Datasheet LTC4311 (Analog Devices) - 7
| Fabricante | Analog Devices |
| Descripción | Low Voltage I2C/SMBus Accelerator |
| Páginas / Página | 12 / 7 — APPLICATIONS INFORMATION. Selecting the values of RS and RP. Low State … |
| Formato / tamaño de archivo | PDF / 142 Kb |
| Idioma del documento | Inglés |
APPLICATIONS INFORMATION. Selecting the values of RS and RP. Low State Noise Margin. Initial Slew Rate. I2C Rise and Fall Time
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LTC4311
APPLICATIONS INFORMATION Selecting the values of RS and RP Low State Noise Margin
The typical confi guration for the data bus for a 2-wire bus A low value of VOL, the low state logic level, is desired is shown in Figure 1. The parameters RP and RS should be for good noise margin. VOL is calculated as follows: chosen carefully. A description of the process for choosing the values of R R • V P and RS follows. V = L CC OL R + R An external pull-up resistor R L P (1) P is required in each bus line to supply a steady state pull-up current if the bus is RL is the series sum of RS and RON, the on resistance of at logic zero. This pull-up current is used for slewing the the open-drain driver. bus line during the initial portion of the positive transition Increasing the value of R in order to activate the LTC4311 pull-up current. P decreases the value of VOL. Increasing RL increases the value of VOL. Using an external pull-up resistor RP to supply steady state pull-up current provides the freedom to adjust rise
Initial Slew Rate
time versus fall time as well as defi ning the low state The initial slew rate, SR, of the bus is determined by: logic-level (VOL). For I/O stage protection from ESD and high voltage spikes V – V SR = CC OL on the bus, a series resistor RS (Figure 1) is sometimes • R C P BUS (2) added to the open drain driver of the bus agents. SR must be greater than SRTHRESH, the LTC4311 slew rate detector threshold (0.5V/μs max), in order to activate the VCC pull-up current. LTC4311 DYNAMIC RP CURRENT
I2C Rise and Fall Time
PULL-UP Bus Rise time of an I2C line is derived using equation 3. CBUS RS t = – R •C • r p BUS DATA IN ⎧ V – V – R ⎪ •I ⎫⎪ ln IHMIN CC p PULLUP AC DATA ⎨ ⎬ R V – V – R •I ON OUT ILMAX CC p PULLUP AC ⎩⎪ ⎭⎪ (3) 4311 F01 Fall time of an I2C line is derived using equation 4.
Figure 1. Typical 2-Wire Bus Confi guration
⎧ VIHMIN • ⎫ (R + R ) – R ⎪⎪ V P L L ⎪⎪ Both the values of RP and RS must be chosen carefully t = R •C •ln CC f T BUS ⎨ ⎬ V to meet the low state noise margin and all bus timing ILMAX ⎪ •(R +R )–R ⎪ requirements. ⎪ V P L L ⎪ ⎩ CC ⎭ A discussion of the electrical parameters affected by the (4) values of RS and RP, as well as the general procedure for where RT is the parallel equivalent of RP and RL. selecting the values of RS and RP follows. 4311fa 7
APPLICATIONS INFORMATION Selecting the values of RS and RP Low State Noise Margin
The typical confi guration for the data bus for a 2-wire bus A low value of VOL, the low state logic level, is desired is shown in Figure 1. The parameters RP and RS should be for good noise margin. VOL is calculated as follows: chosen carefully. A description of the process for choosing the values of R R • V P and RS follows. V = L CC OL R + R An external pull-up resistor R L P (1) P is required in each bus line to supply a steady state pull-up current if the bus is RL is the series sum of RS and RON, the on resistance of at logic zero. This pull-up current is used for slewing the the open-drain driver. bus line during the initial portion of the positive transition Increasing the value of R in order to activate the LTC4311 pull-up current. P decreases the value of VOL. Increasing RL increases the value of VOL. Using an external pull-up resistor RP to supply steady state pull-up current provides the freedom to adjust rise
Initial Slew Rate
time versus fall time as well as defi ning the low state The initial slew rate, SR, of the bus is determined by: logic-level (VOL). For I/O stage protection from ESD and high voltage spikes V – V SR = CC OL on the bus, a series resistor RS (Figure 1) is sometimes • R C P BUS (2) added to the open drain driver of the bus agents. SR must be greater than SRTHRESH, the LTC4311 slew rate detector threshold (0.5V/μs max), in order to activate the VCC pull-up current. LTC4311 DYNAMIC RP CURRENT
I2C Rise and Fall Time
PULL-UP Bus Rise time of an I2C line is derived using equation 3. CBUS RS t = – R •C • r p BUS DATA IN ⎧ V – V – R ⎪ •I ⎫⎪ ln IHMIN CC p PULLUP AC DATA ⎨ ⎬ R V – V – R •I ON OUT ILMAX CC p PULLUP AC ⎩⎪ ⎭⎪ (3) 4311 F01 Fall time of an I2C line is derived using equation 4.
Figure 1. Typical 2-Wire Bus Confi guration
⎧ VIHMIN • ⎫ (R + R ) – R ⎪⎪ V P L L ⎪⎪ Both the values of RP and RS must be chosen carefully t = R •C •ln CC f T BUS ⎨ ⎬ V to meet the low state noise margin and all bus timing ILMAX ⎪ •(R +R )–R ⎪ requirements. ⎪ V P L L ⎪ ⎩ CC ⎭ A discussion of the electrical parameters affected by the (4) values of RS and RP, as well as the general procedure for where RT is the parallel equivalent of RP and RL. selecting the values of RS and RP follows. 4311fa 7