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LTC1694-1

SMBus/I2C Accelerator

Linear Technology 

LTC1694-1

APPLICATIONS INFORMATION

SMBus Rise Time

Rise time of an SMBus line is derived using equations 3,

4 and 5.

tr = t1 + t2

(3)

t1 = – RP • CBUS • ln[(VTHRES – VCC)/

(VILMAX – 0.15 – VCC)]

(4)

if (VILMAX – 0.15) > VTHRES, then t1 = 0µs.

t2 = – RP • CBUS • ln{[VIHMIN + 0.15 – VCC –

(RP • IPULL-UP)]/[VTHRES – VCC – (RP • IPULL-UP)]} (5)

By ignoring the current through RP, a simplified version

of equation 3 is obtained:

t2 = (VIHMIN + 0.15 – VTHRES) • CBUS/IPULL-UP (6)

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

For the LTC1694-1, typically VTHRES = 0.65V and IPULL-UP

= 2.2mA.

CBUS is the total capacitance of the SMBus line.

Increasing the value of RP increases the rise time.

SMBus Fall Time

Fall time of an SMBus line is derived using equation 7:

tf = RT • CBUS • ln{[0.9 • (RP + RL) – RL]/

[(VILMAX – 0.15) • (RP + RL)/VCC – RL]}

(7)

where RT is the parallel equivalent of RP and RL.

The rise and fall time calculation for an I2C system is as

follows.

I2C Bus Rise and Fall Time

Rise time of an I2C line is derived using equation 8.

tr = – RP • CBUS • ln{[VIHMIN – VCC – (RP • IPULL-UP)]/

[VILMAX – VCC – (RP • IPULL-UP)]}

(8)

Fall time of an I2C line is derived using equation 9:

tf = RT • CBUS • ln{[(VIHMIN/VCC) • (RP + RL) – RL]/

[(VILMAX/VCC) • (RP + RL) – RL]}

(9)

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

and VIHMIN = 3V.

For an I2C system with VCC related input levels, VILMAX =

0.3VCC and VIHMIN = 0.7VCC.

CBUS is the total capacitance of the I2C line.

A general procedure for selecting RP and RL is as follows:

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

ment. Generally, an RS of 100Ω is sufficient for high

voltage spike and ESD protection. RON is determined by

the size of the open-drain driver, a large driver will have

a lower RON.

2. Next, the value of RP is determined based on the rise and

fall time requirements using equations 3 to 7 (for an

SMBus system) or 8 and 9 (for an I2C system). The

value chosen for RP must ensure that both the rise and

fall time specifications are met simultaneously.

3. After RP and RL are selected, use equations 1 and 2 to

check if the VOL and SR requirements are fulfilled.

If SR is too low, decrease the value of RP. If VOL is too high,

increase the value of RP.

SMBus Design Example

Given the following conditions and requirements:

VCC = 3.3V nom

VOL = 0.4V max

CBUS = 200pF max

VILMAX = 0.8V, VIHMIN = 2.1V

tr = 0.8µs max, tf = 0.3µs max

If an RS of 500Ω is used and the max RON of the driver

is 200Ω, then RL = 500 + 200 = 700Ω. Using the max

VTHRES of 0.9V and a min IPULL-UP of 1mA.

Using equation 6 to calculate the approximate value of t2:

t2 = (2.1 + 0.15 – 0.9) • [(200 • 10–12)/(1 • 10–3)]

= 0.27µs

t1 = 0.8 – 0.27 = 0.53µs

Using equation 4 to find the required RP to meet tr:

RP = – t1/{CBUS • ln[(VTHRES – VCC)/

(VILMAX – 0.15 – VCC)]} = 27k

RT = (RP • RL)/(RP + RL)

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