LTC1694-1
SMBus/I
2
C Accelerator
FEATURES
s
s
DESCRIPTIO
s
s
s
s
Improves SMBus/I
2
C
TM
Rise Time Transition
Ensures Data Integrity with Multiple Devices
on the SMBus/I
2
C
Improves Low State Noise Margin
Wide Supply Voltage Range: 2.7V to 6V
Tiny 5-Pin SOT-23 Package
Parallel Multiple LTC1694-1 Devices
for Increased Drive
APPLICATIO S
s
s
s
s
s
s
Notebook and Palmtop Computers
Portable Instruments
Battery Chargers
Industrial Control Application
TV/Video Products
ACPI SMBus Interface
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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
I
2
C is a trademark of Philips Electronics N.V.
TYPICAL APPLICATIO
V
CC
5V
C1
0.1µF
1
V
CC
2
GND
SMBus2
SMBus1
LTC1694-1
5
V
CC
5V
Comparison of SMBus Waveforms for
the LTC1694-1 vs Resistor Pull-Up
4
R
P1
R
P2
SCL
SMBus SDA
LTC1694-1
1V/DIV
CLK
IN
CLK
OUT
DEVICE 1
DATA
IN
DATA
OUT
CLK
IN
CLK
OUT
DEVICE N
DATA
IN
DATA
OUT
V
CC
= 5V
C
LD
= 200pF
f
SMBus
= 100kHz
1µs/DIV
1694-1 TA02
1694-1 TA01
LTC1694-1: Patent Pending
U
R
PULL-UP
= 15.8k
U
U
1
LTC1694-1
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW
V
CC
1
GND 2
NC 3
4 SMBus2
5 SMBus1
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
ORDER PART
NUMBER
LTC1694-1CS5
S5 PART MARKING
LTHE
S5 PACKAGE
5-LEAD PLASTIC SOT-23
T
JMAX
= 125°C,
θ
JA
= 256°C/ W
Consult factory for Industrial and Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL
V
CC
I
CC
I
PULL-UP
V
THRES
SR
THRES
t
r
f
MAX
PARAMETER
Supply Voltage Range
Supply Current
Pull-Up Current
Input Threshold Voltage
Slew Rate Detector Threshold
SMBus Rise Time
Standard Mode I
2
C Bus Rise Time
SMBus Maximum Operating Frequency
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.
CONDITIONS
SMBus1 = SMBus2 = V
CC
Positive Transition on SMBus ( Figure 1)
Slew Rate = 0.5V/µs, SMBus > V
THRES
Slew Rate = 0.5V/µs (Figure 1)
SMBus > V
THRES
Bus Capacitance = 200pF (Note 2)
Bus Capacitance = 400pF (Note 3)
(Note 4)
q
q
q
q
q
q
q
MIN
2.7
15
1.0
0.4
TYP
45
2.2
0.65
0.2
0.32
0.30
MAX
6
80
UNITS
V
µA
mA
0.9
0.5
1.0
1.0
100
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/µs, a minimum pull-up current of 1mA and a maximum input
threshold voltage of 0.9V:
Rise Time = [(0.9V – 0.65V)/0.5V/µs] + [(2.25V – 0.9V) • 200pF/1mA]
= 0.77µs
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 = 0.6µs
Note 4:
This parameter is guaranteed by design and not tested.
2
U
W
U
U
W W
W
V
V/µs
µs
µs
kHz
LTC1694-1
TYPICAL PERFORMANCE CHARACTERISTICS
Pull-Up Current
3.50
3.25
3.00
PULL-UP CURRENT (mA)
Pull-Up Current
vs SMBus Voltage
3.5
0.90
0.85
3.0
INPUT THRESHOLD VOLTAGE (V)
PULL-UP CURRENT (mA)
2.5
2.0
1.5
1.0
V
CC
= 6V
V
CC
= 5V
V
CC
= 2.7V
0.5
0
0
1
4
3
5
2
SMBus VOLTAGE (V)
6
7
Slew Rate Detector Threshold
0.50
SLEW RATE DETECTOR THRESHOLD (V/µs)
0.45
0.40
SUPPLY CURRENT (µA)
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
–50
–25
50
0
75
25
TEMPERATURE (°C)
100
125
V
CC
= 5V
V
CC
= 6V
V
CC
= 2.7V
U W
2.75
2.50
2.25
2.00
1.75
1.50
1.25
1.00
–50
–25
50
75
0
25
TEMPERATURE (°C)
100
125
V
CC
= 2.7V
V
CC
= 6V
V
CC
= 5V
1694-1 G01
Input Threshold Voltage
0.80
0.75
0.70 V
CC
= 5V
0.65
0.60
0.55
0.50
0.45
0.40
–50
–25
50
0
75
25
TEMPERATURE (°C)
100
125
V
CC
= 2.7V
V
CC
= 6V
LT1694
G02
1694 G03
Standby Mode Supply Current
80
70
60
50
V
CC
= 6V
40
30
20
10
–50 –25
V
CC
= 5V
V
CC
= 2.7V
0
25
50
75
100
125
TEMPERATURE (°C)
1694 G04
1694-1 G05
3
LTC1694-1
PIN FUNCTIONS
V
CC
(Pin 1):
Power Supply Input. V
CC
can range from 2.7V
to 6V and requires a 0.1µF bypass capacitor to GND.
Supply current is typically 45µA 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 DIAGRAM
TEST CIRCUITS
V
CC
5V
V
CC
C1
0.1µF
SMBus1
4
GND
SMBus2
V
CC
5V
200µA
5
LTC1694-1
200µA
(TYP)
0µA
PULL-UP =
2.2mA (TYP)
I
PULL-UP
=
V
R
1kΩ
–
LT
®
1360
TEST RAMP VOLTAGE
BSS284
+
V
R
1k
–10V
1694-1 F01a
4
W
U
U
U
V
CC
1
2.2mA
SLEW RATE
DETECTOR
CONTROL
LOGIC
SMBus1
5
GND
2
CHANNEL ONE
+
–
VOLTAGE
COMP
0.65V
V
REF
SMBus2
4
CHANNEL TWO
(DUPLICATE OF CHANNEL ONE)
1694-1 BD
HP5082-2080
TEST RAMP
VOLTAGE
0.5V/µs
V
THRES
0V
V
CC
1694-1 F01b
Figure 1
LTC1694-1
APPLICATIONS INFORMATION
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/µs 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/µs.
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
).
R
S
DATA
IN
DATA
OUT
R
ON
U
W
U
U
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.
V
CC
R
P
SMBus
C
BUS
1694-1 F02
Figure 2
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/µs max) in order to activate
the 2.2mA pull-up current.
5
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