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TPS7A4701-EP: Adjustable mode operation

Nishanth B
Prodigy 100 points
Part Number: TPS7A4701-EP

Hi All,

I want to use the TPS7A4701 LDO in adjustable through resistor mode. What should i do with the programmable pins for best design? (Do u suggest me to leave it floating?)

  • 0
    TI__Mastermind 34715 points

    Hello Nishanth,

    If you will not be using the ANY-OUT pins for your output voltage programming, please leave them floating.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    TI__Prodigy 100 points

    Hi Stephen,

    I am using the LDO  to power an RF oscillator and an RF amplifier at frequencies of the range 400Mhz-800Mhz. I am hoping i can use this LDO for this purpose and the noise in the environment wont seep into the ANY-OUT pin inputs and change my output voltage. are both my assumptions correct?

    Thanks and Regards,

    Nishanth A B 

  • 0 in reply to Nishanth B
    TI__Mastermind 34715 points

    Hi Nishanth A B,

    I have used this device in a similar environment in the past, successfully.

    Noise can be divided into 2 categories: Radiated and Conducted.  As explained below, you do not have to worry about conducted EMI if you design the board layout correctly.  Radiated EMI is also unlikely to cause an issue due to the internal low pass filtering of the device, from the ANY-OUT pin to the error amplifier input.

    To protect against Electric Field Coupling EMI, it is absolutely critical that layer 2 of the PWB, underneath the LDO circuitry, be the GND of the LDO.
    This GND will act as an electric field shield, which will prevent coupling of noise from layer 2 directly into the LDO circuitry.  In particular, pay close attention to the feedback (FB) pin.  This is the most susceptible pin in the LDO as it does not have filtering. 

    Severe load transients can couple into the FB pin if the output capacitance is insufficient.
    Using several parallel output capacitors of different values (high to low) can help provide a low impedance to any load transients.
    An excellent resource on this topic is "Electromagnetic Compatibility Engineering" by Henry Ott, pages 441.

    It is unlikely that noise coupling into the floating ANY-OUT pins will cause an issue.

    1. If you have an electric field shield (layer 2 = GND of the LDO), then capacitive coupling cannot exist.
    2. There is no shared impedance with a floating pin, so Conducted EMI cannot exist.
    3. The ANY-OUT pins each have high impedance from the internal series resistors, followed by the NR cap.
      This forms a very low pass filter, which will filter any radiated noise that enters the pins.

    Thanks,

    - Stephen