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CPC5903G Datasheet(PDF) 11 Page - IXYS Corporation |
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CPC5903G Datasheet(HTML) 11 Page - IXYS Corporation |
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11 / 15 page  INTEGRATED CIRCUITS DIVISION CPC5903 R02 www.ixysic.com 11 4 Design Considerations The minimum value of the pull-up resistor, RPU, on the I2C bus is chosen based upon the expected VDD supply voltage range and the weakest load current sinking device on the bus. Note: Systems that do not need maximum bandwidth and busses with lower capacitive loading can use a higher value for the pull-up resistor to reduce power consumption. 4.1 Side A Pull-Up Resistors: RPUA The weakest I2C compliant device on the Side A bus, with RPUA to VDDA, must be able to pull the Side A inputs below 0.4V for outputs rated at 3mA or 0.6V for outputs rated at 6mA when VDDA is at its maximum. For example, if the weakest device is only guaranteed to sink 3mA then the maximum allowed logic low output voltage will be 0.4V. For designs with VDDA_max = 3.6V, the minimum voltage across the pull-up resistor is: Minimum RPUA Voltage = 3.6 - 0.4 = 3.2V For the I2C minimum current sink requirement of 3mA, the minimum value of the pull-up resistor is easily calculated as: RPUA_min = 3.2V / 3mA = 1066.7 Chose a standard value resistor that will not violate this minimum value over tolerance and temperature, such as a 1.1k , 1% tolerance, 100ppm/C temperature coefficient resistor. If all the non-CPC5903 devices on the Side A bus are Fast-mode compliant (400pF capacitive loading capable) with the required 6mA current sink capability, then the bus can be configured for Fast-mode. Resistor selection for Fast-mode is similar to the example given above but because the logic low output level is greater (0.6V) then the voltage across the pull-up resistor will be less. Calculation of the compliant Fast-mode bus minimum pull-up resistor value is given by: RPUA_min = (3.6 - 0.6)V / 6mA = 500 The minimum E96 standard value 1% tolerance, 100ppm/ C temperature coefficient resistor is 511. 4.2 Side B Pull-Up Resistors Calculating the pull-up resistor for Side B is similar to the process used for Side A but with some additional considerations. Before proceeding, it must be pointed out that Side B of the CPC5903 is Fast-mode compliant with VDDB 4.5V. This means the CPC5903 Side B outputs are 6mA capable, allowing bus operation of 400kb/s with up to 400pF of capacitive loading. For VDDB supply levels below 4.5V the CPC5903 outputs are only rated for 3mA, but can be operated at Fast-mode speeds of 400kb/s whenever the bus capacitive loading CLOAD 200pF. Greater capacitive loading of the Side B bus limits the CPC5903 to data rates of 100kb/s. First, it must be determined if the Side B bus will be configured for 3mA or 6mA operation. This is done by evaluating the external (non-CPC5903) devices on the Side B bus and the operational capabilities of the CPC5903. There are three possibilities: 1) One or more of the external devices is limited to 3mA of output current sink. 2) All of the external devices are rated at 6mA of output current sink and the Side B minimum supply voltage VDDB 4.5V. 3) All of the external devices are rated at 6mA of output current sink and the Side B minimum supply voltage VDDB 4.5V. For conditions 1 and 2 above the bus must be configured for 3mA. Condition 3 is the only situation where the bus can be configured for 6mA, a Fast-mode requirement when capacitive bus loading is an issue. 4.2.1 OB Pull-Up resistor: RPU-OB Selecting the pull-up resistor for the OB bus is based upon the manner in which the bus is expected to operate within the restrictions listed above. Although the additional design considerations discussed below for selecting the IOB pull-up resistor, RPUB , are not applicable to the OB pull-up resistor, the Side B pull-up resistors should have the same value to minimize skew between the clock and data. Therefore, setting RPU-OB =RPUB is recommended. |
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