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LM5001, low voltage operation

Dan Savage
Prodigy 50 points
Other Parts Discussed in Thread: LM5001

I am planning on a SEPIC design that operates between 7.6V and 48V and regulates 5V out.  The reason for the SEPIC is automotive type test pulses that are in the direction of a power loss that we are required to "ride through".  The SEPIC design helps alleviate the amount of capacitance needed for bulk energy storage on the input.

The LM5001 datasheet indicates that the Vcc pin is valid down to aboout 3V.  The internal gate drive comes from internal Vcc.  Is the FET gate drive fully reliable down to 3V operation?

I'm trying to better understand the current consumption on the Vin pin.  The table data for the Startup Regulator appears to address this but I'm not certain.  The bias current is specified at 4.5mA but Figure 3 in the data sheet shows the disabled non-switching device consuming about 110uA.  Is there better information on this or am I just missing it in the datasheet?

We don't plan on any external loading on the Vcc pin.  Does this imply that the Vcc Current Limit should never be a concern?

Thanks in advance for any help on this.

 

 

Dan

 

 

  • 0
    TI__Intellectual 1135 points

    Hi Dan,

    At VIN=3.1V, you get higher Rdson, see Figure 5 in the datasheet for details.

    4.5mA is the current when the controller is enabled, but is not switching due to VFB (1.5V) is higher than Vref (1.26V typ).

    Around 110uA shown in Figure 3 is the shutdown current when controller is disabled (EN is 0V).

    If there is no external loading on VCC pin, the VCC current limit should not be a concern.

    Hopefully these answer your questions.

    Thanks.

    Haifeng

  • 0 in reply to Haifeng Fan
    Prodigy 50 points
    Haifeng,
    Thank you. I reviewed the points and am clear on what you state about the Rdson variation with Vcc. I have a followup question.

    How does the Vin pin current get determined when the control loop is actively regulating the output (VFB = Vref)? I need to size a capacitor connected to this pin based on realistic currents under all conditions.

    Thanks for your help.


    Dan
  • 0 in reply to Dan Savage
    TI__Intellectual 1135 points

    Hi Dan,
    VCC rather than VIN is used to provide transient current to drive the FET, so you need to size the capacitor connected to VCC pin instead of VIN pin. Just follow the datasheet to choose the cap for VCC pin, 1uF is typically a good practice. As to VIN pin, you might want to add a small decoupling cap close the PIN, but that is not intended to supply large transient current.
    Thanks.Haifeng

  • 0 in reply to Haifeng Fan
    Prodigy 50 points
    Haifeng,
    I see a problem with this. If the Vin pin falls below the Vcc pin then the body diode depicted on the regulator FET will prevent Vcc from being held up by the capacitor we are talking about.

    Even if that is not a real problem (as I predict) I would need to know the current consumption on the Vcc pin to guarantee meeting our holdup requirements. We have power outages in excess of 20mS that are driving the large energy storage cap and the plans for configuring the LM5001 device.

    I look forward to hearing back from you. Thanks.



    Dan
  • 0 in reply to Dan Savage
    TI__Intellectual 1135 points
    Hi Dan,
    You can esimate the power/current consumption by calculating the driver loss (Ploss=Qg*Vcc*fsw), where typical value of Qg is 4.5nC when VCC=6.9V, fsw is the switching frequency used in your design.
    Thanks.Haifeng
  • 0 in reply to Haifeng Fan
    Prodigy 50 points
    Haifeng,
    Is it accurate to say that when there is no PWM switching that the maximum current draw on the Vin pin is 4.5mA from the other internal circuits that use Vcc?

    And that when there is PWM switching the current draw is 4.5mA plus the gate drive current mentioned in your previous reply?

    Thanks for your help.


    Dan
  • 0 in reply to Dan Savage
    TI__Intellectual 1135 points

    You are right.

    Thanks.

  • 0 in reply to Haifeng Fan
    Prodigy 50 points

    Haifeng,

    Thank you.  I have enough information to move forward.

     

    Dan