i.MX28 with external power supplies

[garyaylward]

Does anyone have any experience of using the iMX28 without using the PMU?

Doubtless this seems like a strange request, but we need to dramatically reduce the capacitance on our board; having 33uF on every power rail is causing real headaches. So the plan is to use external DC/DC converters with much higher switching frequencies and hence smaller decoupling capacitors.

I have read AN4199 but it is only of limited help as it always assumes that the PMU will be used, and simply says 33uF is needed on each power rail (even if the LDOs are used instead of the DC-DC converters?)

So has anyone managed to ignore the PMU completely?

Does the iMX28 work if you just apply the correct voltages to VDDD, VDDIO_33, VDDIO_18 and VDDIO_EMI in the correct sequence?

Will it work with smaller bulk capacitors, assuming the DC-DC converters only require a couple of uF to drive a 600mA load?

Any suggestions/experiences gratefully received!

[JorgeRama_rezRi]

By the way. One important point I was forgetting: In this case, the 15uH inductor between pins DCDC_LP and DCDC_LN has to be removed so the MX28 can be supplied externally.

Best regards.

Jorge.

[autofm]

The inductor removed,all supplies for mx28 used external?

But DCDC_VDDIO is supplied from DCDC.

When modify the OTP,the value of VDDIO should be changed.

How to control it if use external supplies?

Best Regards!

fangmei

[JorgeRama_rezRi · Accepted solution]

Hi Gary,

This is the power-up sequence that has to be followed in the MX28 with external supplies:

1.  BATTERY

2.  VDDD

3.  VDDA

4.  VDDIO33/VDDIO33_EMI

5.  VDDIO18/VDDIO_EMI/VDDIO_EMIQ

Best regards.

Jorge.

[x10]

Hi, Gary,

I just use 10uF capacitors with 1206 pad, the system work fine.

BR

Cheng Shi

[autofm]

You just use a 10uF capacitor? I used a 22uF,but system did not work.

What sequence is right when power-up? I did not use battery and dc-dc_batt pins.

[JorgeRama_rezRi]

Hi Gary,

Just found this discussion today. The documentation of the MX28 was written assuming the processor would supply itself since it was actually planned as a self sufficient device for an easier system development. Let me check if I can find the power up sequence needed for your case and I’ll get back to you very soon.

Best regards.

Jorge.

[Anan]

Hi Gary,

If you are not planning to use internal PMU of IMX28, you need to take care of power sequencing required by the core of the IMX28. As long as you are taking care of power sequencing it should work fine.

Thanks and Regards

-Anand