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TPS62120 active mode current consumption specification

John Peck
Prodigy 130 points
Other Parts Discussed in Thread: TPS62120, TPS62745

I'm using a TPS62120 converter to regulate a 12V input down to a 3V output.  My system draws about 100uA quiescent from its 3V rail, and the datasheet indicates about 70% efficiency there.   I thus expect about 100uA * (3 / 12) * (1 / 0.7) = 36uA of current flowing into the buck converter if the TPS device didn't draw any of its own current.  I actually measure about 200uA.  Is the difference caused by the TPS active-mode current?

The TPS62120 specification calls for a 240uA typical active mode current consumption.  Is this something I would see at Iout = 100uA?  Is this current factored into the overall efficiency? 

  • 0
    TI__Guru*** 147960 points
    Yes, your calculations are correct. This is about what you should measure under those conditions. There may be some differences if different inductors are used for example.

    This app note describes the proper measurement setup for efficiency: www.ti.com/.../slva236.pdf Have you measured Vin, Vout, Iin, and Iout with a meter?
  • 0 in reply to Chris Glaser
    Prodigy 130 points
    Thanks for your help. I had a problem with my original measurement setup, so I repeated some measurements. I went through and measured efficiencies for a range of output currents set with a load resistor. My actual Vin = 12V, and Vout=3.265V, so I calculated efficiencies two ways. First, I tried efficiency = (Iout / Iin) * (3.265/12). Second, I tried efficiency = (Iout / (Iin - 13uA) * (3.265/12). The first method showed an efficiency trending to zero as Iout decreased, and the second showed a stable 90% efficiency from Iout=300uA down to Iout=3uA. It seems like the efficiency at low currents is dominated by the buck controller's 13uA quiescent current, drawn from the input voltage.

    I'm still wondering when this active-mode current specification becomes relevant. It seems like the part only uses 13uA to run itself. Do you know when this higher 240uA active-mode current turns on?
  • 0 in reply to John Peck
    TI__Guru*** 147960 points
    Your first equation is correct. I discuss this exact question in my paper (in the How to use Iq section): www.ti.com/.../slyt412.pdf

    Per the test conditions for the active mode current, this is the 'Iq' when the IC is in 100% mode and holding the high side FET fully on. This is important for a very few applications.
  • 0 in reply to Chris Glaser
    Prodigy 130 points
    > Per the test conditions for the active mode current, this is the 'Iq'
    > when the IC is in 100% mode and holding the high side FET fully on. This
    > is important for a very few applications.

    This sounds like the condition we'd get into at the end of our battery
    life. We're (potentially) using 4 1.5V alkaline batteries to make a Vdd
    of 3.3V. The batteries will be at 0.8V at the end of their lives, and
    the FET will be fully on.

    So...could we stay in the Iq = 13uA regime if we reduced our Vdd? We'd
    like to avoid spending this excessive Iq at the end of the battery life.
  • 0 in reply to John Peck
    TI__Guru*** 147960 points
    Yes, you would be in 100% mode at the very end of your battery life and draw the higher Iq. It could be debated how significant this is, as the batteries will only be that low for maybe 10% of their lifetime. Yes, reducing Vdd would help. You might look at reducing it all the time to save power.

    Finally, if you have 4 AAs (instead of 12V), take a look at the TPS62745 with its 400 nA Iq.
  • 0 in reply to Chris Glaser
    Prodigy 130 points
    Excellent. Thank you for the recommendation.