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Original question:

LMS33460: LMS33460

LMS33460: LMS33460

seungpyo kang
Prodigy 130 points
Part Number: LMS33460
Other Parts Discussed in Thread: TIDA-00173,

Tool/software:

Reference document : TIDA-00173

This circuit can handle an input voltage up to 1200 V. (HV+ point at below circuit)

In that case, the current flowing through the series input stage is 99 µA at 1200 V input. At lower input voltages, a smaller current will flow.

Additionally, the Zener diode will clamp the voltage to ensure there is always 7.5 V between D32 and ground. I have studied the LMS33460 and confirmed that it detects overvoltage when the input voltage exceeds the threshold.

However, I predict that due to the Zener diode, the voltage will remain constant at 7.5 V.

  1. If the input voltage increases to 1300 V, will the voltage between D32 and ground also rise?

  2. If the answer to question 1 is yes, then if the six series resistors of 2M are changed to 1.8M, a larger current will flow at 1200 V. In that case, will overvoltage be detected at a lower voltage?

  3. Last, Please also explain in detail the criteria for selecting the value of the pull-up resistor.

  • 0
    TI__Expert 6660 points

    Hi Seungpyo,

    1. When input voltage increases to 1300V, the voltage at D32 cathode reaches >7.5V and it will begin shunting the extra current to clamp the voltage to 7.5V at the cathode.

    3. The pull-up resistor value of 511kOhms should be fine as the device only needs an input supply of 3.6uA max so the 99uA leaves plenty of extra current to drive the 2N700 FET. 

    Thanks,
    Joshua

  • 0 in reply to Joshua Garcia
    Prodigy 130 points

    Is the value of approximately 100 µA in the figure below corresponding to the knee current? So, if the voltage increases further, the current increases and the voltage corresponding to Vz is applied?

    Also, if a 2M resistor is replaced with a 1.8M resistor, will OV be detected at a lower voltage rather than at 1300 V?

  • 0 in reply to seungpyo kang
    TI__Genius 15811 points

    Hello,

    Due to the US holidays, our team will respond back the week of July 7th.

    Thank you for your patience.

  • 0 in reply to seungpyo kang
    TI__Expert 6660 points

    Hi Seungpyo,

    I cannot comment on the operation of the zener diode. The node at the cathode of the D32 will see ~100uA at ~1300V. If you replace the 2Mohm resistors with 1.8Mohms the OV will be detected at ~1170V.

    Thanks,
    Joshua