ISL88694

FN8239 Rev 1.00

Page 6 of 7

January 21, 2005

Functional Description

SMBus Overview

that more than one device connected to the bus is capable of

controlling it. Master devices communicate to other master or

slave devices using one clock and one data line. These are

both bidirectional.

In order to allow multimaster operation without bus contention,

it is necessary to allow each bus to be connected to a positive

supply voltage via a current-source or pull-up resistor (see

“System Diagram” on page 2). When the bus is free, both lines

are HIGH. The output stages of devices connected to the bus

must have an open-drain or open-collector to perform the

wired-AND function.

Simple pull-up resistors on the clock and data lines work well

unless there are long signal lines or many devices connected

to the bus. Then, the combined capacitance of the bus

increases the rise time on the signal to such an extent that the

communication becomes unreliable or fails to meet the bus

timing specifications. Smaller resistors can sometimes

compensate for the extra capacitance, but this increases the

current consumption when the signal lines are pulled LOW.

ISL88694 Operation

To improve the operation of the SMBus where larger bus

capacitance exists, the ISL88694 provides active pull-up using

switched current sources. When the bus is idle and both lines

are HIGH, a standby pull-up current of 100µA is used to

maintain the signal level while minimizing power consumption.

When either of the two signals is pulled LOW, an active pull-up

When the bus line is released, it is pulled high by the ISL88694

period of time. This voltage-time combination filters out noise

on the signal line. Once the ISL88694 detects a valid rising

edge, a 2.2mA boost current pulls the bus line high very

quickly (see Figure 8). This boost current turns off when the

current returns to the active level. If both inputs are HIGH, the

pull-up current drops to the standby level of 100µA.

FIGURE 8. ISL88694 SMBus SYSTEM BOOST PULL-UP

FIGURE 9. ISL88694 SMBus SYSTEM BOOST PULL-UP

FIGURE 10. NOISE SUPPRESSION. BOOST CURRENT APPLIED

WHEN INPUT SIGNAL EXCEEDS 20Vns (TYPICAL).

With RC

Pullup

With ISL88694 SMA

R=15.8K

C= 200pF

With RC

Pullup

With ISL88694 SMA

R=15.8K

C= 200pF

Gnd

t

SMBus1

>20Vns

SMBus2

With SMA

Without SMA

(Typical)