Datasheet LTC1694-1 (Analog Devices) - 6

LTC1694-1
U U W U APPLICATIONS INFORMATION SMBus Rise Time
For an I2C system with VCC related input levels, VILMAX = 0.3V Rise time of an SMBus line is derived using equations 3, CC and VIHMIN = 0.7VCC. 4 and 5. CBUS is the total capacitance of the I2C line. tr = t1 + t2 (3) A general procedure for selecting RP and RL is as follows: t1 = –RP • CBUS • ln[(VTHRES – VCC)/ 1. RL is first selected based on the I/O protection require- (VILMAX – 0.15 – VCC)] (4) ment. Generally, an RS of 100Ω is sufficient for high if (V voltage spike and ESD protection. RON is determined by ILMAX – 0.15) > VTHRES, then t1 = 0µs. the size of the open-drain driver, a large driver will have t2 = –RP • CBUS • ln{[VIHMIN + 0.15 – VCC – a lower RON. (RP • IPULL-UP)]/[VTHRES – VCC – (RP • IPULL-UP)]} (5) 2. Next, the value of RP is determined based on the rise and By ignoring the current through RP, a simplified version fall time requirements using equations 3 to 7 (for an of equation 3 is obtained: SMBus system) or 8 and 9 (for an I2C system). The t value chosen for R 2 = (VIHMIN + 0.15 – VTHRES) • CBUS/IPULL-UP (6) P must ensure that both the rise and fall time specifications are met simultaneously. For an SMBus system, VILMAX = 0.8V and VIHMIN = 2.1V. For the LTC1694-1, typically VTHRES = 0.65V and IPULL-UP 3. After RP and RL are selected, use equations 1 and 2 to = 2.2mA. check if the VOL and SR requirements are fulfilled. CBUS is the total capacitance of the SMBus line. If SR is too low, decrease the value of RP. If VOL is too high, increase the value of R Increasing the value of R P. P increases the rise time.
SMBus Fall Time SMBus Design Example
Fall time of an SMBus line is derived using equation 7: Given the following conditions and requirements: t VCC = 3.3V nom f = RT • CBUS • ln{[0.9 • (RP + RL) – RL]/ [(V V ILMAX – 0.15) • (RP + RL)/VCC – RL]} (7) OL = 0.4V max CBUS = 200pF max where RT is the parallel equivalent of RP and RL. VILMAX = 0.8V, VIHMIN = 2.1V The rise and fall time calculation for an I2C system is as tr = 0.8µs max, tf = 0.3µs max follows. If an RS of 500Ω is used and the max RON of the driver is 200Ω, then R
I2C Bus Rise and Fall Time
L = 500 + 200 = 700Ω. Using the max VTHRES of 0.9V and a min IPULL-UP of 1mA. Rise time of an I2C line is derived using equation 8. Using equation 6 to calculate the approximate value of t2: tr = –RP • CBUS • ln{[VIHMIN – VCC – (RP • IPULL-UP)]/ t [V 2 = (2.1 + 0.15 – 0.9) • [(200 • 10–12)/(1 • 10–3)] ILMAX – VCC – (RP • IPULL-UP)]} (8) = 0.27µs Fall time of an I2C line is derived using equation 9: t1 = 0.8 – 0.27 = 0.53µs tf = RT • CBUS • ln{[(VIHMIN/VCC) • (RP + RL) – RL]/ [(V Using equation 4 to find the required RP to meet tr: ILMAX/VCC) • (RP + RL) – RL]} (9) For an I2C system with fixed input levels, V R ILMAX = 1.5V P = – t1/{CBUS • ln[(VTHRES – VCC)/ and V (VILMAX – 0.15 – VCC)]} = 27k IHMIN = 3V. RT = (RP • RL)/(RP + RL) 16941fa 6