VR5510 BUCKx electrical specification and LDO2 instability

[enrov]

Hello

I'm looking for information about VR5510 BUCK performances: I would like to use them in analog application where an LDO is the right choose: however VR5510 LDOs can be at limit current in some situation. I'm not able to find any information about voltage ripple specification of the BUCKs (referred to suggested output filter, I'm using). Is it possible to have? I found on this forum that other VRs have this kind of information but VR5510 doesn't.

I have an issue using VR5510 in companion with SJA1110. Short story long: if the SJA1110 is keep in HARD RESET and I power up the VR5510 from external source, the LDO2 connected to VDDIO 1.8V MII power supply rail startup in instability generating 2.2V instead of 1.8V (i think that voltage can rise up to Vin 3.3V if not limited by some SJA1110 internal clamp)

Attached the portion of the schematic from NXP  SJA1110-EVM board that is exactly what I used on my custom board.

I'm monitoring the 1.8V voltage with VR5510 internal voltage monitors, in that case VR5510 PGOOD never change state because of OVERVOLTAGE detected.

Very strange thing is that if I disconnect PGOOD pin from SJA1110 HARD RESET pin, i have no issues and PMIC start successfully together with SJA1110.

Is there the possibility that LDO2 goes in instability due to high output load current and never recovers? OTP program is to autoretry every 4s but VR5510 never retry.

Thank you

enrov

[enrov]

Hello Tomas,

ok thank you. 

Just to close the topic of LDOs, for VR5510 is there any possibility to parallel LDO2 and LDO3 (that are the same hw seems) with external ballast resistor just to let them share a current that can be slightly over the maximum of 400mA (transitory), to keep the voltage within range?

Or, can you tell me if the three BUCKs converter can be used as alternative to LDOs for analog application where noise is important? (this is because i asked the expected output voltage ripple).

I'm starting a new application where I would like to use VR5510 to supply some high-speed ADCs and opamps (tens of MHz range of application).

Thank you

Enrico

[TomasVaverka · Accepted solution]

Hello Enrico,

LDO2 and LDO3 on VR5510 cannot be used in parallel.

In general, BUCKx outputs are not recommended as a direct ADC reference. We recommend adding an external LDO after BUCKx for this purpose.

BRs, Tomas

[TomasVaverka]

Hello Enrico,

Thank you for your question regarding the VR5510.

The BUCKx output ripple specification for VR5510 is peak-to-peak 1%.

Could you please confirm which VR5510 part numbers you are using? If VMONx is not used for LDO2 voltage monitoring, PGOOD cannot be asserted in case of LDO2 overvoltage (OV). Please note that VR5510 does not have internal voltage monitors for LDO rails. 

Does this issue occur on all modules or only on a specific one? If it can be reproduced across all modules, I recommend following up on this with the SJA1110 engineer to check if the issue can also be reproduced on the SJA1110 EVB.

If LDO2 is operating in an overloaded state, the expected fault would be UV (undervoltage), not OV (overvoltage). As a troubleshooting step, please disconnect the LDO2 load and check if the device powers up normally. Additionally, I suggest measuring the impedance of the LDO2 and VPRE network to ensure they are within normal range.

BRs, Tomas

[enrov]

Hello Tomas

thank you for your reply.

Going deeper in analysis seems that LDO2 is driven by outside uncontrolled voltage that rise from 1.8 to 2.2. Due to the fact that I am monitoring LDO2 with VREF, PGOOD is not asserted ever.

The problem seems to be at SJA side, my only curiosity is to understand if LDO2 can be unstable and if, it can forward the input voltage to output. I agree about the point where you say that an overload is related to an UV, but I do not see UVs.

Just for sharing: VR5510 ( I'm using AMMA0 and AMBA0 versions) seems not to be the source of the problem. But any advise on stability of LDOs and instability behavior is well accepted.

Regards

[TomasVaverka]

Hello Enrico,

If the external components for the LDO regulator are within the specified range, there should be no risk of instability. We generally believe that the input of an LDO cannot be directly forwarded to the output, unless the LDO is specifically configured in load switch mode.

BRs, Tomas