LTC4311C データシートの表示（PDF） - Linear Technology

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LTC4311C

Low Voltage I2C/SMBus Accelerator

Linear Technology 

LTC4311

APPLICATIONS INFORMATION

For an I2C system with fixed input levels, VILMAX = 1.5V

and VIHMIN = 3V. For I2C systems with VCC related input

levels, VILMAX = 0.3VCC and VIHMIN = 0.7VCC.

CBUS is the total capacitance of the I2C line.

SMBus Rise and Fall Time

Rise time of a SMBus line is derived using equations 5,

6 and 7.

tr = t1+ t2

(5)

t1 is the time from when the bus crosses the lower slew

rate measurement point, until the bus reaches VTHR and the

accelerators fire. The time from when the accelerators fire

until the bus reaches the upper slew rate measure point is

given by t2. Equations for t1 and t2 are given here:

t1 =

–RP

• CBUS

⎧

• ln ⎨

⎩

VTHR – VCC

VILMAX – 0.15V –

VCC

⎫

⎬

⎭

(6)

If (VILMAX – 0.15V) > VTHR, then t1 = 0

t2 = –RP • CBUS •

ln

⎧

⎨

⎩

VIHMIN + 0.15V – VCC

VTHR – VCC – RP

– RP •I PULLUPAC

•IPULLUP AC

⎫

⎬

⎭

(7)

Fall time of an SMBus line is derived using equation 8:

A general procedure for selecting RP and RL is as fol-

lows:

1. RL is first selected based on the I/O protection

requirement. Generally, an RS of 100Ω is sufficient for

high voltage spikes and ESD protection. RON is

determined by the size of the open-drain driver, a large

driver will have a lower RON.

2. The value of RP is determined based on the VOL and

minimum slew rate requirements. The VOL will determine

the smallest resistance value that can be used in a

system, and the minimum slew requirement will bound

the resistance on the upper end. Generally the largest

value of resistance that meets the minimum slew rate

with some margin will be selected.

3. For I2C systems incorporating the LTC4311, the rise

times are met under most loading conditions, due to

the strong accelerator current. The pull-down drivers

are typically low impedance, and therefore fall times

are not generally an issue. Rise and fall time

requirements must be verified using equations 3 and

4 (for an I2C system) or equations 5 to 8 (for an SMBus

system). The value chosen for RP must ensure that

both the rise and fall time specifications are met

simultaneously.

I2C Design Example

Given the following conditions and requirements:

tf = RT • CBUS •

ln

⎧

⎪⎪

⎨

⎪

⎩⎪

VIHMIN + 0.15V

VCC

VILMAX – 0.15V

VCC

• (RP

• (RP

+ RL ) – RL

+ RL ) – RL

⎫

⎪⎪

⎬

⎪

⎭⎪

(8)

For an SMBus system, VILMAX = 0.8V and VIHMIN = 2.1V.

CBUS is the total bus capacitance of the SMBus line.

VCC = 3.3V NOMINAL

VOL = 0.4V MAXIMUM

CBUS = 600pF

VILMAX = 0.99V,VIHMIN = 2.31V

tr = 0.3µs MAXIMUM,tf = 0.3µs MAXIMUM (9)

If an RS of 100Ω is used and the max RON of the driver is

200Ω, then RL = 200Ω + 100Ω = 300Ω. Use equation 1

to find the required RP to meet VOL.

RP

=

300Ω•(3.3V – 0.4V)

0.4V

RP = 2.175k

(10)

4311fa

8

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