LTC4311CDC-TRMPBF データシートの表示（PDF） - Linear Technology
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LTC4311CDC-TRMPBF Datasheet PDF : 12 Pages
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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
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