Datasheet LTC1694-1 (Analog Devices) - 5

LTC1694-1
U U W U APPLICATIONS INFORMATION SMBus Overview
For I/O stage protection from ESD and high voltage spikes on the SMBus, a series resistor R SMBus communication protocol employs open-drain S (Figure 2) is sometimes added to the open-drain driver of the bus agents. This is drives with resistive or current source pull-ups. This pro- especially common in SMBus-controlled smart batteries. tocol allows multiple devices to drive and monitor the bus without bus contention. The simplicity of resistive or fixed Both the values of RP and RS must be chosen carefully to current source pull-ups is offset by the slow rise times meet the low state noise margin and all timing require- resulting when bus capacitance is high. Rise times can be ments of the SMBus. improved by using lower pull-up resistor values or higher A discussion of the electrical parameters affected by the fixed current source values, but the additional current values of R increases the low state bus voltage, decreasing noise S and RP, as well as a general procedure for selecting the values of R margins. Slow rise times can seriously impact data reli- S and RP follows. ability, enforcing a maximum practical bus speed well VCC below the established SMBus maximum transmission rate. RP SMBus
Theory of Operation
R C S BUS The LTC1694-1 overcomes these limitations by providing a 2.2mA pull-up current only during positive bus transi- DATA IN tions to quickly slew any bus capacitance. Therefore, rise DATA time is dramatically improved, especially with maximum RON OUT SMBus loading conditions. The LTC1694-1 has separate but identical circuitry for 1694-1 F02 each SMBus output pin. The circuitry consists of a positive
Figure 2
edge slew rate detector and a voltage comparator. The 2.2mA pull-up current is only turned on if the voltage
Low State Noise Margin
on the SMBus line voltage is greater than the 0.65V A low value of VOL, the low state logic level, is desired for comparator threshold voltage and the positive slew rate of good noise margin. VOL is calculated as follows: the SMBus line is greater than the 0.2V/µs threshold of the slew rate detector. The pull-up current remains on until the VOL = (RL • VCC)/(RL + RP) (1) voltage on the SMBus line is within 0.5V of VCC and/or the RL is the series sum of RS and RON, the on-resistance of slew rate drops below 0.2V/µs. the open-drain driver.
Selecting the Values of R
Increasing the value of R
S and RP
P decreases the value of VOL. Increasing RL increases the value of VOL. An external pull-up resistor RP is required in each SMBus line to supply a steady state pull-up current if the SMBus
Initial Slew Rate
is at logic zero. This pull-up current is used for slewing the The initial slew rate, SR, of the Bus is determined by: SMBus line during the initial portion of the positive transi- tion in order to activate the LTC1694-1 2.2mA pull-up SR = (VCC – VOL)/(RP • CBUS) (2) current. SR must be greater than SRTHRES, the LTC1694-1 slew Using an external RP to supply the steady state pull-up rate detector threshold (0.5/µs max) in order to activate current permits the user the freedom to adjust rise time the 2.2mA pull-up current. versus fall time as well as defining the low state logic level (VOL). 16941fa 5