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TPS552892-Q1: not working

StefanS
Prodigy 70 points
Part Number: TPS552892-Q1
Other Parts Discussed in Thread: TPS552872, TPS552892

Hi,
Design is assembled on PCB and not working, output (VLED) is 0V, is there error in design?
BR

  • 0
    TI__Mastermind 27800 points

    Hi Stefan,

    Can you probe the Vin, EN and VCC voltage? I don't see the reason in the schematic which can result in the device not working.

    What is the part number of C4, L1 and output capacitors(C6-C8, C19)? 

    BRs,

    Bryce

  • 0
    Prodigy 70 points

    Hi Bryce, thank you for supporting.

    After inspection turns out it was bad solder joint across enable, it was shorted to gnd. Output is working now, on set voltage. But another thing is now popped. Circuit current increase in 1min to draw 200mA on idle open output. Is it problem with F settings for this input/output range 12V/3.8V? Now is around 400kHz.

    L is Murata 1255AY-4R7M#

    Cout is Murata GRM21BC81A226ME44

    BR

    Stefan

  • 0 in reply to StefanS
    TI__Mastermind 27800 points

    Hi Stefan,

    Can you prove the SW1 waveform? I think the large input current is due to FPWM mode operation.

    BRs,

    Bryce

  • +1 in reply to StefanS
    TI__Mastermind 27800 points

    Hi Stefan,

    I think the large output voltage ripple is due to unsuitable compensation parameters, please try to change R20=3.9k, C21=8.2nF.

    And the input current is about tens mA level at FPWM mode with no load when you connect MODE to VCC, because the internal MOSFETs keep switching at every cycle to obtain small output voltage ripple. If you need smaller input current, it is recommend to select PFM mode by connecting MODE pin to GND, but the disadvantage is higher output voltage ripple.

    BRs,

    Bryce

  • 0 in reply to Bryce Xun
    Prodigy 70 points

    Hi Bryce, it's perfect with you suggested values. 92% eff for my case 12Vin, and 3.9Vout 2A. 20mA at idle no load state.This is signal on open load. Ripple while load is less than 50mV.

    BR

    Stefan

  • 0 in reply to StefanS
    TI__Mastermind 27800 points

    Hi Stefan,

    Thanks for feedback. The waveforms looks ok now.

    Please see my other comments about the schematic.

    1. Recommend to choose the 4.7uF capacitor at VCC with voltage rating>10V.

    2. Recommend to add one 100nF between ISP and ISN close to pins.

    3. Recommend to add one more 0.1uF/0402 close to Vout and GND pins.

    4. Recommend to choose the inductor with saturation current>8A. Or you can choose TPS552872 with lower current limit which is p2p to TPS552892.

    5. Follow the layout guideline.

    BRs,

    Bryce