EVC pin on TLF35584

[smadem]

On one of our boards, TLF35584 doesn't seem to turn on the external post regulator. The EVC pin stays low and doesn't seem to toggle at all. Apart from that, the device seems to be working normally. All the pre regulator voltages and LDO voltage outputs seem to be within limits and ROT pin stays high. 

Can you let me know what else is being checked before turning on the EVC pin?

[Polimera]

Hi Smadem,

Thanks for your query. Could you please check the voltage at VCI Pin.   In case of over voltage at VCI or a detection of a short to ground at the external post regulator for µC core supply, the pin EVC will be pulled to low to shut down the regulator.

Regards,

Kranthi

[smadem]

The voltage at VCI is low (around 0.3V), not a short to ground. But according to the datasheet this happens after turning on the external post regulator, which means EVC should be pulled high to turn on the post regulator, voltage at VCI is checked and EVC is pulled low in case of over voltage or short. And while pulling the EVC low, ROT will be pulled low as well. In my case, EVC doesn't toggle at all. It stays low, never goes high to turn on the post regulator. Let me know if I misunderstood something. 

[Polimera]

Hi Smadem,

Yes your understanding is correct. EVC will be pulled low only for VCI over voltage condition. For VCI under voltage ROT will be pulled low.

Could you please check the Vquc voltage?  Can you please probe the voltages of EVC,QUC,Vprereg, Vpost reg?

The external post regulator will be enabled as soon as the pre regulator output voltage VPREREG is present and the LDO_µC output voltage VQUC is above the lower reset threshold. Becasue EVC is internally powered up with Vquc.

Regards,

Kranthi 

[smadem]

Yes, I checked. Vprereg and QUC are stable and as expected. All the other LDO voltage outputs are also stable and as expected. But EVC doesn't seem to toggle at all. It stays flat at 0V. This board was working initially and stopped working after going through an ageing test. Let me know if there is some cause for concern about how the EVC fails to power up and the IC not pulling RTO pin low.

[Polimera · Accepted solution]

Hi Smadem,

To enable the external post reg. the condition is

SEC – Open

VCI – should be connected to feedback output voltage using resistive divider

SYN- optional not used leave open

If you see the power sequencing -at startup Page 62 POWERDOWN to INIT the order is QST-PREREG buck-QUC-ext. post. Reg. -QVR-QCO-QT1-QT2 the ROT(Reset) pin released after successful sequence.

In case of one of the non microcontroller related voltages (Volt_Ref, LDO_Com, Tracker1 or Tracker2) cannot be ramped up (e.g. short to GND), the power sequence is stopped, but the reset output is still released after the power on reset delay time trd. The microcontroller should check the status of the outputs by reading the SPI status register (VMONSTAT)

If the µC related voltage such as external post Reg. via VCI pin has not reached the required threshold then Short- to ground fault is reported and sequence stops with MOVE to FAILSAFE, which will turn OFF all the regulators.

But your statement does not fit to our Datasheet explanation allowing the PMIC to start up with ROT getting High with all LDOs ON, which could be that they don’t satisfy the requirements for enable post regulators like SEC pin open and VCI pin with resistive divider.

Regards,

Kranthi