BQ76PL455A-Q1: VIO power from LDO when using UART

Lukasz Przenioslo

Part Number: BQ76PL455A-Q1

Hello there,

The BQ76PL455A-Q1 datasheet provides the following circuit examples for the devices configured for:

communicating in chain with consequent BQ76PL455A-Q1 (differential lines, additional device),
communicating via "master" module via UART (main device).

It can be noticed that the "main device" config is not powered from the on board LDO for the relevant lines voltage levels to be compatible with the MCU voltage.

In my design, I would still like to power the main device from the on board 5.3 V LDO and use a level shifter to communicate with the MCU. My question is- is there any issue with that approach that I am missing?

I would appreciate all feedback.

over 4 years ago

0 Taylor Vogt over 4 years ago

TI__Genius 17370 points

Hi Lukasz,

You need power from an external source (usually the pack voltage to TOP) so you cant power directly from 5.3V LDO. Perhaps drawing a diagram may help if this is not your intention but 7.3.1.1.4 VP Regulated Output of the datasheet may help clarify.

Regards,

Taylor

0 Lukasz Przenioslo over 4 years ago in reply to Taylor Vogt

Expert 2630 points

Hi Taylor,

But why cannot I use the 5.3 V LDO when I want to use the UAART TX/ RX, Wake up and FAULT signals? Is the limitation the VP current capacity? I want to achieve this functionality:

I would appreciate your further feedback.

0 Taylor Vogt over 4 years ago in reply to Lukasz Przenioslo

TI__Genius 17370 points

Hi Lukasz,

This is exactly why there is on board VIO 5V pin to be used for this purpose. Can you use this pin instead? You can see example in the EVM user guide schematic.

Regards,

Taylor

0 Lukasz Przenioslo over 4 years ago in reply to Taylor Vogt

Expert 2630 points

Hi,

I am reading the EVM power section:

But I don't see any explanation per se. The way I understand it is that whatever goes to the VIO input, it sets the high levels for the TX. RX, Wake up and FAULT pins. So if I connect the VP1 with the VIO (via 1R), should I get exactly that effect?

The issue I am having is that I do not want to create a separate hardware for the PCB with BQ76PL455A-Q1 that is going to be connected with the master controller- so any board from the chain could be replaced as the "1st". I would appreciate your feedback.

0 Taylor Vogt over 4 years ago in reply to Lukasz Przenioslo

TI__Genius 17370 points

Hi Lukasz,

You are correct in understanding of VIO that sets the TX/RX pullup logic, but it must come from external PC/MCU rail because that is what is driving the actual logic to avoid back biasing etc.

Regards,

Taylor

0 Lukasz Przenioslo over 4 years ago in reply to Taylor Vogt

Expert 2630 points

Taylor, thank you for the answer. Could you please explain a bit more about this "back biasing feature"? Because I just cannot understand why cant the VIO be supplied from the on board 5.3 V voltage. I can try to redesign the schematics, so the externally connected master supplies the VIO, but I just don't understand why is that necessary.

I would really appreciate your further feedback!

0 Taylor Vogt over 4 years ago in reply to Lukasz Przenioslo

TI__Genius 17370 points

Lukasz,

It is not a feature, but it means if you try to pullup the RX/TX to both the internal 5.3V and external 5V, they will fight to pull each other and could potentially disrupt the VP device operation. You may try the way you have it, but I cannot recommend or support without following the VIO recommendation given above for proper device operation.

Regards,

Taylor

0 Lukasz Przenioslo over 4 years ago in reply to Taylor Vogt

Expert 2630 points

Taylor, but you csn see in my block diagram example that I would have an isolation IC between the mcu and the bq chip. So it is assured that the rx/tx levels are ewual to VIO. Is it ok in that case?

0 Taylor Vogt over 4 years ago in reply to Lukasz Przenioslo

TI__Genius 17370 points

Lukasz,

But how would you wake up the device in this scenario? VP/VIO would be OFF in shutdown mode so the wakeup pulse would not be received on 455 side of isolator IC. So, I think you would have to use my suggestion or some kind of separate transformer circuit for wakeup pin.

Regards,

Taylor

0 Lukasz Przenioslo over 4 years ago in reply to Taylor Vogt

Expert 2630 points

Ok, now I see the issue! Thank you- is the wake up functionality the only limitation in my scenario?

+1 Taylor Vogt over 4 years ago in reply to Lukasz Przenioslo

TI__Genius 17370 points

Lukasz,

As far as I can tell, yes. Please kindly close the thread by clicking the Resolved my issue button and open a new one if there are further questions.

Regards,

Taylor

Power management

Power management forum

BQ76PL455A-Q1: VIO power from LDO when using UART