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TPS61178: High current at no load (yet another)

Lada Lostak
Intellectual 340 points
Part Number: TPS61178

Hello, I have created 5->11V boost converter using TPS61178. I used this chip previously w/o any problem. This is what I got:

EN=HiZ: no input current at all (few mAmps) - doesn't matter on load (strange, isn't ? Slight smile)

EN=3.3V: 160mA - no load condition

EN=3.3V: 5150mA with Iz=1.52A (Rz=high power wire wound resistor)

I have attached schematics, PCB and scope plot for 0mA and 1.5A (blue=DC output,red=AC output). Power source - 30V/10A low noise non-switched power supply (Vout=5.0V)

PCB details: 6 layers, under TOP layer is solid ground. Nothing special.

Voltage/ripple seems to be OK (except some ringing ?). L8 is physically depopulated. Input voltage physically delivered to solid VCCUSB top solid.

Any idea what could be wrong ? Thank you !

L.

  • 0
    Intellectual 340 points

    And sure, I have found similar issue.. But nothing more.

  • 0 in reply to Lada Lostak
    TI__Expert 4760 points

    Hi Lada,

    When EN is Hiz the behavior is unpredictable and some modules in IC may open to bring few mAmps current. It is normal. And the output voltage ripple seems strange for me. It could be noise coupled from input side.

  • 0 in reply to Yichen Xu
    Intellectual 340 points

    There is internal pull down mentoined in the PDF. But you are right - I didn't notice about HiZ. It is direcly drived by CPU (right now just tied to 3.3V on PCB).

    But I don't have any problem with EN being LOW or HiZ. It works as expected. But no load conditions (EN=3.3V) eats 160mA, which is 0.8W. And it is really strange, because in older designs it was OK as far I remember.

    Do you have any idea why it takes too high power at no load condition ? Can it be over-compensated or some form of oscillation? I will check if I can take shot of another design where I didn't notice this strange behaviour (I don't have EV board). Maybe manufacturer fits bad capacitor/resistor (pretty low probability).

    Thx

    L.

  • +1 in reply to Lada Lostak
    TI__Expert 4760 points

    Hi Lada,

    You are using TPS611781 which works in FPWM so the no load current is large. If you want small no load current, you should use TPS61178 which works in PFM. 

  • 0 in reply to Yichen Xu
    Intellectual 340 points

    Thank you! I didn't notice this information in PDF. I noticed only Figure 4, where is plotted efficiency vs. output current at "Forced PWM". At 1mA it shows 5% efficiency. It means ~20mA. I know I used higher inductor, so it makes sense at the end. Would be nice to note about no-load condition could be really high with higher inductors, because I was looking for that in PDF :)

    L.