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TPS53915: TPS53915 -12V to 3.3V and 12V to 5V not switching

Suresh Reddy1
Prodigy 80 points
Part Number: TPS53915

Hi

We are using TPS53915 in new designs for

12V to 3.3V, 2.5A

and

another config for 12V to 5V, 7A

In both modules o/p is not coming and not switching. The design cross checked with Webench and seems to be fine.

Observations for both DC-DC converters

I/p 12 v is there.

EN pin 3 - 5V thro 100 ohm ; also checked ~3 V

VREG - pin 20 - 5V is there

FB pin 23 - showing 0v,

PGOOd pin2 shows 0V

TRIP at pin 25 shows 0v

VBST shows rises up to 5V but after 100 nf Cap it is zero i,e, o/p

So essentially not switching

Please review and let us know your valuable comments

Attached docs.zip file- it has schematics of both DC-DC converters

ti_dc_3v3.pdfDocs.7z

  • 0
    TI__Genius 16760 points

    The TRIP voltage should be ~200mV.   Can you power down and ohm Trip out the pin and Rtrip components. 

  • 0 in reply to David_Daniels
    Prodigy 80 points

    Hi David

    12V to 5V DC -DC tried

    We have changed to Rtrip to 20K, where we can see

    VTRIP ~200mv ; still no o/p

    also tried with 50K - Vtrip - 0.5V; still no o/p

    VREG - pin 20 - 5V is there

    FB pin 23 - showing 0v,

    PGOOd pin2 shows 0V

    VBST shows rises up to 5V but after 100 nf Cap it is zero i,e, o/p

    Inductor is 1.8uh for 12v to 5v supply

    SER1052-182MLD ; 7.5mohm

    Please suggest what should we do?

    Layout seems to be ok

    Thanks

    Suresh

  • 0 in reply to Suresh Reddy1
    TI__Genius 16760 points

    Suresh,

     

    Does the device communication via PMbus?

    Acting on EN pin, should be the default on the device.  

    But can you read register 02h and 01h? And make sure the registers are set to the defaults.

    Can you send an image of the pcb layout?

    Have you removed and used another device? 

  • 0 in reply to David_Daniels
    Prodigy 80 points

    Hi David

    Please see in line answers

    Does the device communication via PMbus?

    >> Wired up to test from external I2C master. Will update once testing is complete

    Acting on EN pin, should be the default on the device.  

    >> EN ; Logically high on this pin ~3V and also tried with 5V thro series res

    But can you read register 02h and 01h? And make sure the registers are set to the defaults.

    >> We check and update. As of now we are depending this power to power on CFPGA which access this device

    Can you send an image of the pcb layout?

    >> Please find layout and schme for 12v to 3.3V and 12V to 5V supplies

    Have you removed and used another device? 

    >> Both devices are present on board. Tested both of them. O/P is not comingTPS53915_Layout.docx

  • 0 in reply to Suresh Reddy1
    TI__Genius 16760 points

    Please see attached feedback on next steps.

    tejas_e2e.docx

  • 0 in reply to David_Daniels
    Prodigy 80 points

    Hi David

    Both 12V to 5V and 12V to 3.3V are working after removing a capacitor in parallel with upper side resistor of FB. 

    We have removed capacitors across upper side resistors of both supplies. Now fine

    Can you please explain How it works with removal of this cap. Data sheet does not recommend this.

    But we need some technical understanding what would happen at FB with Cap and why o/p will not raise required level?

    In most of our old designs/cards we use this cap, they are working without any issue

    Thanks

    Suresh

  • 0 in reply to Suresh Reddy1
    TI__Genius 16760 points

    The capacitor in parallel with high side feedback resistor, is often referred to as the feedforward capacitor, Cff

    The feedforward capacitor adds a zero in the frequency response which raises the crossover frequency in design that have a lower crossover. 

    Some designs have lower crossover frequencies from using larger value inductor and/or cout and/or the RC ramp setting. 

    I suspect your design is stable without the Cff and using the Cff makes the supply unstable by increasing the crossover frequency thus lowering the phase margin.   The Cff, also, inject sa ripple voltage on the FB pin.

    I suspect the OVP is tripped causing the device to latch off either from the instability or the ripple injection

    Use an oscope and probe the vout or fb pin and see if the output voltage is expected. 

  • 0 in reply to David_Daniels
    Prodigy 80 points

    Thanks David for clarifying that. Yes we have probed and o/p as expected for both supplies and VFB is 0.6V

    I am closing the case

    Thanks

    Suresh