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Dear E2E,
We would like to know if ISO7741 could support i2c signal and connection like photo below.
ISO7741 is push pull type, so we need to remove pull high resistor in the circuit below. Am i right?
BR,
Jason
Hi Jason,
The article that Lucas has shared is the right document to refer to if customer is intending to use a digital isolator to achieve two isolated bidirectional channels (SCL & SDA).
There are a few other devices that have two unidirectional channels SDAI & SDAO instead of a single bidirectional channel SDA. This is probably done to enable unidirectional devices also to be used to interface to such devices. One of such devices is TPS2388 which is a PoE PSE controller. Please find below its simplified schematic from datasheet showing separate SDAI and SDAO pins.
The schematic that you have shared can be directly connected to TPS2388 and other similar devices with separate SDAI & SDAO pins. There is no change required in your schematic and can be directly used to connect to TPS2388.
I would strongly recommend to replace ISO72xx device with ISO77xx to benefit from all the improved performance of ISO77xx devices.
Please let me know if you have any questions, thank you.
Regards,
Koteshwar Rao
Hello Koteshwar,
After confirmed from PoE team, the I2C interface pins are logic I/O pins, so there is significant power consumption to report or measure.
The pin capacitance is 10pF on SCL and 6pF on SDApin which is going to determine how much load your driving circuitry will need to support.
In 12pcs tps23880, the capacitance should be 120pF on SCL pin and 72pF on SDA pin. if so, could ISO77xx support fast I2C data rate without SI issue?
RB,
Jason
Hi Jason,
Thanks for your inputs and for confirming that RST, SCL, SDAI & SDAO are logic I/Os.
Since digital logic inputs typically consume a few uA of current (<10uA for most TI digital isolators), it will be safe to connect such 12 inputs together as the total current only sums upto about 120uA while ISO77xx outputs support max 4mA of current.
I understand the worst-case total cap at SCL & other pins is going to be 120pF, this is only going to affect the rise and fall times. For 3.3V supply, 120pF cap & 2mA output current drive, the typical rise/fall times are going to be about 200ns. This I think is good enough to support upto 1Mbps datarate.
Though logic inputs can be connected together without any issues, logic outputs can't be connected together. One output driving multiple inputs is not a problem but multiple outputs driving together is going to be a problem. If one output is HIGH while one of the other outputs is LOW, then there is a potential VCC-GND short which can cause damage to device.
Fortunately, the SDAO outputs are open drain as it is a requirement for I2C. Since these outputs can only sink current and can't source, 12 such outputs can be connected together to ISO77xx input pin without any issues. Please do note that it is important to use only one pull-up resistor per ISO77xx input, using 12 pull-up resistors coming from all 12 outputs could lead to large sink current that could violate datasheet spec of TPS23880.
Please do check with PoE team to confirm IIL & IIH values for TPS23880 SDAI, SCL & RST inputs. This value is ±10uA for ISO77xx inputs. Thanks.
Regards,
Koteshwar Rao
Hello Rao,
Clear!!
One more question that if data rate is 400kHz, the time period of clock is 2.5us and the duration of ON/OFF time is 1.25us.
If the rising/falling time is 200ns, it will be occupid around 16% of ON/OFF time.
How many percent as maximum is acceptable?
or
How could I check it?
BR,
Jason