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Original question:

LM7480-Q1: Using lm74801 in output clamp mode

LM7480-Q1: LM74801-Q1 versus LM47800-Q1

Trenton Vollendorf
Prodigy 10 points
Part Number: LM7480-Q1
Other Parts Discussed in Thread: LM74800-Q1, TINA-TI

Tool/software:

Hi, I have been doing some simulating of LM74801-Q1 in a common source configuration (using TINA-TI). I am attempting to use this for a relatively high power application (in comparison to the reference design). I am noticing that at large load currents the forward voltage drop of the ideal diode MOSFET is increasing and preventing Vcap from recharging after Vds of Q2 reaches 150mV. I am wondering if this is due to some hidden threshold (I wasn't able to find advertised on the datasheet) that is causing this to happen? Furthermore, is behavior an artifact the 10.5 mV V(ac) linear forward voltage regulation feature which is unique to LM74800-Q1? Might LM74801-Q1 behave differently?

  • 0
    TI__Genius 15145 points

    Hi Trenton,

    V(ac) of 10.5mV is available only in LM74800-Q1 and not LM74801-Q1. In LM74801-Q1, the DGATE is controlled in ON/OFF scheme and FET will operate at RDS_ON all the time.

    VDS will be ILOADxRDS_ON hence VDS of 150mV is possible. This should not alter the behavior of VCAP. Can you share waveforms and schematic when this happens.

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 10 points

    Hi Shiven,

    The file I am simulating can be found on LM7480-Q1 page, specifically the common source transient model. The only modification I made was to add some voltage/current meters (to view some signals of interest), added a Current generator in parallel with the given load to make the part operate at higher load power levels, and increased the source voltage to 28VDC. 

    Simulation screenshot: Cursor tracking V_diode (which is the forward voltage drop of the diode Vds). Also notice VCAP_2 (which is CAP pin measured with respect the VS pin, ie voltage across charge pump capacitor). When Vds or ideal diode MOSFET reaches 150mV VCAP no longer recharges. I have observed this when substituting other MOSFET models with different values of Rds(on), VCAP always stops recharging when Vds of the ideal diode MOSFET reaches 150mV. 

    I am trying to understand why this is happening, is this a problem with the simulation model, or a behavior which is not specified in the datasheet?

  • 0 in reply to Trenton Vollendorf
    TI__Genius 15145 points

    Hi Trenton,

    This behavior is not expected. I will try to simulate it on my end and get back to you by EOD tomorrow.

    Regards,

    Shiven Dhir

  • +1 in reply to Shiven Dhir
    TI__Genius 15145 points

    Hi Trenton,

    I simulated the same on OrCad Pspice and it indeed replicates. 150mV is the threshold for reverse to forward transition.

    Charge pump not charging is just a model artefact and does not happen on the actual IC.

    In reality charge pump functioning is not altered.

    Regards,

    Shiven Dhir