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LDO output voltage is not right

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tony_lth

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Hi. Gurus,
I designed a SCH as the following picture. I used a 1.8V fixed voltage LDO, and linked its output to STM32 VDDA_VREF+ directly.
Now the issue is the LDO output is 2.25V, not 1.8V.
If I broke the trace into the VDDA_VREF+, the LDO output is right, i.e. 1.8V.
So what's the reason for that? Is the hardware circuit design right or not?
Best,
Tony Liu
 

Attachments

  • LDO_as_Vref.JPG
    LDO_as_Vref.JPG
    49.7 KB · Views: 147

Hi,

It seems to caused by the STM32....
But to find out we need to read it's datasheet .... for this we need it's exact partname

Klaus
 

    tony_lth

    Points: 2
    Helpful Answer Positive Rating
Hi, KlausST,
PLS Check the attachment.
Best,
Tony Liu
 

Attachments

  • STM32L431XX-DS11453_单片机数据手册_DM00257211_ENV3.pdf
    2.6 MB · Views: 109

Hi,

I don't see anything problematic in the datasheet.

Can we see a photo of your PCB, maybe the PCB layout?

Klaus
 

Hi,

I only had a quick scan of the datasheet... I am confused as VDDA looks like a ~2.4V regulator/reference output whose 'degraded' output is VDDA - 150 mV.

Connecting the two regulator outputs together would explain the 2.25V (2.4V - 0.15V) seen at the 1.8V LDO output when it is connected to VDDA pin.

What is VDDA_VREF+ pin, please? Input, output, configurable input or output?
 

Hi,

I can't spot any failure in your schematic, and the V_DDA/Vref pin should be connected correctly, according to the DS. I have connected other types of STM32 MCUs in the same way.

Which LDO do you actually use, it seems there is an other part number shown below the "SPX5205" - LDO in your schematic.

As Klaus suggested, a sketch of your lyout might be of help.

BR
 

It seems you didn't disable the internal voltage reference generator. It's not clear how to do it, possibly you have to act on a DAC register
 
Depending on what part you are using there are devices that have programmable
Vref, one of the voltages is 1.8. So would beg the question you using external because
more accuracy needed ?

It does appear the Vref is an internal bandgap derived reference.



Regards, Dana.
--- Updated ---

Looks like you have to flip some register bits to use an external Vref, otherwise
you may be tying two regulators outputs together, the internal one and the external.


Regards, Dana.
 
Last edited:
Hi, Gurus,
It seems the Vref+ voltage varies with the VCC, when LDO output trace was broken.
When VCC is 3.3V, Vref+ is 2.4V,
When VCC is 2.7V, Vref is about 1.7V.
So Vref+ seems as 2/3 of VCC, maybe the resistors fucntion.
The STM32L431 has several package, and the package we selected, STM32L431CCU6, should not have internal LDO.
Pls comments, many thanks.
Best,
Tony Liu
--- Updated ---

PS:
And another LDO in my SCH is just replacement pin-to-pin,
I had replaced the SPX5205 with that, and get the same performance as the SPX5205.
The both LDO output are 1.8V, but linked to Vref, have the same volatge 2.25V.
 

This part has an internal buffer that can be enabled/disabled. This would
indicate if unbuffered the Vref is sensitive to load. I know of parts in other
manufacturers where these Vrefs are only good for a few uS of load current
when unbuffered.

You might consider talking directly to a ST FAE on this as manuals I looked at
seem not coherent on the Vref system.


Regards, Dana.
 

    tony_lth

    Points: 2
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Hi,

It seems the Vref+ voltage varies with the VCC, when LDO output trace was broken.
When VCC is 3.3V, Vref+ is 2.4V,
When VCC is 2.7V, Vref is about 1.7V.
So Vref+ seems as 2/3 of VCC, maybe the resistors fucntion.

if I understood you correct, the LDO output voltage varies with th applied input voltage [V_out(V_in)], even when the LDO output is not connected to the MCU?

BR
 

Disconnect the 1.8v regulator from the VDDA pin.

Now connect a 22k resistor from VDDA to ground. Measure the voltage across the resistor, what do you get?
 

Hi, Gurus,
Finally the SW found a solution from web:
using the VDDINT(1.2V) as ADC input as Vref, but this voltage is 1.05V~1.35V in datsheet.
So no need external LDO anymore.
Any comment?
Best,
Tony Liu
 

Hi,

I only could find VREFINT in the attached datasheet in post #3, and its voltage ranges from 1.182 V to 1.232 V for the temperature range from -40 °C to +130 °C (page 91). Page 92 includes a plot of the VREFINT voltage as a function of the ambient temperature.

Depending on your requirements by means of minimum detectable voltage change and ambient temperature, the variation might already be visible in your result. E.g. for a nominal reference voltage of 1.2 V and an ADC resolution of 12 Bit a voltage change of ~300 µV corresponds to one LSB. According to table 25 the internal typical voltage reference variation is 5 mV over the temperature range (have also a look at Fig. 20). So this variation might already influence your aimed result.

BR
 

    tony_lth

    Points: 2
    Helpful Answer Positive Rating
Hi, stenzer,
30mV is OK for me.
Many thanks.
Best,
Tony Liu
 

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