of propagation delay

through a twisted pair based on cable length is shown in

Equation 2. Equation 2 is useful for rough estimates, but

when designing applications always calculate propaga-

tion delay based on the actual physical properties of the

cabling used for the I2CAN bus lines.

tCABLE =

ICABLE

0.15m/ns

(2)

Fast-mode (400kHz capable) I2C devices are allowed 0.9µs

to acknowledge a valid data byte, even while ACK times

(tVD;ACK) are often much shorter in practice. A tVD;ACK of

0.9µs would limit the data transmission rate of a LT3960

application to under 400kHz for even one meter of twisted

pair. For this reason, it is recommended that all I2C slaves

be Fast-mode Plus (1MHz) devices instead of Fast-mode

(400kHz) devices when attempting to maximize trans-

mission rate. The shorter maximum tVD;ACK (450ns) of

Fast-mode plus devices allows for communication across

a greater distance at any given clock speed. Figure 14

consolidates the information above, plotting maximum

clock speeds for a given bus length for applications with

fast-mode and fast-mode plus devices.

Figure 14.

FAST–MODE SLAVE DEVICES

FAST–MODE PLUS SLAVE DEVICES

BUS LENGTH (m)

5 10 15 20 25 30 35 40 45 50

200

225

250

275

300

325

350

375

400

425

450

3960 F14

Maximum I2CAN Clock Speed

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