This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LMZ20502: Shorted output to ground

Part Number: LMZ20502
Other Parts Discussed in Thread: TPS82130, LMZ10503, LMZ30604, LMZ30606

We are using the LMZ20502SILT and on 4 of our boards the part "goes bad" after a few power cycles. The failure mode is a very low ohm reading to ground at the VOUT pin and the voltage is stuck around 0.5V.

This mode is consistent with all 4 failures. The output is supposed to be 1.35V. Rtop is  69.8K, Rbot is 56.0K and the cap across Rtop is 56pF. Cin is 47uF/10V and Vin is 3.3V. Cout is pretty high at 825uF and the rail is prebiased at startup. On top of that my enable pin is controlled by a buffer that has no power for about 10mS when the 3.3V is applied. I suspect the output capacitance is way too high. I have 4 failures out of 20 boards. This is a ASIC application which is driving the Cout qty. I might be able to remove some of the Cout and I added a pull down resistor on the enable pin. What is my Cout limit? Is there a better part for the job? 

  • Hi Craig,

    Can you please provide the schematic and PCB layout for review? How much current are you pulling for your application of 3.3V to 1.35V conversion? 

    As you've suspected this device is limited to an effective output capacitance range of 100-200uF. Even after 1.35 DC bias at the output, the effective Cout at Vout of the LMZ20502 is still above 200uF. Do you also have a pull down resistor on EN? It's never recommended to have EN floating. In your case, if you have a pull down resistor on EN when the buffer has no power for about 10mS the EN will not be floating.  

    What is the minimum output capacitance requirement on the downstream ASIC? It might be worth looking into the TPS82130. This is a 17V/3A device in uSiL package(3 x 2.8 mm2) that can take up to 400uF output capacitance. 

    The beginning descriptions(Vout short to GND) sounds like an EOS issue. You might want to send the failed boards in for FA  through this link if that is the case.  

    Regards,

    Jimmy 

  • We are only pulling 50mA or so. The enable pull down will be on the next rev of the board. When the parts fail there is about 4-8 ohms to ground at the LMZ20502 output
  • I was looking at the LMZ10503. How much Cout can that pat handle?
  • Hi Craig,

    I'm curious to know why you would need so much output capacitance if you are only sinking about 50mA. Can you confirm how much capacitance is required in your application?

    Regarding prebias, this device has a negative current limit of 750mA to prevent reverse current from damaging the internal power FETS.

    In the case that you absolutely need 800+uF of output capacitance, the LMZ10503 would not be a good option because that high of an output capacitance would affect the compensation of the part. An alternative to this is the LMZ30604 which should be able to support high output capacitance in your application. This device does not have hiccup mode, so higher output capacitance can be used. It also has a pin-to-pin compatible device(LMZ30606) in the case that you want to drive more current in your design.

    Regards,
    Jimmy
  • My mistake, We are using 500mA. Still relatively low for 800uF+. The 800uF comes from the reference design which is pretty specific, its a NXP LS1021A. Does the LMZ30604 tolerate the prebias rail?
  • Concerning our existing design with the LMZ20502, We have the floating enable for 10mS after the Vin comes up, 825uF of capacitance and the prebiased rail. Would any of those three conditions cause the parts to fail? I should note that 470uF of that lump sum is a single Panasonic EEF-GX0D471R, 470µF 2V Aluminum Polymer Capacitor 2917 (7343 Metric) 3 mOhm 2000 Hrs @ 105°C capacitor. It should be more than 2V probably.
  • Hi Craig,

    Internally the LMZ30604 has reverse over current protection in the event of power cycling and pre-biased output startup. Since LMZ30604 allows larger output capacitance to be used, I'd recommend changing to that device since you are fixed on 825+uF.

    Concerning the LMZ20502 operation, 825uF of capacitance is definitely outside of the recommendation for that device. This could result in unstable design and adversely affect the stability and regulation of the device. It could also be that the device sees a short circuit since too large of an output capacitance would required considerable longer to charge the output capacitors up to final voltage. Perhaps you can test if this is the case in your system by removing output capacitance until the effective Cout is between 100-200uF range to see if this goes away.

    Regards,

    Jimmy