5

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

µA of pull-up

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/

µs threshold of the

slew rate detector. The boosted pull-up current remains on

and/or the slew rate drops below 0.2V/

µs.

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

µA, thereby lowering the

system power consumption.

(Figure 2) is sometimes added to the open-drain driver of

the bus agents. This is especially common in SMBus-

limited by the low state noise margin and timing require-

ments of the SMBus specification. The maximum value for

sources are used.

protection while meeting both the low state noise margin

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

IN

DATA

OUT

Figure 2

Low State Noise Margin

low pull-up current (350

µA maximum) of the LTC1694.

R

VNM

I

R

S MAX

OL MAX

L

PU

MAX

ON MAX

()

()

()

()

=

−

−

LL-UP

(1)

tion is 0.4V