LTC4311 APPLICATIONS INFORMATION For an I2C system with fi xed input levels, VILMAX = 1.5V A general procedure for selecting RP and RL is as fol- and VIHMIN = 3V. For I2C systems with VCC related input lows: levels, VILMAX = 0.3VCC and VIHMIN = 0.7VCC. 1. RL is fi rst selected based on the I/O protection CBUS is the total capacitance of the I2C line. requirement. Generally, an RS of 100Ω is suffi cient for high voltage spikes and ESD protection. RON is SMBus Rise and Fall Time determined by the size of the open-drain driver, a large driver will have a lower R Rise time of a SMBus line is derived using equations 5, ON. 6 and 7. 2. The value of RP is determined based on the VOL and minimum slew rate requirements. The V t = t + t (5) OL will determine r 1 2 the smallest resistance value that can be used in a t1 is the time from when the bus crosses the lower slew system, and the minimum slew requirement will bound rate measurement point, until the bus reaches VTHR and the the resistance on the upper end. Generally the largest accelerators fi re. The time from when the accelerators fi re value of resistance that meets the minimum slew rate until the bus reaches the upper slew rate measure point is with some margin will be selected. given by t2. Equations for t1 and t2 are given here: 3. For I2C systems incorporating the LTC4311, the rise ⎧ V – V ⎫ THR CC times are met under most loading conditions, due to t = –R •C •ln 1 P BUS ⎨ ⎬ V – 0.15V – V the strong accelerator current. The pull-down drivers ⎩ ILMAX CC ⎭ (6) are typically low impedance, and therefore fall times If (V are not generally an issue. Rise and fall time ILMAX – 0.15V) > VTHR, then t1 = 0 requirements must be verifi ed using equations 3 and 4 (for an I2C system) or equations 5 to 8 (for an SMBus t = –R •C • 2 P BUS system). The value chosen for RP must ensure that ⎧V + 0.15V – V – R ⎫ both the rise and fall time specifi cations are met •I ln IHMIN CC P PULLUPAC ⎨ ⎬ simultaneously. V – V – R •I ⎩ THR CC P PULLUP AC ⎭ (7) I2C Design Example Fall time of an SMBus line is derived using equation 8: Given the following conditions and requirements: t = R •C • V f T BUS = 3.3V NOMINAL CC ⎧ V + 0.15V V ⎫ = 0.4V MAXIMUM IHMIN OL • (R + R ) – R ⎪⎪ V P L L ⎪ ⎪ C CC = 600pF BUS ln ⎨ ⎬ V – 0.15V ILMAX V = 0.99V,V = 2.31V ⎪ ⎪ ILMAX IHMIN • (R + R ) – R ⎪ V P L L ⎪ ⎩ CC ⎭ t = 0.3µs MAXIMUM,t = 0.3µs MAXIMUM r f (9) (8) If an RS of 100Ω is used and the max RON of the driver is For an SMBus system, VILMAX = 0.8V and VIHMIN = 2.1V. 200Ω, then RL = 200Ω + 100Ω = 300Ω. Use equation 1 to fi nd the required RP to meet VOL. CBUS is the total bus capacitance of the SMBus line. 300Ω•(3.3V – 0.4V) R = P 0.4V R = 2.175k P (10) 4311fa 8
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