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TLV703: TLV703 is tracking the input voltage

Mickaël Porte
Prodigy 200 points
Part Number: TLV703
Other Parts Discussed in Thread: TPS560430,

Hi,

We designed a custom PCB that use one TPS560430, one TLV70235 and one TLV70333.

Here is the schematic (U4 is the TLV70235) :

The TPS560430 is configured to output 5V. In specific condition the TLV70333 fails to regulate the output to 3.3V and the input voltage is forwarded to output voltage.

This problem occurs if the input voltage on the TPS560430 is greater than 9V and the output current on TPS560430 is greater than 15mA.

Here is the pcb layout of power supplies (V5 is highlighted):

I did not see this problem on the TLV70235 even if I increase the TPS560430 output current above 100mA.

Here are my tests in order to understand the problem :

  • At start I tought that the ground was not good on the TLV70333 so I added wires between the ground pins of TLV70x and a better ground on the PCB. It didn't change anything.
  • If I remove the TPS560430 and I power the TLV70x from a labs power supplies there is not problem, both LDO are working fine.
  • I tried to replace the TLV70333 with an equivalent component from another manufacturer. This didn't solve the issue
  • I cut the 5V trace that is running under the TLV70333. It didn't help..
  • I placed the TLV70333 on the same footprint as the TLV70235 (one above one without connecting the outputs) and then the problem disapear.


I have 3 prototypes that behaves the same way so I don't think it is an assembly problem. I suspect that the problem is related to some EMC issues but I don't understand why the LDO is failing to regulated the output and how I can fix this problem.

Please can you provide me help to understand and fix this problem ?

Regards,

Mickaël

  • 0
    TI__Mastermind 45070 points

    Hi Mickael,

    The fact that removing the TPS560430 and using a lab supply make the issue go away suggests to me that the DC/DC converter is generating a lot of ground noise or EMI that is impacting the TLV703.  One trick that sometimes works is to put a 1-10Ohm resistor in the boot-strap cap of the DC/DC to slow down the turn-in of the high side FET. This is a shot in the dark as I am not familiar with our DC/DC products. Are you matching the recommended layout as best you can for this?

    As for the LDO, I am wondering if a newer LDO might be more immune to external influence. One idea is if you can order samples of the TPS7A20 to see if this has the issue or not. 

    I am curious to hear what you find.

  • 0 in reply to JCHK
    Prodigy 200 points

    Hi John,

    Thanks for support. The ground noise was the right suggestion, the bypass capacitor on the TPS560430 was missing in my schematic. I added it on two of my prototype after that the TLV70333 was working fine.

    I also try a PTPS7A2033 without the bypass capacitor on TPS560430 but it didn't work as expected. It had almost the same behavior than the TLV70333 :

    • No load on TPS the PTPS regulate to 3.3V
    • 30 mA load on TPS the PTPS regulate to 4.5V.

    When the TPS560430 is loaded, the PTPS output is 4.5V when in the same condition the TLV output is 5V. I don't know if it is enough to claim that the TPTS is more immune.

    Regards,