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TPS780 Ground pin current in dropout and when Vout > Vin

Bernhard Boehmer
Expert 1020 points
Other Parts Discussed in Thread: TPS780

Hello TI,

1. What is the TPS780 Ground pin current when in dropout?

2. What is the TPS780 Ground pin current when in Vout >= Vin (Vout & Vin down to 0V)?

Will the part stay well-behaved in this way or would it be necessary to switch it into and out of the circuit in these conditions?

  • 0
    TI__Guru 55940 points

    Hi Bernhard,

    1. The ground pin current for TPS780 series will increase slightly in dropout to 5.6uA.  Please note that I did have a load of 150mA during the measurements.
    2. By Vout, do you mean the voltage at which Vout is set?  If so, this is dropout.  If not, it is not recommended to have Vout greater than Vin.
    TPS780 will behave as described on pg 20 of the datasheet when in dropout.  Essentially the device will behave as a resistor.  PSRR and transient response are degraded in dropout.  During a reverse voltage the current is not limited.
    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Expert 1020 points

    Hello Ryan,

    > The ground pin current for TPS780 series will increase slightly in dropout to 5.6uA.  Please note that I did have a load of 150mA during the measurements.

    Thank you, this helps. We have planned an important micropower (MSP LPM3) mode with the TPS78033022 in which the load current is 4.5uA -  do you expect a significant decrease in ground pin current in dropout?

    > By Vout, do you mean the voltage at which Vout is set?  If so, this is dropout.  If not, it is not recommended to have Vout greater than Vin.

    In a particular micropower mode we intend driving Vout between 3.6V and 1.8V, with Vin floating (the intention is to bypass the device but not to have to switch it out), EN tied to IN. Since Vout is connected to Vin via the pass FET's parasitic diode, Vin could be expected to be a diode drop below Vout, with approximately the same ground current as when in dropout. Can you perhaps confirm this?

    Many thanks,

    Bernhard

  • 0 in reply to Bernhard Boehmer
    TI__Guru 55940 points

    Hi Bernhard,

    I would not expect a significant reduction in ground pin current due to the reduction in load.  In fact I went back to the bench just to check and did not see a change for this device.

    I am still not sure I understand what you mean by "bypass."  From my understanding of how you are intending to bypass the LDO, Vin would remain greater than Vout due to the capacitors.  This would cause the same dropout condition.  Could you share a schematic or maybe block diagrams illustrating the condition you intend to use the LDO in?

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Expert 1020 points

    Hello Ryan,

    We have in the meantime also managed to get hold of a device and can confirm your measurements. When Vout is below the setpoint, we measured up to 6.4uA of GND current in dropout (and let's simplify "bypass" to mean Vout is tied to Vin).

    However, when Vout is greater that the setpoint, the GND current increases dramatically (e.g. Ignd is over 600µA when Vout = 3.6V with the device set for 3.3V). This might be due to the output discharge FET turning on (as it is also documented to do, e.g. for the TPS780330220 when changing from the setpoint from 3V3 to 2V2).

    It is a pity these characteristics aren't documented (as some other manufacturers have done for their parts), as it would have saved many hours.

    Regards,

    Bernhard