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LTC4311IDC

Low Voltage I2C/SMBus Accelerator

Linear Technology 

LTC4311

APPLICATIONS INFORMATION

This is the lowest resistor value that may be chosen and

still meet VOL. Next calculate the largest value of RP that

will satisfy SR, the minimum slew rate requirement. Us-

ing VOL = 0.4V and SR = 0.5V/μs calculate the value of

RP with equation 2.

RP

= 3.3V − 0.4V

600pF •0.5V / µs

RP = 9.667k

(11)

This is approximately the largest value of RP that will satisfy

the minimum slew rate requirement. Since RP is larger

than 2.175k the VOL will be below 0.4V, and the slew rate

will actually be faster than calculated. Choosing RP = 10k ,

VOL and SR are recalculated.

VOL

=

300Ω • 3.3V

300Ω +10kΩ

=

96mV

SR = 3.3V – 96mV = 0.534V / µs

10kΩ • 600pF

(12)

The rise and fall times need to be verified using equations

3 and 4.

tr = –10kΩ • 600pF •

In

⎧ 2.31V

⎨⎩ 0.99V

–

–

3.3V

3.3V

– 10kΩ

– 10kΩ

• 2.5mA

• 2.5mA

⎫

⎬

⎭

=

0.297µs

(13)

tf = 291Ω • 600pF •

In

⎧

⎪

⎨

⎪

2.31 •(10kΩ + 300Ω) – 300Ω

3.3V

0.99V •(10kΩ + 300Ω) – 300Ω

⎫

⎪

⎬

⎪

=

0.158µs

⎩ 3.3V

⎭

(14)

Both the rise and fall times meet the 0.3μs I2C requirement

and the VOL is satisfied, while meeting the minimum slew

rate requirement, so RP is chosen to be 10k.

If tr is not met, RP should be decreased and if tf is not met

then RP should be increased.

SMBus Design Example

Given the following conditions and requirements for a low

power SMBus system:

VCC = 3.3V NOMINAL

VOL = 0.4V MAXIMUM

CBUS = 400pF

VILMAX = 0.8V,VIHMIN = 2.1V

tr = 1µs MAXIMUM,tf = 0.3µs MAXIMUM

(15)

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

(16)

Calculate Maximum RP from equation 2.

RP

=

3.3V – 0.4V

400pF •0.5V / µs

RP = 14.5k

(17)

Choose RP = 13k and recalculate VOL and SR.

VOL

=

300Ω • 3.3V

300Ω + 13kΩ

=

74mV

SR = 3.3V – 74mV = 0.62V / µs

13kΩ • 400pF

(18)

The rise and fall times need to be verified using equations

5 to 8.

t1 = –13kΩ • 400pF •

ln

⎧ 0.9V–3.3V

⎨⎩ 0.8V–0.15V–3.3V

⎫

⎬

⎭

=

0.515µs

(19)

4311fa

9

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