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LM5109B-Q1: VDD below 8V

Chuqiang Wen
Expert 5260 points
Part Number: LM5109B-Q1
Other Parts Discussed in Thread: LM5109B

Hi Expert,

There have a question regarding to the VDD of LM5109B-Q1. The recommended operating voltage for VDD is 8V to 14V. Customer would like to use a simple linear regulator to supply VDD. The input of linear regulator is 12V battery, the operating range is about 9V to 16V. When the Vbat is under 9V condition. The output of linear regulator to VDD may lower than 8V(7~8V). 

My question is that when VDD lower than 8V in some cases, does this will have some impact on functional of LM5109B-Q1? In other words, does LM5109B-Q1 can work normally when VDD lower than 8V? 

Thanks!

Ethan Wen

  • 0
    TI__Mastermind 21860 points

    Hi Ethan,

    I'm also Ethan.

    The LM5109B has an under voltage lockout protection of 6.7 volts at 25 C. This could be 6.0 V at -40 C and 7.4 V at 125 C. The 7.4 V UVLO at 125 C could be a problem. This means your device will not handle hot temperatures well.

    The input thresholds will also change depending on the supply voltage. Figure 13 covers this.

    The rise and fall times are also dependent on the supply voltage. The lower the supply voltage, the slower the part will switch.

    I think it might be possible, but I would recommend powering the device off a buck-boost converter or another power rail. It might also be feasible to power the device off the battery itself.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    TI__Expert 5260 points

    Hi Ethan,

    Nice to meet you! 

    The operating battery voltage is 9~16V. And the battery voltage rail may noisy because many ECU supply from it. If not consider the noise on 12V bat, do you foresee any risks to use the dynamic range(9~16V) to drive MOSFET?

    Thanks!

    Ethan Wen

  • +1 in reply to Chuqiang Wen
    TI__Mastermind 21860 points

    Hi Ethan,

    I wouldn't want to go above 14 volts. Sorry I missed the fact that you already said the battery voltage.

    I tested an EVM in the lab. The EVM will work down to 6.3 volts at room temperature.

    Also, I reread the datasheet on the UVLO section. The value listed is the rising threshold. At 125 C, the chip will enter UVLO at about 6.9 volts.

    It seems like the chip will work fine if it is above 7 volts. I would not try to operate the chip below 7 volts though.

    By the way, TI can't guarantee functionality outside the recommended operating conditions of course.

    Best Regards,
    Ethan Galloway