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LMP8640HV: CMRR in PWM Mode

Part Number: LMP8640HV
Other Parts Discussed in Thread: LMP8640, LMP8601, INA240

Hi everyone,

while running a current controlled valve, we noticed some unexpected signals at the output of the LMP8640HVMK-T. There are significant spikes every time the PWM switches the high-side-switched valve on and off.

The circuit we use applies a 24V PWM to control current, which is measured by a 25mOhm Shunt, which in turn is used with the LMP8640HV-T to supply the current signal to an µC ADC.

A zoom on the negative and positive going edge provides details;

This seems to be some effect that has to do with CMRR, but it does not correlate to datasheet value of 103 dB.

Additional info:

  • A competitor IC with similar function performs as expected
  • When using the LMP8640 in a low side switching scenario, these voltage dips do not occur.

Can someone explain this behaviour and how it could have been seen from the datasheet that the LMP8640 might act this way in this particular application.

Thanks in advance and a nice weekend!

Simon Bauer

  • Hello Simon,

    Thanks for considering to use Texas Instruments in your design. This behavior is something that is revealed through simulation or bench tests. CMRR does contribute as you noted that the dips disappear with the low-side configuration. As for calculating the offset from the CMRR, you cannot use the DC value of 103dB. You will need to use the AC CMRR vs frequency curve (Figure 24), which for 2KHz signal will be around 60dB.

    Typically for this application, we would recommend the LMP8601 or INA240. Of the two, INA240 will have the best performance.