LTC4311ISC6-TRMPBF datasheet(7/12 Pages) LINER

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LTC4311

4311fa

Selecting the values of RS and RP

The typical conﬁguration for the data bus for a 2-wire bus

is shown in Figure 1. The parameters RP and RS should be

chosen carefully. A description of the process for choosing

the values of RP and RS follows.

An external pull-up resistor RP is required in each bus

line to supply a steady state pull-up current if the bus is

at logic zero. This pull-up current is used for slewing the

bus line during the initial portion of the positive transition

in order to activate the LTC4311 pull-up current.

Using an external pull-up resistor RP to supply steady

state pull-up current provides the freedom to adjust rise

time versus fall time as well as deﬁning the low state

logic-level (VOL).

For I/O stage protection from ESD and high voltage spikes

on the bus, a series resistor RS (Figure 1) is sometimes

added to the open drain driver of the bus agents.

VCC

CBUS

Bus

RON

4311 F01

DATA

LTC4311

DYNAMIC

CURRENT

PULL-UP

DATA

OUT

Figure 1. Typical 2-Wire Bus Conﬁguration

APPLICATIONS INFORMATION

Low State Noise Margin

A low value of VOL, the low state logic level, is desired

for good noise margin. VOL is calculated as follows:

OL =

L • VCC

L + RP

(1)

RL is the series sum of RS and RON, the on resistance of

the open-drain driver.

Increasing the value of RP decreases the value of VOL.

Increasing RL increases the value of VOL.

Initial Slew Rate

The initial slew rate, SR, of the bus is determined by:

CC –VOL

P • CBUS

(2)

SR must be greater than SRTHRESH, the LTC4311 slew rate

detector threshold (0.5V/μs max), in order to activate the

pull-up current.

I2C Rise and Fall Time

Rise time of an I2C line is derived using equation 3.

r = – Rp • CBUS •

IHMIN –VCC –Rp •IPULLUP AC

ILMAX –VCC –Rp •IPULLUP AC

⎧

⎨

⎪

⎩⎪

⎫

⎬

⎪

⎭⎪

(3)

Fall time of an I2C line is derived using equation 4.

f = RT • CBUS • ln

IHMIN

•(R

P + RL )–RL

ILMAX

•(R

P + RL )–RL

⎧

⎨

⎪⎪

⎩

⎪

⎫

⎬

⎪⎪

⎭

⎪

(4)

where RT is the parallel equivalent of RP and RL.

Both the values of RP and RS must be chosen carefully

to meet the low state noise margin and all bus timing

requirements.

A discussion of the electrical parameters affected by the

values of RS and RP, as well as the general procedure for

selecting the values of RS and RP follows.

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