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LTC1694

SMBus/I

C Accelerator

Improves SMBus Rise Time Transition

Ensures Data Integrity with Multiple Devices
on the SMBus

Improves Low State Noise Margin

Auto Detect Low Power Standby Mode

Wide Supply Voltage Range: 2.7V to 6V

Tiny 5-Pin SOT-23 Package

The LT C

1694 is a dual SMBus active pull-up designed to

enhance data transmission speed and reliability under all
specified SMBus loading conditions. The LTC1694 is also
compatible with the Philips I

Bus.

The LTC1694 allows multiple device connections or a
longer, more capacitive interconnect, without compro-
mising slew rates or bus performance, by using two
bilevel hysteretic current source pull-ups.

During positive bus transitions, the LTC1694 current
sources provide 2.2mA to quickly slew the SMBus line.
During negative transitions or steady DC levels, the cur-
rent sources decrease to 275

A to improve negative slew

rate and improve low state noise margins. An auto detect
standby mode reduces supply current if both SCL and
SDA are high.

The LTC1694 is available in a 5-pin SOT-23 package,
requiring virtually the same space as two surface mount
resistors.

Notebook and Palmtop Computers

Portable Instruments

Battery Chargers

Industrial Control Application

TV/Video Products

ACPI SMBus Interface

LTC1694

GND

C1
0.1

SMBus1

SMBus2

SCL

SDA

DEVICE 1

CLK

OUT

SMBus

DATA

OUT

DEVICE N

1694 TA01

CLK

OUT

DATA

OUT

, LTC and LT are registered trademarks of Linear Technology Corporation.

C is a trademark of Philips Electronics N.V.

= 5V

s/DIV

= 200pF

SMBus

= 100kHz

1694 TA02

Comparison of SMBus Waveforms for

the LTC1694 vs Resistor Pull-Up

LTC1694

PULL-UP

= 15.8k

LTC1694: Patent Pending

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

1V/DIV

LTC1694

ORDER PART

NUMBER

LTC1694CS5

(Note 1)

Supply Voltage (V

) ................................................. 7V

SMBus1, SMBus2 Inputs ............ 0.3V to (V

+ 0.3V)

Operating Ambient Temperature Range ....... 0

C to 70

Junction Temperature ........................................... 125

Storage Temperature Range ................. 65

C to 150

Lead Temperature (Soldering, 10 sec.)................. 300

GND 2

NC 3

5 SMBus1

4 SMBus2

TOP VIEW

S5 PACKAGE

5-LEAD PLASTIC SOT-23

JMAX

= 125

= 256

C/ W

Consult factory for Industrial and Military grade parts.

LTEE

S5 PART MARKING

The

denotes specifications that apply over the full operating temperature range, otherwise specifications are at T

= 25

= 2.7V to 6V unless otherwise noted.

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

Supply Voltage Range

2.7

Supply Current

SMBus1 = SMBus2 = Open

100

PULL-UP

Pull-Up Current

SMBus1 = SMBus2 = 0V

125

275

350

Boosted Pull-Up Current

Positive Transition on SMBus ( Figure 1)

1.0

2.2

Slew Rate = 0.5V/

s, SMBus > V

THRES

Input Threshold Voltage

Slew Rate = 0.5V/

s (Figure 1)

0.4

0.65

0.9

THRES

Slew Rate Detector Threshold

SMBus > V

THRES

0.2

0.5

SMBus Rise Time

Bus Capacitance = 200pF (Note 2)

0.32

1.0

Standard Mode I

C Bus Rise Time

Bus Capacitance = 400pF (Note 3)

0.30

1.0

MAX

SMBus Maximum Operating Frequency

(Note 4)

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 pull-up current of
125

A, a minimum boosted pull-up current of 1mA and a maximum input

threshold voltage of 0.9V:

Rise Time = [(0.9V 0.65V)/125

A + (2.25V 0.9V)/1mA] · 200pF

= 0.67

Note 3: The rise time of an I

C bus line is calculated from V

IL(MAX)

IH(MIN)

or 1.5V to 3V (with V

= 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

Note 4: This parameter is guaranteed by design and not tested.

ABSOLUTE AXI U RATI GS

PACKAGE/ORDER I FOR ATIO

ELECTRICAL CHARACTERISTICS

LTC1694

TEMPERATURE (

PULL-UP CURRENT (

350

325

300

275

250

225

200

175

150

125

100

1694 G01

100

125

= 6V

= 5V

= 2.7V

TEMPERATURE (

BOOSTED 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

1694 G02

100

125

= 6V

= 5V

= 2.7V

Boosted Pull-Up Current vs
SMBus Voltage

TEMPERATURE (

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

1694 G04

100

125

= 6V

= 2.7V

= 5V

TEMPERATURE (

SLEW RATE DETECTOR THRESHOLD (V/

0.50

0.45

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

1694 G05

100

125

= 6V

= 2.7V

= 5V

Slew Rate Detector Threshold

Standby Mode Supply Current

TEMPERATURE (

SUPPLY CURRENT (

100

1694 G06

100

125

= 6V

= 2.7V

= 5V

Pull-Up Current at SMBus = 0V

Boosted Pull-Up Current

SMBus VOLTAGE (V)

3.5

3.0

2.5

2.0

1.5

1.0

0.5

1694 G03

BOOSTED PULL-UP CURRENT (mA)

= 6V

= 5V

= 2.7V

Input Threshold Voltage

TYPICAL PERFOR A CE CHARACTERISTICS

(Pin 1): Power Supply Input. V

can range from 2.7V

to 6V and requires a 0.1

F bypass capacitor to GND.

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.

PI FU CTIO S

LTC1694

SMBus Overview

SMBus communication protocol employs open-drain
drivers with resistive or current source pull-ups. This
protocol 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 they afford 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 reliability, enforcing a maximum practical
bus speed well below the established SMBus maximum
transmission rate.

Theory of Operation

The LTC1694 overcomes these limitations by using bilevel
hysteretic current sources as pull-ups. During positive
SMBus line transitions, the pull-up current sources typi-
cally provide 2.2mA to quickly slew any parasitic bus
capacitance. Therefore, rise time is dramatically improved,
especially with maximum SMBus loading conditions.

The LTC1694 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 LTC1694 nominally sources only 275

A of pull-up

current to maintain good V

noise margin. The 2.2mA

boosted pull-up current is only turned on if the voltage on
the SMBus line voltage is greater than the 0.65V compara-
tor 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 boosted pull-up current remains on
until the voltage on the SMBus line is within 0.5V of V

and/or the slew rate drops below 0.2V/

Auto Detect Standby Mode

The LTC1694 enters standby mode if the voltage on both
the SCL and SDA lines is high (idle state). In standby mode,
the pull-up currents drop to 100

A, thereby lowering the

system power consumption.

Maximum R

Considerations

For ESD protection of the SMBus lines, a series resistor R

(Figure 2) is sometimes added to the open-drain driver of
the bus agents. This is especially common in SMBus-
controlled smart batteries. The maximum value of R

limited by the low state noise margin and timing require-
ments of the SMBus specification. The maximum value for
R

is 700

if resistive pull-ups or fixed value current

sources are used.

In general, an R

of 100

to 200

is sufficient for ESD

protection while meeting both the low state noise margin
and fall time requirement. If a larger value of R

is required,

take care to ensure that the low state noise margin and
timing requirement of the SMBus specification is not
violated. Also, the fall time of an SMBus line will also be
increased by using a high value series resistor.

APPLICATIO S I FOR ATIO

SDA

1694 F02

DATA

OUT

Figure 2

Low State Noise Margin

An acceptable V

noise margin is easily achieved with the

low pull-up current (350

A maximum) of the LTC1694.

The maximum value of R

is calculated from a desired low

state noise margin (NM

S MAX

OL MAX

MAX

ON MAX

(

)

(

)

(

)

(

)

LL-UP

(1)

OL(MAX)

The maximum V

of the SMBus specifica-

tion is 0.4V

Part Number LTC1694