Power supply differences between Apalis design guide and Evaluation/Ixora schematics


I’m reviewing the Apalis Carrier Board Design Guide as well as the Evaluation Board and Ixora Carrier Board schematics as reference for my custom carrier board design. I’ve noticed a few minor differences between the Design Guide power supply schematics and those of the Evaluation/Ixora boards, and I just want to clarify them.

The below is from the Design Guide (Figure 63, Page 70):

This image is from the Ixora schematics (Sheet 4):

The Design Guide shows to connect TPS51120 EN (pin 12) to VREG5 (pin 21), whereas the Ixora (and Evaluation Board) schematics show EN (pin 12) connecting to PMIC_EN. Is the cause of this difference due to the fact that the Evaluation and Ixora boards have various power jumpers, etc., where the Design Guide does not? I’ve not compared the schematics in detail, so there may be other slight variations, hopefully due to the same reason.

Dear @jars121,

thank you very much for the interest in our product and for using the Toradex Developer Community.

The difference you noticed is due to the fact that the design guide is showing a generic approach in which the carrier board power-up sequence starts as soon as the 12V_FILT_UNREG voltage is provided, while on the Ixora and Apalis Evaluation board there is a push button controller IC which is responsible for enabling the DC/DC through the PMIC_EN signal.

The same voltage compatible signal has been then used to enable the 3.3V part of the DC/DC.

I really hope this helps to clarify your doubts, please don’t hesitate to use again the community the community.

Thanks @diego.tx, your clarification is much appreciated as always. That makes perfect sense to me. I’ve followed the Design Guide accordingly, as I want my board to power up as soon as the input voltage is provided.

Dear @jars121,
it is a pleasure to help!

@diego.tx A quick follow up question if you don’t mind. If my carrier board is hard wired to a power source (i.e. 12V_FILT_UNREG is always enabled), and I have an active high signal to turn on the device (i.e. essentially providing the PMIC_EN signal), can I use this signal as PMIC_EN directly, rather than having to use the LTC2954 IC? If so, I could implement a small RC circuit on each of the required EN pins, so that when the PMIC_EN goes low, the board remains powered for a couple of seconds to allow the Toradex module to shut down (I’d route the PMIC_EN signal to a GPIO on the Apalis module and register an interrupt for this purpose). Does that all make sense, or would the LTC2954 with inverter (convert my active high signal to the active low signal expected by a pushbutton controller IC) be the better option?

Dear @jars121,
So, if I rightly understood, you would like to generate externally a PMIC_EN (maybe with a power button?) and use this PMIC_EN signal to create two EN pins which must behave differently from the PMIC_EN signal.

Then, when PMIC_EN goes to 0V, the module gets an interrupt and initiate the shutdown while the two EN signals are kept high.

I am not sure if it overcomplicates the design and if anyway it is easier to use the features (KILL, INT, on and off timings) of the LTC2954. This really depends on the way you would like to control the signal PMIC_EN and on the desired turn on and turn off behavior of your product.

Anyway, please consider that you probably can use the DC/DC power good signals and the internal LDOs to enable different power supplies. That might be helpful and reduce the effort to create EN signals which are not the same as PMIC_EN you are planning to use for the interrupt.

Please evaluate carefully how long the module needs to remain powered after the interrupt has been triggered. The required time might be related to the SW status at that moment so it might vary a lot.

Thanks again @diego.tx. I’ve given it some further thought, and I think the LTC2954 approach is going to be the most robust. I can play with the value of the PDT capacitor as needed once I know roughly the maximum amount of time I need for the SW to shutdown, which would be tricky to do relying on external RC circuits alone.

Dear @jars121, this is probably the best approach and the most flexible one. You can then use the interrupt and the kill signals to implement a proper power off. Please have a look at this page, it might provide further help with this implementation: https://developer.toradex.com/knowledge-base/gpio-(linux)#GPIO_PowerOff I wish you a nice day.