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TPS5405 problems with switching frequency

Muris Trax
Prodigy 30 points
Other Parts Discussed in Thread: TPS5405

Hi all,

Is there any way to define the TPS5405's switching frequency using Rosc resistor so it remains fixed? I have placed 100k resistor and I have around 270kHz switching frequency when I measure it across Schottky diode. Any frequency above 270kHz is OK for my end circuit.

The problem begins when a relay turns on in my circuit (more current gets consumed) - so the switching frequency of TPS5405 drops and tends to interfere with another oscillator in my circuit that operates at 40-150kHz.

How can I stop TPS5405 from dropping the switching frequency below 270kHz or even more?

EDIT: I am using 1N5817 diode and 82uH coil from Coilcraft (MSS1048-823ML).

Thanks in advance!

  • 0
    TI__Guru**** 191445 points

    TPS5405 uses fixed frequency operation.  But there are conditions that can cause it to change.  Most likely you are entering pulse-skipping at light load conditions.  The datasheet says that the peak inductor current must be greater than 300 mA to avoid this.  I think that value is somewhat unpredictable though.

  • 0
    Mastermind 7395 points

    Rosc programs the device frequency, but it skips pulses for light loads. If that is a problem for you, you can try a higher switching frequency or to use another controller than doesnt have the pulse skipping feature.

  • 0 in reply to Avi Chami1
    Prodigy 30 points

    Thank you both for your answers.

    I thought it was the pulse skipping option that is causing me trouble, but strangely I don't have any heavy loads in my circuit. Maybe relay draws too much current when it first turns ON so TPS5405 thinks it is the short circuit condition and starts prescaling the frequency...

    When I put resistor value of around 68k it oscillates at 900kHz-1.2MHz but even then it reduces the frequency to make me problems.

    Thanks!

  • 0 in reply to Muris Trax
    TI__Guru**** 191445 points

    There is also frequency foldback.  If the feed back voltage falls, then frequency is reduced.  You should look at the output voltage when the relay is switched.  Maybe you can add some additional output capacitance if that is your issue rather than pulse skipping.  In any case it always helps to post your schematic and waveforms.