Datasheet LTC4311 (Analog Devices) - 9

LTC4311
APPLICATIONS INFORMATION
This is the lowest resistor value that may be chosen and
SMBus Design Example
still meet VOL. Next calculate the largest value of RP that Given the following conditions and requirements for a low will satisfy SR, the minimum slew rate requirement. Us- power SMBus system: ing VOL = 0.4V and SR = 0.5V/μs calculate the value of RP with equation 2. V = 3.3V NOMINAL CC V = 0.4V MAXIMUM R = 3.3V − 0.4V OL P 600pF •0.5V / µs C = 400pF BUS R = 9.667k P V = 0.8V,V = 2.1V ILMAX IHMIN (11) t = 1µs MAXIMUM,t = 0.3µs MAXIMUM r f (15) This is approximately the largest value of RP that will satisfy If an RS of 100Ω is used and the max RON of the driver is the minimum slew rate requirement. Since RP is larger 200Ω, then RL = 200Ω + 100Ω = 300Ω. Use equation 1 than 2.175k the VOL will be below 0.4V, and the slew rate to fi nd the required RP to meet VOL. will actually be faster than calculated. Choosing RP = 10k , V 300Ω•(3.3V – 0.4V) OL and SR are recalculated. R = P 0.4V R = 2.175k V = 300Ω•3.3V =96mV P OL (16) 300Ω+10kΩ Calculate Maximum RP from equation 2. SR = 3.3V – 96mV = 0.534V / µs 10kΩ•600pF 3.3V – 0.4V (12) R = P 400pF •0.5V / µs The rise and fall times need to be verifi ed using equations R = 14.5k P 3 and 4. (17) t = –10kΩ• 600pF • r Choose RP = 13k and recalculate VOL and SR. ⎧ 2.31V – 3.3V – 10kΩ • 2.5mA ⎫ In ⎨ ⎬ = 0.297µs V = 300Ω•3.3V = 74mV 0.99V – 3.3V – 10k ⎩ Ω • 2.5mA ⎭ OL 300Ω + 13kΩ (13) SR = 3.3V – 74mV = 0.62V / µs t = 291Ω• 600pF • f 13kΩ• 400pF (18) ⎧ 2.31 • ⎫ (10kΩ + 300Ω) – 300Ω ⎪ ⎪ The rise and fall times need to be verifi ed using equations In 3.3V ⎨ ⎬ = 0.158µs 5 to 8. 0.99V ⎪ •(10kΩ + 300Ω)– 300Ω ⎪ t = –13kΩ• 400pF • 1 ⎩ 3.3V ⎭ (14) ⎧ 0.9V–3.3V ⎫ ln ⎨ ⎬= 0.515µs 0.8V–0.15V–3.3V ⎩ ⎭ (19) Both the rise and fall times meet the 0.3μs I2C requirement and the VOL is satisfi ed, while meeting the minimum slew rate requirement, so RP is chosen to be 10k. If t r is not met, RP should be decreased and if tf is not met then RP should be increased. 4311fa 9