LTC1694CS5 datasheet(6/8 Pages) LINER

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LTC1694

RON(MAX):

The maximum on resistance of the open-

drain driver

IPULL-UP(MAX): The maximum LTC1694 low pull-up cur-

rent is 350

µA

Fall Time

Fall time is a function of the SMBus capacitance, RS, RON

and the pull-up current. Figure 3 shows the maximum

allowed (RS + RON) based on the Intel SMBus fall time

requirement of 300ns with a 50ns safety margin.

BUS CAPACITANCE (pF)

1.4

1.2

1.0

0.8

0.6

0.4

0.2

300

1694 F03

100

200

400

500

VCC = 5V

Figure 3. Maximum Value of RS + RON as a Function of Bus

Capacitance for Meeting the SMBus tf(MAX) Requirement

The maximum value of RS, based on fall time require-

ments, can also be calculated by rearranging equation 6.

Given below are some equations that are useful for calcu-

lating rise and fall time and for selecting the value of RS.

Initial Slew Rate

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

SR = IPULL-UP(MIN)/CBUS

(2)

CBUS is the total capacitance of the SMBus line.

IPULL-UP(MIN) is the LTC1694 minimum pull-up current

(125

µA).

SR must be greater than SRTHRES, the LTC1694 slew rate

detector threshold (0.5V/

µs max) in order to activate the

2.2mA boosted pull-up current. This limits the maximum

SMBus capacitance.

SMBus Rise Time

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

4 and 5.

tr = t1 + t2

(3)

t1 = (VTHRES – VIL(MAX) + 0.15) •

CBUS/IPULL-UP

(4)

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

t2 = (VIH(MIN) + 0.15 – VTHRES) • CBUS/IPULL-UP(B) (5)

IPULL-UP(B) is the LTC1694 boosted pull-up current (2.2mA

typ).

For an SMBus system, VIL(MAX) = 0.8V and VIH(MIN) = 2.1V.

For the LTC1694, typically VTHRES = 0.65V and

IPULL-UP = 275µA.

CBUS is the total capacitance of the SMBus line.

SMBus Fall Time

Fall time of an SMBus line is derived using equation 6.

tf = RT • CBUS • ln{[(0.9 • VCC) – (RL • IPULL-UP(LOW))]/

[VIL(MAX) – 0.15 – (RL • IPULL-UP(LOW))]}

(6)

where RL is the sum of RS and RON (see Figure 2).

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 7.

tr = (VIH(MIN) – VIL(MAX)) • CBUS/IPULL-UP(B)

(7)

Fall time of the I2C line can be derived using equation 8.

tf = RT • CBUS • ln{[VIH(MIN) – (RL • IPULL-UP)]/

[VIL(MAX) – (RL • IPULL-UP)]}

(8)

For an I2C system with fixed input levels, VIL(MAX) = 1.5V

and VIH(MIN) = 3V.

For an I2C system with VCC related input levels, VIL(MAX) =

0.3 • VCC and VIH(MIN) = 0.7 • VCC.

CBUS is the total capacitance of the I2C line.

APPLICATIO S I FOR ATIO

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