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LM74670-Q1: Cannot get device to switch FET

Peter Young
Prodigy 10 points
Part Number: LM74670-Q1
Other Parts Discussed in Thread: LM74610-DQEVM

We are developing a bridge rectifier with ideal diodes. Max current is low (<100mA),  AC input voltage 10Vpp.

We were originally going to use the LM74610 device because the datasheet showed it could cope with a 60Hz AC input (see pic below). This is obviously a mistake in the datasheet because we learnt that this device is too slow for our application which is around the 200Hz mark. We were able to get the device switching correctly at 1Hz max only.

We then replaced one of the LM74610 devices on the LM74610-DQEVM PCB with a LM74670 device, but have not been able to produce any rectification ...we just see the AC input voltage at the output i.e. no rectification at all. (Note, we are not powering the 2nd LM74610 device on the PCB at all). We tried several values for VCAP i.e. 0.47uF to 2.2uF with little effect. 

Can you please provide some suggestions on what to do?

  • 0
    TI__Mastermind 33765 points

    Hi Peter,

    Thanks for reaching out to us.

    Can you share your schematics and waveforms?

    Thanks and Regards

    Shiven Dhir.

  • 0 in reply to Shiven Dhir
    Prodigy 10 points

    Hi Shiven,

    This is our test circuit below, and waveforms below that at 2, 10, 50 Hz input supply.

    As you can see, it is not switching correctly especially when the frequency is increased. We have tried many values of VCAP ranging from 2.2uF to 10nF without success (large DC offset above 10Hz).

    Does this mean that the LM74670 device cannot handle changes in input frequency? 

    Our application has changing input supply frequency ...from 1Hz to 100Hz range. Is this device suitable?

  • 0 in reply to Peter Young
    TI__Mastermind 33765 points

    Hi Peter

    The schematic looks okay to me.

    May I know the output capacitance and the load here?

    Looks like the output capacitance is charged here and there is very little load to discharge it when the FET is in OFF state due to which you see this Offset.

    Thanks and Regards

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 10 points

    Does this mean that the LM74670 device cannot handle changes in input frequency? 

    Our application has changing input supply frequency ...from 1Hz to 100Hz range. Is this device suitable?

  • +1 in reply to Peter Young
    TI__Mastermind 33765 points

    Hi Peter

    This device can handle changes in input frequency.

    If you test with low output capacitance or higher load the offset should reduce.

    Here output capacitance is not getting time to discharge and next cycle of sine wave charges it back because of which you see the offset.

    Thanks and Regards

    Shiven Dhir