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LM5046: Current sense input using allegro current sensor instead of current sensing resistor

Kihyang Kim
Prodigy 30 points

Part Number: LM5046

Hello team.

I want to design LM5046 CS input using 200A rating current sensor (Allegro - ACS759ECB-200B-PFF-T).

This current sensor has a conversion ratio of 6.6mV/A and an offset of 1.65V.

So, the offset removal and gain tuning circuit and the absolute value circuit were made using several op-amps (TL084BIYDT).

The offset removal and gain tuning circuit output will be... (Vcs - Vcomp3)*R34/R35 => (Vcs - 1.65V)*0.75, and

the absoulte circuit output will be... -0.75V ~ +0.75V   to   0V ~ 0.75V.


Will the CS pin work normally even if the circuit is configured as shown below?

Any answers would be helpful for me, thanks.

Many Thanks

Kim.

  • 0
    TI__Mastermind 33135 points

    Kim,

    Will the CS work normally when the signal is conditioned and applied through a a network of op amps? I'm not sure since this is not how the CS signal is normally sensed and processed. If your signal conditioning circuit removes the offset and sets the gain accordingly, such that the resultant CS signal is within the limits of the LM5046, it should work. However, when considering possible time delay through your network, leading edge blanking, CS filtering, etc will the circuit respond on a cycle-by-cycle basis and give the performance we expect from peak current mode control?  You will have to build the circuit and let us know?

    Regards,

    Steve M

  • 0 in reply to Steven Mappus1
    Prodigy 30 points

    Hello, Steve. Thank you for the answer.
    Currently, I have not built and tested the circuit, it is in the design process. As the primary current of the transformer is expected to be about 120A, it is difficult to find a suitable current transformer, so a current sensor is used.

    To explain the design a little more, we are going to use the voltage mode of LM5046, and the switching frequency is 4~50kHz. So the current will be around 80-100 kHz. The bandwidth of the op-amp is 4Mhz, there will be some delay(propagation, rising time, etc), but I am going to use the CS pin to prevent excessive current from flowing through the MOSFET and destroying the device.

    As you said, since the CS pin operates cycle-by-cycle (initialized with the MOSFET CLK and LEB inside the CS pin), I am curious as to whether the circuit using the op-amp will work.

    Regards,

    Kim

  • 0 in reply to Kihyang Kim
    TI__Mastermind 33135 points

    Kim,

    Understood. From the LM5046 point of view, it should not care how the current is sensed and translated. As long as your sensed current is less than 750mV during normal operation, it should work as you describe. For VMC, the CS function is for OCP only so it can be more tolerant of any delay or distortion, introduced by your signal conditioning network, since CS is not part of the closed loop control.

    Steve M