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Add delay to LM5006 enable

David Dean
Expert 2305 points
Other Parts Discussed in Thread: LM5006

I am using the LM5006, and I need to add a delay of ~10 seconds from when input voltage is applied until the regulator generates an output (5V in my case). 

Timing accuracy is not very important.  +/- 50% should be acceptable.

Normally I would use an RC on the regulator enable pin, but it doesn't appear to be that simple for this part - the RT/SD pin performs two functions, and I wouldn't want to effect the switching frequency.  Also, the shutdown is active low, which makes it tricky to control with a voltage that is ramping UP.

Perhaps the correct pin to use is actually UV, rather than SD?  My relatively long delay would require a pretty big capacitor if I am stuck using the ~200K resistor connected to the UV pin - not so great.

I am trying to keep the circuit as simple as possible.  A correctly placed RC would be best.  If needed, a discrete transistor or two could be OK.  I would like to avoid any reset or timer ICs, if possible.

  • 0
    TI__Expert 6365 points

    Hey David,

    There is no harm using the RT/SD pin to set the delay of the IC startup with a RC.  The switching frequency of the LM5006 device is depended on the RT resistor current so adding a fairly large cap should just prolongs the startup until the cap is fully charged.

     

  • 0 in reply to Howard Chen-WEBENCH Design Center
    Expert 2305 points

    Howard,

      Interesting!  As the voltage on the RT/SD ramps up, will the device begin to switch at a lower frequency, eventually speeding up to the frequency set by the Rt resistor?  Or will it not switch until a certain threshold voltage, and then begin to switch at the specified frequency?

      If I were to go with this approach, what value can I use as the shutdown voltage of the Rt pin?  The datasheet says that the part is put in shutdown by "taking the pin to ground", but it does not specify the shutdown voltage threshold.  I assume it is actually some non-zero value, and I would design my RC to work with that threshold.

    Thanks,

    Dave

  • 0 in reply to David Dean
    Expert 2305 points

    Howard,

      Another potential issue is that my Vin is not constant - it will often be changing to anywhere within the valid Vin range.  Suppose Vin is ~10V and my capacitor on Rt ramps up and the regulator turns on.  If Vin then goes up to 30V, what will happen?  Will the regulator shut down?  Or will it switch at a different frequency until the voltage at Rt catches up?  This seems like a potentially bad situation...

    Thanks,

    Dave

  • 0 in reply to David Dean
    Expert 2305 points

    Hi Howard,

      Just checking in.  I have been working on this and I really like the idea of implementing the delay with a single capacitor attached to the RT/SD pin, as long as my changing Vin won't affect the regulator operation.  Any update on that question?

    Thanks,

    Dave

  • 0 in reply to David Dean
    TI__Expert 6365 points

    Hey Dave,

    The shutdown threshold level is in the datasheet, Page 4; in the On Time Generator section and the typical level is at 0.9V.  The regulator will wait on the cap to charge to the threshold and then it will begins to switch.

  • 0 in reply to Howard Chen-WEBENCH Design Center
    Expert 2305 points

    Yes, you are correct about the shutdown threshold...I just missed it.

    My only remaining concern is what will happen when there is a change in Vin.  Normally the voltage at RT would change right away, to follow the graph on page 7.  But since the capacitor will slow the voltage change at RT, will the regulator switch at a different frequency until the RT pin charges (or discharges) to the correct voltage? 

    I guess my concern is because the datasheet does not mention much about how the RT resistor is used to control the switching frequency.  I am worried that the capacitor will cause problems when Vin changes.

    Thanks for all the help,

    Dave

  • 0 in reply to David Dean
    TI__Expert 6365 points

    Hey Dave,

    Adding the cap will not impact the operation of the device but the delay time will change with Vin level.  The shutdown threshold is well below the Rt threshold so it will have no impact with the device operating conditions.

    Regards,
    Howard

  • 0 in reply to Howard Chen-WEBENCH Design Center
    Expert 2305 points

    Great!

    I guess this is one last question - is this a "normal" way to achieve a startup delay for these parts?  I intend to add a 100uF cap from RT to ground, and along with the 200K RT resistor, I should get something in the neighborhood of a 10s delay (depending on VIN).  In your opinion, is this a pretty decent solution or is it a bit of a hack?

    Thanks,

    Dave

  • 0 in reply to David Dean
    TI__Expert 6365 points

    Hey Dave,

    Since this device does not have its own softstart pin, it's a good solution to create the delay for this type of devices.