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LM6142: The output delay for current sensing application

Expert 4240 points

Replies: 4

Views: 1037

Part Number: LM6142

Hi Team,

   The customer met output delay issue when design LM6142 in their current sensing application.

   The schematic is shown in below. 

When the current ramp up,the output is not follow with the current,it will come out after several us as below figure.

Could you help check why there is the delay and which spec will cause such issue?

Best Regards,

Nick Dai

  • Hi Nick,

    Is the scope capture you have shown during startup of the above system? If so, the delay may be arising because you are generating the op amp supply voltage from the same rail that is sourcing the load. This means the op amp may not be powered up during the initial rise in load current. The rise time of the 12V rail will depend on the value of the resistors in your divider and the bypass cap used. 

    Regards,

    Zak Kaye

    Precision Amplifiers Applications

    Regards,

    Zak Kaye
    Precision Amplifiers Applications 

  • In reply to Zak Kaye:

    Hi Zack and Nick,

    Was it determined that the 12V rise time was responsible for the initial delay of the LM6142 output (~ 4us) to track the load current as Zack had suggested?

    I also think that with no load current, the way the circuit is configured, the LM6142 output would need to be at 0V jamming into the bottom supply rail. There may be some delay associated with coming out of this "overload" condition responsible for this delay. Unfortunately, the LM6142 datasheet does not have any overload recovery plots to see if this 4us delay can be caused by that. As an example, here is an overload recovery plot taken from another device (LMH6642 which is a much faster device) as an example to consider:

    To see if this is the root cause, one could run the LM6142 from a slightly negative (e.g. -1V source) as an experiment to see if the delay can be eliminated?

    Please let me know if these suggestions resolved your issue or not?

    Regards,

    Hooman

  • In reply to Hooman Hashemi:

    I've closed the thread for now, unless I hear otherwise.

    Regards,
    Hooman
  • In reply to Hooman Hashemi:

    This is a result of the LM6142 "overload recovery time". Overload recovery defines the amount of time it takes for the output to come out of saturation and to begin to react to a change in input voltage. Unfortunately the LM6142 does not include an overload recovery time specification but a value in the range of 1-10us is typically for this type of device.

    Regards,
    Collin Wells
    Precision ADC Applications

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