LTC1694 UUWUAPPLICATIO S I FOR ATIOSMBus OverviewMaximum RS Considerations SMBus communication protocol employs open-drain For ESD protection of the SMBus lines, a series resistor RS drivers with resistive or current source pull-ups. This (Figure 2) is sometimes added to the open-drain driver of protocol allows multiple devices to drive and monitor the the bus agents. This is especially common in SMBus- bus without bus contention. The simplicity of resistive or controlled smart batteries. The maximum value of RS is fixed current source pull-ups is offset by the slow rise limited by the low state noise margin and timing require- times they afford when bus capacitance is high. Rise ments of the SMBus specification. The maximum value for times can be improved by using lower pull-up resistor RS is 700Ω if resistive pull-ups or fixed value current values or higher fixed current source values, but the sources are used. additional current increases the low state bus voltage, In general, an R decreasing noise margins. Slow rise times can seriously S of 100Ω to 200Ω is sufficient for ESD protection while meeting both the low state noise margin impact data reliability, enforcing a maximum practical and fall time requirement. If a larger value of R bus speed well below the established SMBus maximum S is required, take care to ensure that the low state noise margin and transmission rate. timing requirement of the SMBus specification is not violated. Also, the fall time of an SMBus line will also be Theory of Operation increased by using a high value series resistor. The LTC1694 overcomes these limitations by using bilevel hysteretic current sources as pull-ups. During positive SDA SMBus line transitions, the pull-up current sources typi- cally provide 2.2mA to quickly slew any parasitic bus RS capacitance. Therefore, rise time is dramatically improved, DATA especially with maximum SMBus loading conditions. IN DATA The LTC1694 has separate but identical circuitry for each RON OUT SMBus output pin. The circuitry consists of a positive edge slew rate detector and a voltage comparator. 1694 F02 The LTC1694 nominally sources only 275µA of pull-up current to maintain good VOL noise margin. The 2.2mA boosted pull-up current is only turned on if the voltage on Figure 2 the SMBus line voltage is greater than the 0.65V compara- tor threshold voltage and the positive slew rate of the Low State Noise Margin SMBus line is greater than the 0.2V/µs threshold of the An acceptable VOL noise margin is easily achieved with the slew rate detector. The boosted pull-up current remains on low pull-up current (350µA maximum) of the LTC1694. until the voltage on the SMBus line is within 0.5V of VCC The maximum value of RS is calculated from a desired low and/or the slew rate drops below 0.2V/µs. state noise margin (NML): Auto Detect Standby Mode V − NM ( ) R OL MAX L = − R The LTC1694 enters standby mode if the voltage on both S MAX ( ) ON MAX ( ) (1) IPULL-UP MAX ( ) the SCL and SDA lines is high (idle state). In standby mode, the pull-up currents drop to 100µA, thereby lowering the VOL(MAX): The maximum VOL of the SMBus specifica- system power consumption. tion is 0.4V 1694fa 5
