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LP5907 Vdropout Spec

Tom McKinney
Prodigy 560 points

Scenario, Simplified

We want to use the LP5907MFX-3.3/NOPB with a 3.3V-5.5V input (and “3.3V” output).  It is an LDO and we are unsure about how its Voutput will behave under “3.3V input -> 3.3V output” scenarios.

 

Question

If we supply 3.3V to this chip and use 50mA of power, will we get a consistent dropout voltage (e.g. 3.28V at 50mA load, 3.18V at 225mA load)?   Or does supplying less than “3.3V + Vdropout” go out of spec and cause unpredictable voltage output/current supply issues?

 

More information on why we’re asking this

In our application, our power supply is 3.3V to 5.5V.  Our PCB is tiny and we need to minimize board space requirements.  Most critically, our MCU requires a “3.3V” power supply and, for its reflashing mode, has very tight tolerance requirements.

 

Alternative TI parts?

If the LP5907MFX will not provide a predictable output voltage under this circumstance, what other “buck/drop with voltage passthrough” devices you have that occupy a similarly tiny amount of space?

  • 0
    TI__Prodigy 245 points

    Hello Tom,

    Under "3.3V input -> 3.3V output" scenarios, the device behaves resistively as if the pass-transistor is the resistor between the input and the output instead of a normal LDO regulation mode. Under this condition, the output voltage tracks the input voltage with a consistent voltage drop until the input voltage is lower than the minimum input operating voltage of 2.2 V. So the output voltage and current are predictable and consistent.

    Please note that the output tolerance is +/- 2%, and the dropout for SOT-23 package is guaranteed at 250mV @ 250mA (so pass-transistor Rdson is approximately ~ 1 Ohm). Therefore, the output voltage could be as low as 3.234V, and the dropout voltage at 50mA load could be 50mV or lower. Please also note that, under these conditions (where device is near or below dropout voltage), AC performance will not be as specified in the EC table especially PSRR. Input ripple will also be dropped resistively and proportionally to the output capacitance.

    So it depends on what is critical and tolerable for the MCU reflashing mode. What would be helpful for us to understand is if there is any particular constraint and requirement for MCU reflashing mode.

    Hope this helps answer your questions. Please feel free to let me know if you have any questions or I can be of more support for you.


    Sincerely,

    Zarni Tun

    Applications Engineer

    RF Power Division (LDO)