ltc1694-1is5-trpbf Linear Technology Corporation, ltc1694-1is5-trpbf Datasheet - Page 6

• Manufacturer Part Number: ltc1694-1is5-trpbf
• Description: Smbus/i?c Accelerator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC1694
R
I
Fall Time
Fall time is a function of the SMBus capacitance, R
and the pull-up current. Figure 3 shows the maximum
allowed (R
requirement of 300ns with a 50ns safety margin.
The maximum value of R
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 R
Initial Slew Rate
The initial slew rate, SR, of the Bus is determined by:
C
I
(125 A).
SR must be greater than SR
detector threshold (0.5V/ s max) in order to activate the
2.2mA boosted pull-up current. This limits the maximum
SMBus capacitance.
6
PULL-UP(MAX)
PULL-UP(MIN)
BUS
ON(MAX)
SR = I
Figure 3. Maximum Value of R
Capacitance for Meeting the SMBus t
is the total capacitance of the SMBus line.
PULL-UP(MIN)
:
1.4
1.2
1.0
0.8
0.6
0.4
0.2
S
0
0
+ R
: The maximum LTC1694 low pull-up cur-
is the LTC1694 minimum pull-up current
V
The maximum on resistance of the open-
drain driver
rent is 350 A
CC
ON
= 5V
100
) based on the Intel SMBus fall time
U
BUS CAPACITANCE (pF)
/C
BUS
200
U
S
, based on fall time require-
THRES
S
300
+ R
, the LTC1694 slew rate
ON
W
f(MAX)
400
as a Function of Bus
1694 F03
Requirement
500
U
S
, R
(2)
ON
S
.
SMBus Rise Time
Rise time of an SMBus line is derived using equations 3,
4 and 5.
if V
I
typ).
For an SMBus system, V
For the LTC1694, typically V
I
C
SMBus Fall Time
Fall time of an SMBus line is derived using equation 6.
where R
Rise and fall time calculation for an I
follows.
I
Rise time of an I
Fall time of the I
For an I
and V
For an I
0.3 • V
C
PULL-UP(B)
PULL-UP
2
BUS
BUS
C Bus Rise and Fall Time
t
[V
t
t
C
t
f
t
t
[V
r
1
IL(MAX)
2
r
f
= R
BUS
IL(MAX)
= R
= t
= (V
= (V
= (V
IL(MAX)
is the total capacitance of the SMBus line.
is the total capacitance of the I
IH(MIN)
CC
1
L
2
2
/I
L
L
C system with V
C system with fixed input levels, V
= 275 A.
IH(MIN)
+ t
PULL-UP
• C
THRES
IH(MIN)
and V
• C
is the sum of R
– 0.15 > V
is the LTC1694 boosted pull-up current (2.2mA
2
– 0.15 – (R
BUS
– (R
BUS
= 3V.
IH(MIN)
– V
• ln{[(0.9 • V
– V
2
+ 0.15 – V
2
• ln{[V
L
C line can be derived using equation 8.
C line is derived using equation 7.
• I
IL(MAX)
IL(MAX)
PULL-UP
THRES
= 0.7 • V
L
IH(MIN)
IL(MAX)
CC
• I
S
) • C
+ 0.15) •
, then t
THRES
PULL-UP(LOW)
related input levels, V
and R
)]}
CC
– (R
= 0.8V and V
BUS
) – (R
CC
) • C
ON
.
1
THRES
/I
L
= 0 s.
PULL-UP(B)
(see Figure 2).
2
• I
L
2
BUS
C line.
C system is as
• I
PULL-UP
)]}
/I
PULL-UP(LOW)
PULL-UP(B)
= 0.65V and
IL(MAX)
IH(MIN)
)]/
IL(MAX)
= 2.1V.
= 1.5V
1694fa
)]/
(3)
(4)
(5)
(6)
(7)
(8)
=