ADM1266 PDIO during startup

I would like to use an ADM1266 to control the Run/SS inputs of LTM8032 and LTM8032.

To get soft-start, I would like to use the PDIO outputs as open drain outputs with internal pullup to 3V3 with an external 0.22 uF  cap to give a slow rising edge and hence soft start.

My idea was that the first start of the state machine would disable all the power supplies actively pulling the signals low using the open drain outputs.

Then in sequence I would enable the PSU by releasing the open drain outputs so the internal pull-up "slowly" enables the PSU as it charges the 0.22 uF cap.

If I need to shutdown due to over temperature or unexpected shutdown of the Enclustra Zynq module I can disable all the PSUs and the open drain will pull down Run/SS actively (so quicker than the soft start)

In  ADI Power Studio I would set each PSU to have its control be "Open Drain with 20 kOhms pullup to AVDD"

My problem is understanding what happens during power up.

For the unprogrammed ADM1266, my understanding is that all the PDIO output will be 20 k pulldown. With no external pullup resistors my power supplies should stay disabled.

Once I program the ADM1266, with the outputs configured as "Open Drain with 20 k Pull up" what will happen during startup?

From the datasheet I see that  from 1.5 to 2.7 V the outputs will be pulled down by 20k, so this is safe to keep them turned off.

From 2.7 v onwards the signals stay pulled down by 20 k until the device read the EEPROM.

"The PDIOx pin then goes to the state demanded by the configuration. This configuration provides a known condition for the PDIOx pins during power-up."  The required state will be 20 k pullup, but does this set the state to open, with 20 k pull up, or driven low (disabled)?

"If the pin is configured to output, after downloading the configuration and before the sequence is run, the ADM1266 senses the voltage on the pin and drives the pin to the same level as the voltage sensed on the pin." Does this mean the that depending on to voltage level the signal reaches during power up it could be held low or released high?

My concern is that the power supplies could turn on for a short time until the sequencer starts?

Do I need to disable all the power supplies in the first start of my sequencer, or this the default?

Thank you

William.

  • 0
    •  Analog Employees 
    on Feb 19, 2020 12:09 AM over 1 year ago

    Hello William,

    Initially the Cap will be completely discharged. ADM1266 will wakeup and PDIO pin will be in Hi-Z and once it crosses 1.5V the internal 20k pull down will become activated and keep PDIO low. Also at this time it will quickly sense the voltage on the PDIO pin and that should be low too. After ~200ms when the configuration is loaded from the EEPROM, it will configure the PDIO to Open-drain with internal 20k pull-up, at the same time since it had sensed the voltage on the pin to be low, it will actively drive that pin low. This will make sure that your power supply dosent accidentally get turned on. 

    The only issue I envision, is if you power down the board and power it back quickly, without giving enough time for the Cap to discharge, and the charged cap can accidentally turn the power supply on, during the initial ~200ms period.