LTC4311ISC6 データシートの表示（PDF） - Linear Technology

部品番号

コンポーネント説明

一致するリスト

LTC4311ISC6

Low Voltage I2C/SMBus Accelerator

Linear Technology 

LTC4311

APPLICATIONS INFORMATION

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

LTC4311

DYNAMIC

CURRENT

PULL-UP

RP

CBUS

Bus

RS

DATA

IN

DATA

OUT

RON

4311 F01

Figure 1. Typical 2-Wire Bus Configuration

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.

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:

VOL

=

RL • VCC

RL + 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:

SR = VCC – VOL

RP • 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.

tr = – Rp • CBUS •

ln

⎧⎪

⎨

⎩⎪

VIHMIN

VILMAX

–

–

VCC

VCC

– Rp

– Rp

•IPULLUP AC

•IPULLUP AC

⎫⎪

⎬

⎭⎪

(3)

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

tf

= RT

⎧ VIHMIN

•

CBUS

•

ln

⎪⎪

⎨

⎪

VCC

VILMAX

⎩⎪ VCC

•(RP + RL ) – RL

•(RP + RL ) – RL

⎫

⎪⎪

⎬

⎪

⎭⎪

(4)

where RT is the parallel equivalent of RP and RL.

4311fa

7

Share Link: