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Replies: 8
Views: 434
Part Number: ISO7740
The output of the ISO7740F is listed as "Undetermined" in table 2 of the datasheet when Vcco < 2.25V.
Is there any way to arrange the device (perhaps with external components) so that there is no glitch (going high) when Vcco is applied? (assume EN is held low and Vcci may be powered down or powered up with input Low)
In reply to Manuel Chavez:
In reply to Steve Widener:
Steve,You're welcome! The datasheet table doesn't cover cases with external components, like pull-down or pull-up resistors. The addition of a pull-down connects the output path to ground even if the voltage on the output pin is indeterminable.What amplitude and duration are the glitches we're looking to prevent? Bench testing on the previously mentioned table condition shows voltage on the output pin to be 300mV or less during the moment of startup and on the timing order of microseconds and nanoseconds as shown in the following picture. These amplitudes are not expected to turn on any external components since they are lower than most thresholds.I will await your response.Thank you,Manuel ChavezP.S. Post was text was edited for clarity
You're welcome, Steve!Thanks for the details in this response. High outputs are not expected on the ISO7740F outside of data transfers, but please give me 2-3 days to check out the voltage supervisor and verify with the isolation team. Insights to the questions below will be helpful meanwhile.When is Vcco expected to be lower than 2.25V?What is the turn-on time of the FETs?Respectfully,Manuel Chavez
Hi Steve,
The waveform attached in a previous post was captured during powerup, but since then I have not been able to reproduce it in the lab to test out our proposed solutions. Wiggles on the output vary based on device-to-device variations as well as the PCB environment the ISO7740F is in. Our team suggests using a 10kOhm pulldown resistor on the output pin to help reduce voltage waves and adding a small capacitor of ~0.1nF in parallel can help reduce any voltage wiggles even further. The supervisor IC is a valid solution, it just may be overkill.We can continue testing to check if these solutions are good; please let me know if you or our customer would like us to proceed.Have a good weekend!Manuel Chavez