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LMZ23608: What will happen if SYNC input is stuck high?

Thomas Schlöter
Prodigy 10 points
Part Number: LMZ23608
Other Parts Discussed in Thread: LM2842,

Hello,

I am using four LMZ23608 in current share mode in my design in order to supply a high number of addressable LEDs. The microcontroller which generates the input signal to the LEDs is supplied independently by an LM2842 from the same 24 V power rail which is also operating as a 5 V standby supply.

When LED activity is requested the microcontroller controls the enable input of all LMZ23608 and also generates 90 degree phase shifted clocks for the modules.

From the datasheet I have read in chapter 7.3.1 "Synchronization Input":

Applying a sustained logic 1 corresponds to 0-Hz PWM frequency and will cause the module to stop switching.

What exactly does "stop switching" mean in this case? Assuming the microcontroller would fail or have a software bug that leads to enable pins and sync inputs being stuck high, will the module shut down safely or could it stop switching with the FET conducting and connect my 5 V power plane to the 24 V rail?

I am asking because in this case a software bug could lead to a catastrophic failure of my design by blowing the polymer capacitors on the 5 V rail.

Thanks in advance for any clarification.

Thomas

  • 0
    TI__Guru 50120 points

    Hi Thomas,

    Can you help provide just a schematic or block level diagram of the LMZ23608 power module just for my own visual interpretation? Where are the polymer caps connected to? 

    My understanding here is that if there is a sustained logic HI (1) the device would stop switching and shutdown. This means the output voltage of the LMZ23608 will be zero since there is no switching characteristic to regulate a voltage. The input voltage to the LMZ23608 will still be high. 

    Regards,

    Jimmy

  • 0 in reply to Jimmy Hua
    Prodigy 10 points

    Hello Jimmy,

    thank you for your reply. The caps are indeed connected to the output of the LMZ23608 modules (aluminum polymer caps). The output is regulated to 5 V while the input rail is 24 V. So, obviously if the input and output of the module got constantly connected driving the output rail to 24 V, my application circuit would be seriously damaged.

    I was afraid if „stop switching“ could mean that the high side switch inside the module stays enabled, continuously connecting the output to the input rail in case the SYNC input is constantly driven high.

    If „stop switching“ just means that the output voltage will be shut down to 0V, everything is fine. The fact that the input rail stays powered is clear to me. Also in my application if the output voltage goes down unexpectedly nothing serious will happen. The output just never shall drastically exceed 5 V in case the clock signal fails.

    Regards,

    Thomas

  • 0 in reply to Thomas Schlöter
    TI__Guru 50120 points

    Hi Thomas,

    My reasoning here is that the internal monitoring circuitry for a condition where it "stop switching" would try to shut the part down meaning output voltage shutdown to 0V. It would be a flaw in design if during an invalid switching frequency/SYNC injection that VOUT = VIN.  

    However just to be safe I'll double-check with the engineer who has previously worked on this. I'd expect the same reply but want to confirm.

    Regards,

    Jimmy

  • 0 in reply to Jimmy Hua
    Prodigy 10 points

    Hello Jimmy,

    thank you for double-checking. In the end I would also assume that the module would monitor a stuck signal and shut down safely. In the datasheet a maximum duty cycle of 85% for the clock is given in the electrical characteristics, so it would be technically possible to shut down if the signal is high for longer than 85% of the lowest allowed frequency.

    I just don‘t want to try it in my design without asking before because the prototype I am building has quite an expensive BOM on the PCB.

    Regards,

    Thomas

  • 0 in reply to Thomas Schlöter
    TI__Guru 50120 points

    Hi Thomas,

    Understood. I've contacted the systems engineer who previously worked on this part. I hope to provide feedback by end of today and close out this post. When I get more information I'll get back to you on this.

    Thanks for your patience.

    Regards,

    Jimmy 

  • 0 in reply to Jimmy Hua
    TI__Guru 50120 points

    Hi Thomas,

    It will just stop clocking, so once the last PWM commands finishes it will stop switching. This would mean the device output voltage should remain 0V and off. 

    Regards,
    Jimmy 

  • 0 in reply to Jimmy Hua
    Prodigy 10 points

    Hi Jimmy,

    thank you very much for clarification. In this case I can safely use the module in my design.

    Have a nice weekend!

    Regards,

    Thomas