Datasheet

LTC4311
9
4311fa
APPLICATIONS INFORMATION
This is the lowest resistor value that may be chosen and
still meet V
OL
. Next calculate the largest value of R
P
that
will satisfy SR, the minimum slew rate requirement. Us-
ing V
OL
= 0.4V and SR = 0.5V/μs calculate the value of
R
P
with equation 2.
R
P
=
3.3V −0.4V
600pF •0.5V / µs
R
P
= 9.667k
(11)
This is approximately the largest value of R
P
that will satisfy
the minimum slew rate requirement. Since R
P
is larger
than 2.175k the V
OL
will be below 0.4V, and the slew rate
will actually be faster than calculated. Choosing R
P
= 10k,
V
OL
and SR are recalculated.
V
OL
=
300Ω•3.3V
300Ω+10kΩ
=96mV
SR =
3.3V – 96mV
10kΩ•600pF
=0.534V / µs
(12)
The rise and fall times need to be verifi ed using equations
3 and 4.
t
r
= –10kΩ•600pF •
In
2.31V – 3.3V – 10kΩ • 2.5mA
0.99V – 3.3V – 10kΩ • 2.5mA
⎧
⎨
⎩
⎫
⎬
⎭
= 0.297µs
(13)
t
f
= 291Ω•600pF •
In
2.31
3.3V
•(10kΩ+300Ω) – 300Ω
0.99V
3.3V
•(10kΩ+300Ω) – 300Ω
⎧
⎨
⎪
⎩
⎪
⎫
⎬
⎪
⎭
⎪
= 0.158µs
(14)
Both the rise and fall times meet the 0.3μs I
2
C requirement
and the V
OL
is satisfi ed, while meeting the minimum slew
rate requirement, so R
P
is chosen to be 10k.
If t
r
is not met, R
P
should be decreased and if t
f
is not met
then R
P
should be increased.
SMBus Design Example
Given the following conditions and requirements for a low
power SMBus system:
V
CC
= 3.3V NOMINAL
V
OL
= 0.4V MAXIMUM
C
BUS
= 400pF
V
ILMAX
= 0.8V,V
IHMIN
= 2.1V
t
r
= 1µs MAXIMUM,t
f
= 0.3µs MAXIMUM
(15)
If an R
S
of 100Ω is used and the max R
ON
of the driver is
200Ω, then R
L
= 200Ω + 100Ω = 300Ω. Use equation 1
to fi nd the required R
P
to meet V
OL
.
R
P
=
300Ω•(3.3V – 0.4V)
0.4V
R
P
= 2.175k
(16)
Calculate Maximum R
P
from equation 2.
R
P
=
3.3V – 0.4V
400pF•0.5V / µs
R
P
= 14.5k
(17)
Choose R
P
= 13k and recalculate V
OL
and SR.
V
OL
=
300Ω•3.3V
300Ω+13kΩ
= 74mV
SR =
3.3V – 74mV
13kΩ•400pF
= 0.62V / µs
(18)
The rise and fall times need to be verifi ed using equations
5 to 8.
t
1
= –13kΩ•400pF•
ln
0.9V–3.3V
0.8V–0.15V–3.3V
⎧
⎨
⎩
⎫
⎬
⎭
= 0.515µs
(19)