SN74CB3Q3305: Prevent reverse bias

Björn Hellerstedt

Part Number: SN74CB3Q3305
Other Parts Discussed in Thread: BQ25790, , SN74LVC2G17

Hello

I just want to confirm.

Charger BQ25790 is active when the MCU ( STM32 ) has no power.

I have seen that there is a risk to reverse biasing the MCU ( via GPIO clamp diodes ) and I want to prevent this by using SN74CB3Q3305.

See schematic below. The signals "BQ_QON ( from BQ25790 ) and KILL_SWITCH ( smart switch) " are just simple control pins ( high 3.3v ) but the will be high when STM32 is at 0v.

What I want to do:

So when +3v3_SYS is close to 0.0v ( MCU has no power & SN74CB3Q3305 no power ) , BQ_QNON and KILL_SWITCH signals will be disconneted and NO reverse current will appear.

When +3v3_SYS is 3.3V ( MCU ON ), BQ_QNON and KILL_SWITCH will be connected to STM32 pins and current will flow.

I have another application in the same design where the logic level is more than 3.3v. But the target is the same.

Please let me know your thoughts.

Thank you for your help!

over 1 year ago

0 Alex Paul over 1 year ago

TI__Expert 5655 points

Hi Björn,

I will let our experts comment whether this is a feasible solution.

However, I would like to show you our e-fuses which might be a fit here :

TPS2521 seems to be a suitable solution:

It offers features like: Input reverse polarity protection, Overvoltage clamping, Thermal shutdown

with integrated switches.

Regards,

Alex

0 Björn Hellerstedt over 1 year ago

Prodigy 190 points

Hello Alex

Thanks, I will wait for your experts to comment.

Thank you for link to TPS2521 it is a nice product but not for this application.

+1 Clemens Ladisch over 1 year ago in reply to Björn Hellerstedt

Guru 320490 points

This would work. But you are using the SN74CB3Q3305 only because it has overvoltage-tolerant inputs; you are not actually switching the signals with the OE pins. The same function can be done with any buffer with overvoltage-tolerant inputs, e.g., SN74LVC2G17/34.

0 Björn Hellerstedt over 1 year ago in reply to Clemens Ladisch

Prodigy 190 points

Hello Clemens

Thanks,

You are correct. But I only want to use one type of IC to prevent reverse bias current, in other applications on this board I have 5 V and also I2C so I need bidirectinal switching. I guess SN74CB3Q3305 is suitable.

Thanks

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SN74CB3Q3305: Prevent reverse bias