imx535qsb ,mc34708 's output is vsrtc=1.2 instead of 1.3v?

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imx535qsb ,mc34708 's output is vsrtc=1.2 instead of 1.3v?

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ylw
Contributor II

i I finished  a board full of references to imx53qsb reference schematic. I used the MC34708 PMIC as the demo does that.

in this design I  use  MC34708 to power the iMX535.The system should be  automaticaly startup when the power comes up,but i fail.

in the design ,pums[5:1]=00111;ictest=gnd;

and i have tested  these parameters:

vccbp=3.8v

VCOREDIG =1.496v;

vcore=2.775v;

vREFCORE =1.19v;

valways =3.785v;

vddlp=1.476v;

vsrtc=1.234v

PWNON1=1.496v;

PWNON2=0;

PMIC_ON_REQ=0v?

so ,i have some questions :

1)vsrtc is 1.2v instead of 1.3v?

2)vpll,sw2,sw1,and so on ,  approach to 0v?

3)but i tested qsb (tp12),i find PMIC_ON_REQ=1.3v and PMIC_STBY_REQ(tp5)=0.and i want to know the difference between the qsb(the qsb  system has no sd card to boot yet) and my design .and this difference hanppened beacuse vsrtc is different ?

Is there anybody who has experiance with auto startup the MC34708 ?


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JorgeRama_rezRi
NXP Employee
NXP Employee

Hi Yan,

I had a similar issue, in my case I was seeing a voltage drop at startup. The VSRTC voltage actually reached 1.3V, but then it dropped to 1.2V. The problem was my external circuitry in that regulator was making it unstable under certain conditions. Anyways, I fixed it by changing the output capacitor to 0.1uF and I used an extra 0.1uF for decoupling close to the i.MX. As far as I remember, the MC34708 datasheet used to say this cap should be 1.0uF, but in the latest datasheet I saw 0.1uF, so it must have changed. I also placed a 10K resistor in paralell with the VSRTC output cap (from VSRTC output to GND) to bleed the regulator in case of short power cycles, this improved the performance a lot. Hopefully you have the same issue and you can fix it easily.

Another possibility is that you have an old version of the MC34708. I kind of remember 1.2V used to be the default VSRTC voltage, so please ask for a new version of the PMIC just to verify.

Hope this helps!

Best regards.

Jorge.

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ylw
Contributor II

hI ,,jorge,

because some reasons ,i have to advance this project ahead,so i have to force the net of NVCC_SRTC is 1.3v regardless of vsrtc ,so the net  PMIC_ON_REQ_ARM that is i.mx' pin w14 output , is high level ,and the PWRON2 is high ,the pmic is on mode ,so the  pmic continues to work ?

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ylw
Contributor II

Hi ,jorge ,

The extra capacitor on my board is 0.1uf  but i find it placed  cloed to pmic ,how about the affect ?by the wany ,i want to consult you whether 1.2v is not changed  ( old version of the MC34708 you mention)  regardless of pums[5:1]=00111?The mark of mc34708 on my board is mc34708vm dbtbtbh1141g ,I don't know whether it is old version , and  i plan to ask one or two samples from agent .thanks a lot .

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NicolasIzquierd
Contributor III

Hi Yan,

Can you check you voltages levels with the Table 11 of the pmic datasheet?  you should have at least those outputs in the default state, also if you can share the schematic that can help.

-ni

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