Problem with working on 2.5V

[Artem Dmitriev]

Posted on March 01, 2018 at 11:10

Hello!

On the PCB from LDO(

MCP1700T-3302E

) 3.3V are supplied 2 chips:

GPS

:

SIM

68

M,

STM32L475RET6.

GPS power supply is controlled by STM.

When STM turn on GPS module voltage temporarily drops to 2,5V.

At this time MCU works incorrectly if frequency is 80MHz (hang or HardFault handler are called). If frequency 40Mhz all fine.

What could be the problem?

Note: this post was migrated and contained many threaded conversations, some content may be missing.

[AvaTar]

Posted on March 02, 2018 at 11:13

STM32L475xx Datasheet named 'en.DM00172872.PDF', DocID027692 Rev 3, pages 95 .. 97.

Max current at 80MHz according to datasheet is about 12mA, but this might suffice to push your power supply 'over the edge', together with the GPS module current surge.

I had such a case a while ago, with a LCD display on a F4 discovery board.

It started up fine, and then got lost when accessing the display.

The issues disappeared when I switched from USB supply to an external supply.

Pretty sure your power supply is inappropriate.

[Andrew Neil]

Posted on March 02, 2018 at 11:31

AvaTar wrote:

Pretty sure your power supply is inappropriate.

No question about that at all - it is clearly inadequate!!

:(

[Andrew Neil]

Posted on March 02, 2018 at 12:37

aa bb wrote:

At startup,

GPS draw a lot of current.

Indeed - see:

https://community.st.com/0D50X00009XkWxZSAV

aa bb wrote:

you could add a slow inrush current with mosfet..

Butstudy the GPS documentation carefully - it may not like a slow supply rise ...

[Artem Dmitriev]

Posted on March 02, 2018 at 12:58

We tried to power MCU & GPS from Laboratory power supply, situatuion still the same.

If we turn on GPS voltage drops to 2.5V.  We tried to connect 100 Ohm resistor instead of GPS and in this case all works fine.  We tried to use 1 Ohm resisitor instead of L? bead and it helps to lower voltage drop but voltage drop still exists.

[AvaTar]

Posted on March 02, 2018 at 13:44

I would short this coil too, perhaps with a 1..10 Ohm resistor.

If the voltage drop is still too severe, you might consider separating the 3.3V supplies for the MCU and the GPS module.

And perhaps the GPS supply from regulator with an enable input, switched by the STM32, instead of the MOSFET.

Or a hardware guy can suggest a cheaper solution.

[Andrew Neil]

Posted on March 02, 2018 at 14:55

Sorry - I can't read Russian.

Artem Dmitriev wrote:

We tried to power MCU & GPS from Laboratory power supply, situatuion still the same.

So use your scope to look at the Lab supply's output before it reaches your board - does it have the droop?

If it does, then you have a problem with

the Lab supply!

If it doesn't, t

hen follow through the circuit to see where the droop appears.

meyer.frank

‌wrote:

consider separating the 3.3V supplies for the MCU and the GPS module.

+1

GPS supply from regulator with an enable input, switched by the STM32, instead of the MOSFET.

+1

[Andrew Neil]

Posted on March 02, 2018 at 15:09

AvaTar wrote:

a hardware guy can suggest...

artem.dmitriev

‌ - do you not have a hardware guy on your team? Or someone with HW experience you can call on?

Who designed the board?

[AvaTar]

Posted on March 02, 2018 at 17:34

Not so good with Russian technical terms, but the coil seems to be 100mH.

IMHO at least two orders of magnitude too much.

[Dvorak.Peter]

Posted on March 03, 2018 at 23:45

Artem, I would add 1 megohm resistor to the gate of VT9 transistor. This will slow down the turn on slope.

For even slower turn on, add 100nF cap between gate and output  of VT9. Adjust R C values to taste.

Peter

[Dvorak.Peter]

Posted on March 04, 2018 at 00:47

Using a 0.05 ohm switch to connect a discharged 4.7 uF capacitor to your uC supply is asking for trouble.

Try removing the C29 (

 4.7 uF) 

capacitor. The module may have additional internal caps.

The better solution is the R C modification to the VT9 switch mentioned earlier.

See 

https://electronics.stackexchange.com/questions/9307/how-to-limit-inrush-current

 

Peter