LTC1694-1CS5#PBF Linear Technology, LTC1694-1CS5#PBF Datasheet - Page 5

• Manufacturer Part Number: LTC1694-1CS5#PBF
• Manufacturer: Linear Technology

Specifications of LTC1694-1CS5#PBF

• Operating Temperature (min): 0C
• Operating Temperature Classification: Commercial
• Operating Temperature (max): 70C
• Rad Hardened: No
• Lead Free Status / RoHS Status: Compliant

SMBus Overview
SMBus communication protocol employs open-drain
drives with resistive or current source pull-ups. This pro-
tocol allows multiple devices to drive and monitor the bus
without bus contention. The simplicity of resistive or fixed
current source pull-ups is offset by the slow rise times
resulting when bus capacitance is high. Rise times can be
improved by using lower pull-up resistor values or higher
fixed current source values, but the additional current
increases the low state bus voltage, decreasing noise
margins. Slow rise times can seriously impact data reli-
ability, enforcing a maximum practical bus speed well
below the established SMBus maximum transmission rate.
Theory of Operation
The LTC1694-1 overcomes these limitations by providing
a 2.2mA pull-up current only during positive bus transi-
tions to quickly slew any bus capacitance. Therefore, rise
time is dramatically improved, especially with maximum
SMBus loading conditions.
The LTC1694-1 has separate but identical circuitry for
each SMBus output pin. The circuitry consists of a positive
edge slew rate detector and a voltage comparator.
The 2.2mA pull-up current is only turned on if the voltage
on the SMBus line voltage is greater than the 0.65V
comparator threshold voltage and the positive slew rate of
the SMBus line is greater than the 0.2V/ s threshold of the
slew rate detector. The pull-up current remains on until the
voltage on the SMBus line is within 0.5V of V
slew rate drops below 0.2V/ s.
Selecting the Values of R
An external pull-up resistor R
line to supply a steady state pull-up current if the SMBus
is at logic zero. This pull-up current is used for slewing the
SMBus line during the initial portion of the positive transi-
tion in order to activate the LTC1694-1 2.2mA pull-up
current.
Using an external R
current permits the user the freedom to adjust rise time
versus fall time as well as defining the low state logic level
(V
APPLICATIONS
OL
).
U
P
to supply the steady state pull-up
INFORMATION
U
S
and R
P
is required in each SMBus
P
W
CC
and/or the
U
For I/O stage protection from ESD and high voltage spikes
on the SMBus, a series resistor R
added to the open-drain driver of the bus agents. This is
especially common in SMBus-controlled smart batteries.
Both the values of R
meet the low state noise margin and all timing require-
ments of the SMBus.
A discussion of the electrical parameters affected by the
values of R
selecting the values of R
Low State Noise Margin
A low value of V
good noise margin. V
R
the open-drain driver.
Increasing the value of R
Increasing R
Initial Slew Rate
The initial slew rate, SR, of the Bus is determined by:
SR must be greater than SR
rate detector threshold (0.5/ s max) in order to activate
the 2.2mA pull-up current.
L
V
SR = (V
is the series sum of R
OL
= (R
CC
S
L
L
• V
and R
– V
DATA
DATA
OUT
increases the value of V
IN
OL
CC
OL
, the low state logic level, is desired for
)/(R
)/(R
P
P
, as well as a general procedure for
and R
OL
L
P
+ R
S
is calculated as follows:
Figure 2
• C
R
R
R
S
P
S
ON
P
and R
V
and R
S
P
CC
BUS
decreases the value of V
)
THRES
must be chosen carefully to
1694-1 F02
)
S
P
ON
(Figure 2) is sometimes
follows.
, the LTC1694-1 slew
, the on-resistance of
C
LTC1694-1
OL
BUS
SMBus
.
16941fa
5
(1)
OL
(2)
.