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LM5117: LM5117 no load output voltage is issue

Part Number: LM5117
Other Parts Discussed in Thread: CSD18563Q5A

           I used LM5117 to design two outputs, input voltage 25~40V, output1 is 12V/10A,  output2 is 22V/4A, output 1 is ok, but output2 is less than 22V without load and is ok with load. They're on the same circuit board and the same frequency.

  • Hello,

    Please send the schematic and a completed quickstart calculator file for the 22V/4A design. Are the PCB layouts the same? What about diode emulation mode - is that enabled or disabled?

    Regards,

    Tim

  • They are the same PCB layouts, is there distraction between two ICs with the same frequency on a board. The quick recovery diode has been changed, only no load is issue and load is ok.

  • DEMB is open here, so it's pulled low by the internal 50k resistor to GND, implying diode emulation mode. Try it with DEMB tied to VCC (to get constant frequency FPWM operation). My suspicion is that the boot cap is difficult to recharge at the higher VOUT setting when operating in diode emulatoin mode when very few pulse occur at no load. This depends on the boot diode and boot cap - you can lower the boot cap from 470nF to 100nF so that it charges quickly during the ow-side FET conduction internal when the SW voltage is low.

    As an aside here, note that the NCE6080 is an old technology FET with super high capacitance and gate charge (Qg = 70nC at Vgs =7.6V). This means the DC gate drive current from VCC is Igate = Qg*Fsw = 225kHz*70nC = 16mA. Two FETs will be 32mA plus the IC quiescent current of 4.8mA puts the total quite close to the VCC current limit of 42mA (and well above the min spec of 30mA). Recommendation is to use more efficient 5 x 6mm FETs with lower Qg, such as the CSD18563Q5A, for this 22V/4A design.

    Regards,

    Tim