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1
LTC1694-1
SMBus/I
2
C Accelerator
s
Improves SMBus/I
2
C
TM
Rise Time Transition
s
Ensures Data Integrity with Multiple Devices
on the SMBus/I
2
C
s
Improves Low State Noise Margin
s
Wide Supply Voltage Range: 2.7V to 6V
s
Tiny 5-Pin SOT-23 Package
s
Parallel Multiple LTC1694-1 Devices
for Increased Drive
The LTC
1694-1 is a dual SMBus active pull-up designed
to enhance data transmission speed and reliability under
all specified SMBus loading conditions. The LTC1694-1 is
also compatible with the Philips I
2
C Bus.
The LTC1694-1 allows multiple device connections or a
longer, more capacitive interconnect, without compro-
mising slew rates or bus performance, by supplying a high
pull-up current of 2.2mA to slew the SMBus or I
2
C lines
during positive bus transitions
During negative transitions or steady DC levels, the
LTC1694-1 sources zero current. External resistors, one
on each bus line, trigger the LTC1694-1 during positive
bus transitions and set the pull-down current level. These
resistors determine the slew rate during negative bus
transitions and the logic low DC level.
The LTC1694-1 is available in a 5-pin SOT-23 package.
s
Notebook and Palmtop Computers
s
Portable Instruments
s
Battery Chargers
s
Industrial Control Application
s
TV/Video Products
s
ACPI SMBus Interface
, LTC and LT are registered trademarks of Linear Technology Corporation.
I
2
C is a trademark of Philips Electronics N.V.
V
CC
= 5V
1
s/DIV
1694-1 TA02
C
LD
= 200pF
f
SMBus
= 100kHz
Comparison of SMBus Waveforms for
the LTC1694-1 vs Resistor Pull-Up
LTC1694-1
1V/DIV
R
PULL-UP
= 15.8k
LTC1694-1: Patent Pending
LTC1694-1
V
CC
GND
V
CC
5V
C1
0.1
F
SMBus1
SMBus2
1
2
5
4
SCL
SDA
DEVICE 1
CLK
IN
CLK
OUT
SMBus
V
CC
5V
DATA
IN
DATA
OUT
DEVICE N
1694-1 TA01
CLK
IN
R
P2
CLK
OUT
DATA
IN
DATA
OUT
R
P1
DESCRIPTIO
U
FEATURES
APPLICATIO S
U
TYPICAL APPLICATIO
U
2
LTC1694-1
ABSOLUTE
M
AXI
M
U
M
RATINGS
W
W
W
U
PACKAGE/ORDER I
N
FOR
M
ATIO
N
W
U
U
ORDER PART
NUMBER
LTC1694-1CS5
(Note 1)
Supply Voltage (V
CC
) ................................................. 7V
SMBus1, SMBus2 Inputs ............ 0.3V to (V
CC
+ 0.3V)
Operating Ambient Temperature Range ....... 0
C to 70
C
Junction Temperature ........................................... 125
C
Storage Temperature Range ................. 65
C to 150
C
Lead Temperature (Soldering, 10 sec.)................. 300
C
V
CC
1
GND 2
NC 3
5 SMBus1
4 SMBus2
TOP VIEW
S5 PACKAGE
5-LEAD PLASTIC SOT-23
T
JMAX
= 125
C,
JA
= 256
C/ W
Consult factory for Industrial and Military grade parts.
LTHE
S5 PART MARKING
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
C. V
CC
= 2.7V to 6V, unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
CC
Supply Voltage Range
2.7
6
V
I
CC
Supply Current
SMBus1 = SMBus2 = V
CC
q
15
45
80
A
I
PULL-UP
Pull-Up Current
Positive Transition on SMBus ( Figure 1)
q
1.0
2.2
mA
Slew Rate = 0.5V/
s, SMBus > V
THRES
V
THRES
Input Threshold Voltage
Slew Rate = 0.5V/
s (Figure 1)
q
0.4
0.65
0.9
V
SR
THRES
Slew Rate Detector Threshold
SMBus > V
THRES
q
0.2
0.5
V/
s
t
r
SMBus Rise Time
Bus Capacitance = 200pF (Note 2)
q
0.32
1.0
s
Standard Mode I
2
C Bus Rise Time
Bus Capacitance = 400pF (Note 3)
q
0.30
1.0
s
f
MAX
SMBus Maximum Operating Frequency
(Note 4)
q
100
kHz
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: The rise time of an SMBus line is calculated from (V
IL(MAX)
0.15V) to (V
IH(MIN)
+ 0.15V) or 0.65V to 2.25V. This parameter is
guaranteed by design and not tested. With a minimum initial slew rate of
0.5V/
s, a minimum pull-up current of 1mA and a maximum input
threshold voltage of 0.9V:
Rise Time = [(0.9V 0.65V)/0.5V/
s] + [(2.25V 0.9V) 200pF/1mA]
= 0.77
s
Note 3: The rise time of an I
2
C bus line is calculated from V
IL(MAX)
to
V
IH(MIN)
or 1.5V to 3V (with V
CC
= 5V). This parameter is guaranteed by
design and not tested. With a minimum boosted pull-up current of 1mA:
Rise Time = (3V 1.5V) 400pF/1mA = 0.6
s
Note 4: This parameter is guaranteed by design and not tested.
3
LTC1694-1
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
TEMPERATURE (
C)
50
PULL-UP CURRENT (mA)
3.50
3.25
3.00
2.75
2.50
2.25
2.00
1.75
1.50
1.25
1.00
0
50
75
1694-1 G01
25
25
100
125
V
CC
= 6V
V
CC
= 5V
V
CC
= 2.7V
Pull-Up Current
vs SMBus Voltage
TEMPERATURE (
C)
50
INPUT THRESHOLD VOLTAGE (V)
0.90
0.85
0.80
0.75
0.70
0.65
0.60
0.55
0.50
0.45
0.40
0
50
75
1694 G03
25
25
100
125
V
CC
= 6V
V
CC
= 2.7V
V
CC
= 5V
TEMPERATURE (
C)
50
SLEW RATE DETECTOR THRESHOLD (V/
s)
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
0
50
75
1694 G04
25
25
100
125
V
CC
= 6V
V
CC
= 2.7V
V
CC
= 5V
Slew Rate Detector Threshold
Standby Mode Supply Current
TEMPERATURE (
C)
50
SUPPLY CURRENT (
A)
100
1694-1 G05
0
50
80
70
60
50
40
30
20
10
25
25
75
125
V
CC
= 6V
V
CC
= 2.7V
V
CC
= 5V
Pull-Up Current
SMBus VOLTAGE (V)
0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
3
5
LT
1694 G02
1
2
4
6
7
PULL-UP CURRENT (mA)
V
CC
= 6V
V
CC
= 5V
V
CC
= 2.7V
Input Threshold Voltage
4
LTC1694-1
PI
N
FU
N
CTIO
N
S
U
U
U
V
CC
(Pin 1): Power Supply Input. V
CC
can range from 2.7V
to 6V and requires a 0.1
F bypass capacitor to GND.
Supply current is typically 45
A when the SMBus or I
2
C
lines are inactive (SCL and SDA are a logic high level).
GND (Pin 2): Ground.
NC (Pin 3): No Connection.
SMBus2 (Pin 4): Active pull-up for SMBus.
SMBus1 (Pin 5): Active pull-up for SMBus.
BLOCK DIAGRA
M
W
+
SLEW RATE
DETECTOR
CONTROL
LOGIC
0.65V
V
REF
VOLTAGE
COMP
2.2mA
CHANNEL ONE
CHANNEL TWO
(DUPLICATE OF CHANNEL ONE)
1694-1 BD
1
V
CC
5
SMBus1
SMBus2
2
GND
4
TEST CIRCUITS
Figure 1
LTC1694-1
V
CC
GND
V
CC
5V
C1
0.1
F
SMBus1
SMBus2
5
4
HP5082-2080
TEST RAMP VOLTAGE
BSS284
V
R
1k
10V
1694-1 F01a
+
LT
1360
V
CC
5V
200
A
PULL-UP =
2.2mA (TYP)
200
A
(TYP)
TEST RAMP
VOLTAGE
0
A
V
CC
1694-1 F01b
0.5V/
s
0V
V
THRES
I
PULL-UP
=
V
R
1k
5
LTC1694-1
APPLICATIO
N
S I
N
FOR
M
ATIO
N
W
U
U
U
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
CC
and/or the
slew rate drops below 0.2V/
s.
Selecting the Values of R
S
and R
P
An external pull-up resistor R
P
is required in each SMBus
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
P
to supply the steady state pull-up
current permits the user the freedom to adjust rise time
versus fall time as well as defining the low state logic level
(V
OL
).
For I/O stage protection from ESD and high voltage spikes
on the SMBus, a series resistor R
S
(Figure 2) is sometimes
added to the open-drain driver of the bus agents. This is
especially common in SMBus-controlled smart batteries.
Both the values of R
P
and R
S
must be chosen carefully to
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
S
and R
P
, as well as a general procedure for
selecting the values of R
S
and R
P
follows.
Figure 2
V
CC
R
S
C
BUS
SMBus
R
ON
1694-1 F02
DATA
IN
DATA
OUT
R
P
Low State Noise Margin
A low value of V
OL
, the low state logic level, is desired for
good noise margin. V
OL
is calculated as follows:
V
OL
= (R
L
V
CC
)/(R
L
+ R
P
)
(1)
R
L
is the series sum of R
S
and R
ON
, the on-resistance of
the open-drain driver.
Increasing the value of R
P
decreases the value of V
OL
.
Increasing R
L
increases the value of V
OL
.
Initial Slew Rate
The initial slew rate, SR, of the Bus is determined by:
SR = (V
CC
V
OL
)/(R
P
C
BUS
)
(2)
SR must be greater than SR
THRES
, the LTC1694-1 slew
rate detector threshold (0.5/
s max) in order to activate
the 2.2mA pull-up current.
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