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LM25011: LM25011A IC destruction - would back-EMF diode prevent?

Peter Baxter
Intellectual 390 points
Part Number: LM25011
Other Parts Discussed in Thread: LM2941

I’m developing a LM25011A low-drop-out-voltage step-down converter.

However, this question is about protecting the LM25011A from destruction during development – rather than how to get low-drop-out (If I need to – I’ll ask about that later).

I appear to have destroyed about 5 of these ICs. My latest effort has the LM25011A current limiting the bench power supply and being to hot to touch. Possibly the internal Mosfet is destroyed.

 I’m pondering whether the Mosfet has been damaged by excessive current to 0V? Unlikely in my circuit.

Or is it back EMF? On the 7.2 Functional block Diagram – it doesn’t show a Mosfet diode.

I’m trying different inductors and normal converter operation stops happening. I find that it pulses the inductor at a slower frequency (maybe at - 0.2A 0.62 ohm - Current Limit frequencies).

Under proper operation, the SW node has shortish pulses towards ground.

Possibly once the IC is destroyed, I get an inversion of that - with low level having high level pulses.

Should I put a reverse Schottky diode between Vin and SW?

Is there any other typical reason why these ICs get destroyed? I noticed other customers comments on these ICs failing.

Thanks

Peter

  • 0
    TI__Genius 10820 points

    Hello Peter,

    What are your intended operating conditions? I'm looking for input voltage, output voltage, and output current.

    Also, can you please send over your schematic and layout for me to review?

    Regards,

    Harrison Overturf

  • 0
    Intellectual 390 points

    Low-Drop-Out Voltage Step-Down Converter.

    My application is to take a 12V SLA Battery (12.5V to 14.7V) (DIODE-ORed with a 15V SMPS) and step it down to say 12.2V. I do have room to move the 12.5V upwards – but I want to see what drop-out voltage I can achieve.

    To get a low-drop-out voltage, the Toff/Ton ratio becomes 0.024/0.976 (93ns/3361ns). Given Toff min will be 93ns (from the data sheet), the converter Frequency will need to be around 500kHz (rather than 1MHz). This suggests that with low Vin, the Mosfet Switch is nearly always on (97%).

    The TI downloadable LM25011 spreadsheet won’t process this application.

    I created an excel spreadsheet from the data sheet calculations. The LM25011A suggests an 78uH inductor.  The closest I have is 47uH.

    In developing this circuit, I appear to have destroyed quite a few controllers. Probably most of them were due to something I did. But as they are now out-of-stock, I can’t afford to lose any more.

    The upper circuit is my prototype PCB. Currently the output voltage settles at 14V (rather than 12V). This is probably due to not enough ripple voltage. I currently don’t have the Minimum Output Ripple circuit section loaded. I will try that again later.

    My original question was about, “why is the converter getting damaged?”

    My new question will be….

    Is my approach and the values, suitable to get low drop out voltage?

     

    Even though I have tried to follow the data sheet maths, my excel flow is a bit confusing. I’ve found that it requires looping around and repeating the calculations based upon the inductor value and the RT value.  

     

    There is data on the data sheet that I am having trouble understanding.

    On page 17 under the Prefix RT:

    It mentions minimum Ton times of 90ns and 150ns. These two will come from page 6. I presume the 90ns is the OFF TIMER value from 90(min) 150(typ) 208(max) line.

    Wouldn’t the 208(max) have been a safer choice?

    The other value of 150 is mentioned.

    Is that from the ON TIMER RT PIN ton-1 values?  

    On page 18, mention is made of the nominal current limit value of 1.63A.

    Is that the peak current of 1736mA (mentioned in the 3rd paragraph), with ½ of 200mAp-p (100mA) subtracted from it?

    The schematics show the current design and what I feel that I will need to do to get the bootstrap capacitor section working properly wit the short turn off period.

    LM25011A Low Drop Out Converter.pdfLM25011A LDO 12V.xlsx

  • 0 in reply to Peter Baxter
    TI__Genius 10820 points

    Hi Peter,

    In terms of why the components are getting damaged it is hard to say at this time. Have you taken any scope images of the devices operating normally? Is there a particular input voltage at which the devices are getting damaged? 

    It sounds like you aren't taking the efficiency of the converter into account when calculating the lowest possible input voltage. 

    Use the following equation to estimate the minimum input voltage:

    toff = (1-D)*Tsw

    where D=Vo/(Vin*efficiency)

    Tsw = period of desired switching cycle 

    toff=93ns

    efficiency = ~80% (estimation)

    Regarding your confusion with information on page 17, the 90ns is referring to the minimum on-time. The 150ns refers to the off-time due to that 'respectively'.

    In terms of your question: Is that the peak current of 1736mA (mentioned in the 3rd paragraph), with ½ of 200mAp-p (100mA) subtracted from it? 

    I'll have to look into this more closely and get back to you. 

    Regards,

    Harrison Overturf

  • 0 in reply to Harrison Overturf
    Intellectual 390 points

    Your response made me realise that I was trying to solve the task using the wrong way.

    It also made me think that it would be better to use a Linear LDO regulator such as the LM2941.

    Thanks

    Regards,

    Peter