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LM5176: Webench design problem

Part Number: LM5176
Other Parts Discussed in Thread: CSD18540Q5B

Hello,

We are using LM5176 in out power supply. Vin is 48VDC Vout is 24V with 5.5A. 

The circuit is similar to your webench design.

Power inductor is XAL1010-472MEB, Buck MOSFETs M1 & M2 are BSC340N08NS3-G, boost MOSFETs M3 & M4 are CSD18540Q5B.

The problem we are facing is, with no load the inductor is getting very hot. if we connect small load, the LM5176 blows. BOOT1 (pin 26) getting short with AGND.

Please let us know how to solve this problem?

Our circuit diagram is shown below,

XAL1010-472MEB
  • Hi Bala,

    Thank you for designing with the LM5176.  I took a look of your schematic and nothing obvious can explain the problem.  I am afraid there may be some layout issues.  The layout plays a critical role for the four switch buck-boost circuit.  Please refer to our evm or the datasheet layout guidelines. 

    To help you debug, please tell us what is the exact operating condition when you cause the IC damage:  Vin, Iout, etc.   Also please show the following waveforms: 

    SW1, SW2, LO1, LO2 on the same scope picture.

    Thanks,

    Youhao Xi, Applications Engineering

  • Hi,

    Please check the waveform, Ch1-SW1, Ch2-SW2, Ch3-LO1 and LO2 is 0V. This waveform is at no load condition,

    The IC getting damage at condition Vin is 48V, Voutis 24V and Iout is 1A.

    We used inductor XAL1010-472MEB, but in eval board XAL1510-472MEB used. Is this under rating inductor cause the problem?

    For diode D17(200V, 10A) and D18(60V, 2A), is this part OK for this application.

    In no load condition within 1 min duration, inductor reaches 80degC. is it normal.

    Thanks & Regards,

    Bala

  • Hi Bala,

    Thank you for the info.  I don't have the datasheet of the inductor that you uses. It seems to be a smaller size.  However, an improper inductor may damage the FETs, when it gets saturated, but it should not damage the IC if your FETs are not damaged.    Can you confirm that your FETs are okay?

    The waveforms shows the circuit runs in buck mode, which is the right mode. No problem there.

    D17 is excessive. Why chose such a big part there?  D17 is for the boot strap capacitor and the working current is less than 1A, and 60V rating diode should be okay if your circuit does not ring much, or be safe to chose an 100V 1~2A diode. 

    The no load condition still has circulating current to warm up the inductor.  It may be because your inductor size is too small not able to handle the thermal? Can you change to the one like the EVM  used and see if there is any improvement.

    Thanks,

    Youhao Xi, Applications Engineering

  • Hi Youhao,

    Please find datasheet for the inductor XAL1010-472ME in link:https://www.coilcraft.com/pdfs/xal1010.pdf. This inductor's Irms 24A, instead of eval board inductor is 29A. We use Webench recommended part. And FETs not damaging, all in good condition.

    D17 rating we got from Webench design (Type: Schottky  VRRM: 200 V  Io: 10 A). if it is a huge current, will it create a problem?

    We will trying to get the same eval board inductor.

    In Webench our design input is 9-52 Vin, because of that Rsense if 5.45mΩ. if i change the design with 45- 52 Vin Rsence is 12mΩ. Is this sense resistor creates the problem?

    What is the correct value for Rcomp, Ccomp and Ccomp2 for this application voltage?

    Thanks & Regards

    Bala

  • Hi Bala,

    Thank you.  We also published a design calculator to help customer to get a good start point in design. Could you try it?

    www.ti.com/.../snvc208

    Thanks,

    Youhao

  • Hi Bala,

    Can I assume the issue is resolved?  Let me close it here but you can re-open it by adding a new post.  If you have any updates, please let us known.

    Thanks,

    Youhao