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LM61480-Q1: Design Sanity Check after first implementation failure

Sam Pratt
Prodigy 10 points
Part Number: LM61480-Q1
Other Parts Discussed in Thread: LM61480

Hello, 

I recently designed an LM61480-Q1 based DC-DC converter (actually for powering the display board of a DLP3010-LC EVM, but that is another story) and built up my first board based on the schematic attached below. The 3.3v power supply based on a different switcher (and downstream of the LM61480) is a system I've used many times before, and works satisfactorily. In the first test board, the LM61480 did not appear to be regulating at all; in poking around on the board, it seems to be switching with very nearly a 100% duty cycle at about 100kHz, and is just passing the supply voltage straight through. 

The behavior I'm seeing out of the test board is simple; if I apply say, 12V to the board, the output side of the regulator rises linearly to ~200mV below that supply and stays there. It's possible that I just killed the device somehow during my initial bring-up, but I was careful to limit input current below 50mA (and never saw it draw more than 30mA, mostly when I loaded the 3.3v downstream supply).

I wanted to check that my basic design is sound before I populate another board, since if I've made a gross design error somewhere and need to re-spin the boards, then I'd like to do that before I cook another populated board. Do you see any major issues in the schematic I should look at? On digging through my design and the datasheet, my component values (in particular the inductor, which is this unit from Wuerth) differ some from table 9-2 in the datasheet, but I think that is based on me mistaking the frequency configuration; I must have read the top row of the table (for 400kHz) vs the 2.2MHz line. Is my inductor selection to blame?

Thanks,

Sam

LM61480_Schem.pdf

  • 0
    TI__Guru 50120 points

    Hi Sam,

    The schematic symbols looks okay assuming that your switching frequency is set to 400kHz. Is this your intended switching frequency? Looking at your schematic it looks like the RT pin is connected to the FREQ_SEL netlist. Please make sure that this pin is not floating. 

    • RT, SPSP and SYNC/MODE should not be floated. 

    For your application of 12Vin|5Vout|400kHz, the datasheet Table 9-4 shows a 2.4uH which is similar to your 2.7uH. 

    As for the part, I would suggest picking a device that has appropriate current rating (8A) and saturation current rating following inductor selection guidance in Section 9.2.2.3.

    I would suggest isolating the U1 circuit by itself first to make sure that it can startup at 12Vin|5Vout, which is a very common system application. If the device does not properly regulate 5V and you'd see the expected switching behavior on SW_OUT then I would try another new IC to retest board layout. 

    Regards,

    Jimmy

  • 0 in reply to Jimmy Hua
    Prodigy 10 points

    Jimmy,

    Thanks for the info. I had written out a long reply with scope captures of startup and more questions and design info when I happened to glance at my microscope again, looking at the part and noticed there "6148Q3" on the package. A quick glance at the datasheet shows me... this is the fixed 3.3v variant; of course it's going to give me higher-than-5V output voltages when I run it through a 5:1 voltage divider. 

    It was just an ordering foul-up. I removed the feedback network entirely and shorted the feedback pin to the output. Sure enough, 3.301V on the output, rock solid. Thanks very much for your help; I have a much better feel for this device now. I wish I could have my two days back.

    -Sam

  • 0 in reply to Sam Pratt
    TI__Guru 50120 points

    Hi Sam,

    Okay that make sense and I am glad you were able to find the root cause for this lower output voltage than expected.

    I will be closing this thread now. 

    Thank you for using the E2E forum.

    Regards,

    Jimmy