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Design a IBB that meeting -55V/1.4A output when input = +12V

Lin Chih-Min
Prodigy 10 points
Other Parts Discussed in Thread: LM5088, LM5116, CSD19534Q5A

I want to design a inverting buck boost converter that meeting -55V/1.4A output when input = +12V.

I was using Webench to do this design, and chose LM5088 & LM5116.  I had tried both of  IC but are failed.

I was wondering if other IC have ability can achieve ?

Best regards,

Min Lin

  • 0
    TI__Guru 72622 points

    Both controllers should be feasible here. The max voltage seen by the controller is 55V + 12V = 67V, so a 75V controller (LM5088) may be a bit tight in terms of voltage margin (just ensure that the SW voltage does not have overshoot). If you need further assistance, please forward the schematic.

    Regards,

    Tm

  • 0 in reply to Timothy Hegarty
    Prodigy 10 points

    When test the LM5088 design of webench, practically I saw a sawtooth wave at Vout with an oscilloscope. (Vmax = -4V, Vmin = -38V)

    I also check SW node without overshoot condition (Vmax = +14V, Vmin = -38V)

    And test the LM5116 design of webench, Vout is around -5V, high side MOSFET is almost on.

    Can give me any help about this design ? 

    Best regards,

    Min Lin

  • 0 in reply to Lin Chih-Min
    TI__Guru 72622 points

    Hi MIn,

    I can't open thses links - can you send as PDFs or image files?

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 10 points

    Hi Tm,

    LM5088 & LM5116 PDFs as below

    2671.LM5088.pdf8284.LM5116.pdf

    Best regards,

    Min Lin

  • 0 in reply to Lin Chih-Min
    TI__Guru 72622 points

    Hi Min,

    No need for a 150V FET with the LM5088 75V controller. In fact, its gate voltage Miller plateau is 6V, which is high for the LM5088's 7.8V gate drive amplitude (implying very little overdrive, especially at cold). A 100V FET (e.g. CSD19534Q5A) and 100V diode should work fine.

    Furthermore, the Fsw with a 10k RT resistor is ~550kHz, which is really high for this 77W design. The 6mOhm shunt sets up a very high current limit (120mV CS threshold), so check that the inductor is not saturating. The input current will be ~7A at full load, so the inductor current is approximately 1.4A + 7A. Finally, ensure the input filter is well damped (use an electrolytic).

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    TI__Guru 72622 points

    Similar considerations apply to the LM5116 design. The current limit seems much higher than the inductor current rating.

    The output capacitance at 2 x 470uF is huge and it will be difficult to startup at -55V. Also, the boot cap should be 100nF, not 1nF. Use a 2.2uF ceramic cap across the FETs (from VIN to -VOUT) as this provides the switching current for the FETs and minimizes the power loop parasitic inductance.

    Change to the CSD19534Q5A FET for the low side to reduce Qrr losses at 350kHz (=Vin *Qrr*Fsw).

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 10 points

    I have changed boot cap to 100nF, 2.2uF ceramic cap across the FETs, replace output cap 100uF or 220uF and Rcs become 10mR. 

    It seems doesn't work for my design, the output voltage become approximately -0.5V, and gate driver is disable, internal regulator seems doesn't work (VCC pin voltage is 0.15V ). Once D5 & Q3 have been burned after replacing the original.  Is the IC burned?

    Best regards,

    Min Lin

  • 0 in reply to Lin Chih-Min
    TI__Guru 72622 points

    Hi Min,

    Please verify the controller is good - it may have been damaged if the FET blew. If problems persists, please check your layout - see app note snva803 for power stage layout best practice.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 10 points

    Hi Tim,

    I have change another LM5116 and test the circuit, Vout become around -14V.

    I also change the divided resistor combination (24.9K & 1.21K), this work as i expected Vout = -25V although it will fail soon that seems inductor to overheat or saturate resulting in Vout drop to -17V.

    These two experiments make me think it’s a duty cycle problem. LM5116 couldn't work as high duty cycle like my requirement ? 

    Best regards,

    Min Lin

  • 0 in reply to Lin Chih-Min
    TI__Guru 72622 points

    Hi Min,

    The duty cycle for IBB is D = Vout/(Vin + Vout) = 55V/(12V + 55V) = 82%. Please ensure the duty cycle doesn't violate the toff-min constraint of the controller for the chosen switching frequency (toff-min for the LM5116 is 450ns typ, 580ns max -- see the tHO(OFF) parameter in the EC table of the LM5116 datasheet). Also, you can verify if the inductor is adequately rated in terms of current (the DC inductor current for IBB is Iin + Iout, which is quite high given the power level).

    Regards,

    Tim