Datasheet LTC1694 (Analog Devices) - 6

LTC1694
U U W U APPLICATIO S I FOR ATIO
RON(MAX): The maximum on resistance of the open-
SMBus Rise Time
drain driver Rise time of an SMBus line is derived using equations 3, IPULL-UP(MAX): The maximum LTC1694 low pull-up cur- 4 and 5. rent is 350µA tr = t1 + t2 (3)
Fall Time
t1 = (VTHRES – VIL(MAX) + 0.15) • C Fall time is a function of the SMBus capacitance, R BUS/IPULL-UP (4) S, RON and the pull-up current. Figure 3 shows the maximum if VIL(MAX) – 0.15 > VTHRES, then t1 = 0µs. allowed (RS + RON) based on the Intel SMBus fall time t requirement of 300ns with a 50ns safety margin. 2 = (VIH(MIN) + 0.15 – VTHRES) • CBUS/IPULL-UP(B) (5) IPULL-UP(B) is the LTC1694 boosted pull-up current (2.2mA 1.4 typ). V ) CC = 5V Ω 1.2 For an SMBus system, V (k IL(MAX) = 0.8V and VIH(MIN) = 2.1V. ON 1.0 For the LTC1694, typically VTHRES = 0.65V and + R S IPULL-UP = 275µA. 0.8 C 0.6 BUS is the total capacitance of the SMBus line. 0.4
SMBus Fall Time
0.2 MAXIMUM VALUE OF R Fall time of an SMBus line is derived using equation 6. 0 0 100 200 300 400 500 tf = RL • CBUS • ln{[(0.9 • VCC) – (RL • IPULL-UP(LOW))]/ BUS CAPACITANCE (pF) [VIL(MAX) – 0.15 – (RL • IPULL-UP(LOW))]} (6) 1694 F03 where RL is the sum of RS and RON (see Figure 2).
Figure 3. Maximum Value of RS + RON as a Function of Bus Capacitance for Meeting the SMBus t
Rise and fall time calculation for an I2C system is as
f(MAX) Requirement
follows. The maximum value of RS, based on fall time require- ments, can also be calculated by rearranging equation 6.
I2C Bus Rise and Fall Time
Given below are some equations that are useful for calcu- Rise time of an I2C line is derived using equation 7. lating rise and fall time and for selecting the value of RS. tr = (VIH(MIN) – VIL(MAX)) • CBUS/IPULL-UP(B) (7)
Initial Slew Rate
Fall time of the I2C line can be derived using equation 8. The initial slew rate, SR, of the Bus is determined by: tf = RL • CBUS • ln{[VIH(MIN) – (RL • IPULL-UP)]/ SR = IPULL-UP(MIN)/CBUS (2) [VIL(MAX) – (RL • IPULL-UP)]} (8) CBUS is the total capacitance of the SMBus line. For an I2C system with fixed input levels, VIL(MAX) = 1.5V and V I IH(MIN) = 3V. PULL-UP(MIN) is the LTC1694 minimum pull-up current (125µA). For an I2C system with VCC related input levels, VIL(MAX) = 0.3 • V SR must be greater than SR CC and VIH(MIN) = 0.7 • VCC. THRES, the LTC1694 slew rate detector threshold (0.5V/µs max) in order to activate the CBUS is the total capacitance of the I2C line. 2.2mA boosted pull-up current. This limits the maximum SMBus capacitance. 1694fa 6