APX811 Reset Supervisor question

[bpiquette]

Your description of the reset circuit in the OSD3558-SM-RED schematics (page 1) indicates that the APX811 has an internal pull-up to it’s VCC (SYS_VDD1_3P3V), but that is not shown in the datasheet for that part anywhere. What’s pulling up that signal between S1, U3 and U4?

[Erik Welsh]

We had the same issue with the APX811 datasheet.  The typical application circuit in the APX811 implies an internal pull-up resistor, but the datasheet never explicitly says that there is an internal pull-up resistor, nor does it list its value.  However, the APX811/APX812 is a footprint compatible replacement for the MAX811/MAX812  (https://datasheets.maximintegrated.com/en/ds/MAX811-MAX812T.pdf) which does list an internal pull-up resistance on the MR# pin of 20kOhms.  Since the APX811 was a cheaper part than the MAX811, we purchased a part and measured the resistance between MR# and VCC and found it was approximately 20kOhms.  Therefore, we felt confident using that part as the reset supervisor but we included the note in the reference design schematics since the APX811 datasheet was not explicit about the pull-up resistor.

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