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LM2621 Vdd during startup

Jay Paxman
Prodigy 50 points
Other Parts Discussed in Thread: LM2621

I am using the LM2621 to boost 1.8v-3v input to 7v output.  The datasheet says Vdd must be 2.5v-5v.  

Most implementations I have seen online just power Vdd through a 500ohm resistor from Vout (and the Boot pin directly from Vout), even if Vout is above 5v, and nobody has reported a problem.  I am using it that way in my prototype (and it's okay so far), because the datasheet suggested method did not work .  

In the Bootstrapping section, the datasheet says when Vout is over 5v, VDD can be connected to Vout stepped down using a linear regulator.  Can you suggest a regulator that would work?  I used an LDO, but it did NOT work.  The DC-DC could never even get started, because as it raised Vout, that did NOT go through the LDO to get back to Vdd, so the bootstrapping didn't work.  The 3.3v LDO I chose, the output stayed at 0v because its input, Vout, never got high enough.  And Vout never got high enough because Vdd was never there.  Both waiting for the other.  

So, two questions:

1.  Is it OK to run Vdd from 7v through a 500 ohm resistor, just as it is done from 5v?  According to the datasheet, it won't damage anything, but will the circuit no longer function within specs?  Which specs?  Datasheet says:

Absolute maximum ratings indicate limits beyond which damage to the device may occur. Electrical specifications do not apply when operating the device outside of its rated operating conditions.

The absolute max for Vdd is 10v.  The operating conditions ar 2.5v-5v.

2.  Please suggest a simple inexpensive "linear regulator" that will work (as suggested in the datasheet Bootstrapping section) - one that will pass enough voltage and current through it during startup to allow startup to occur -- Vout is connected to BOOT and to the LDO input, LDO output going back to Vdd.

Thank you.

Jay

  • 0
    TI__Guru 72845 points

    7V on the Vdd pin is probably OK.  As long as you do not exceed the data sheet abs max.

    Exceeding the operating ratings may mean that some of the data sheet specs will not be met.

    FD

  • 0 in reply to Frank De Stasi
    Prodigy 50 points

    Thank you for your response.  This design is for a high quantity product, and your answer with "probably" and "may" isn't really assuring.  Could someone in engineering responsible for the LM2621 be contacted to find out for sure?  What affect will Vdd at 7 volts have on the operation of the LM2621?  If it will run out of spec, which specs?  If they cannot assure me it will function properly with a continuous 7v on Vdd, then I will need to add the expense and space on the PCB for a linear regulator.

    Note: I found that the datasheet for T.I.'s LP2980 includes a graph of Vout vs Vin (Fig 9), and it looks like that LDO would work.  As long as the input is over 1v, the output is just slightly below the input until it reaches the regulation voltage.  So, with a 1.8v minimum input in my product, it seems like Vdd (LDO output) should rise with the LM2621 output (LDO input) as the bootstrapping gets the DC-DC started.  Can engineering verify that would work reliably (they suggested a linear regulator -- is this one they would suggest)?

    Thank you again.

  • 0 in reply to Jay Paxman
    TI__Genius 15810 points

    Hello Jay,

    The datasheet does have an abs max rating of 10V on Vdd, but the EC table values and operation testing would have been done with max 5V on Vdd. We can't say for sure what the device will undergo when you apply 7V at Vdd for a considerable amount of time. We do not have data. Quite likely it's life will be shortened. Unfortunately this is the answer we can give you.

    The LP2980 does seem like a viable option for the step down of 7V to 5V for Vdd. Your minimum Vin seems to be 1.8V which should be enough to keep the LDO in dropout mode and keep Vdd happy. 

    Sorry for the brevity in response.

    Regards,
    Akshay