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LMZ34002: Load driving issues using a -10V DAC output range

Carl Trudeau
Prodigy 60 points
Part Number: LMZ34002
Other Parts Discussed in Thread: DAC81416,

Greetings community, 

I'm currently having a ripple issue using the LMZ3004 as a inverter to generate -12V from a 12V input. It is current limited at 700 mA by a current limiter IC. The -12V output is used to feed the VSS supply pins of DAC81416. 

I'm driving the 16 output channels with a 2k load on each (5 mA at +/-10V range). I'm using a parallel load with a dip switch to toggle some of the channels between 5mA and 25mA, as the datasheet mentions it can drive up to 25mA on each output. The problem I'm observing goes as follow: With the 10V output range with the DAC, I'm observing that the limiter is effectively working after enabling up to 7 switches for changing the loads. 

I'm using 4 DACs in my actual design, the mentioned with up to 7 switches toggles to drive a load of 25 mA and the rest is driven at 5mA on each outputs. So that makes a total of : 

-9 channels at 5mA == 285 mA

- seven switched channels at 25mA == 175 mA ,

- the supply current drawn by each of the 4 DACs Vdd , 25 mA each according to the datasheet (100mA total)

- the supply current drawn by the Vss supplies of the 4 DACs, 60 mA and since the the efficiency of the LMZ34002 is about 85% : roughly 110 mA 

- 285 + 175 + 100 + 110 == 670 mA 

Considering the precision of the loads, this amount of current seems like the threshold for my current limiter. 

But then, when I switch the range to -10V, the issue occurs : 

The overcurrent fault occurs at a much lower current threshold, as I'm only able to toggle 3 switches, which makes roughly a difference of 80 mA.  

Also, the ripple observed at the LMZ input is about twice as bad , about 200 mV pp. 

I'm using 10uF ceramic low ESR decoupling capacitor as per recommend by the datasheet, tried a 4.7uF also, the ripple remains the same. 

I'm wondering if somebody might have a clue about what I'm observing, I can provide more details if necessary :) 

Kind Regards

  • 0
    TI__Mastermind 31550 points

    Hello Carl,

    Thanks for sharing a detailed description of your system and the problem you are encountering.

    With SMPS, you will have some ripple and with particular modes of the converter that ripple can become worse by the switching nature.

    For your given application requirements we can finesse the operation of the converter to meet those requirements.

    First, I need to visualize the ripple you are seeing and confirm with the given power conditions: Vin, Vout. Iout, Fsw, BOM, the output ripple makes sense.

    Can you provide waveforms of: Vin voltage, PH voltage, and Vout voltage?

    Also, can your provide the DC load current for the given waveform?

    Lastly, can you provide the schematic and BOM utilized in your evaluation?

    From inspection of the above materials I hope to give corrective action to reduce ripple assuming I am able to recognize the problem.

  • 0 in reply to Marshall_Beck
    Prodigy 60 points

    Hi Marshall, 

    thanks for your reply. The first screenshot below is the 12V input voltage of the LMZ whilst the DAC range is at 10V. 

    Second screenshot is the 12V input while the DAC range is at -10V. Last one, the output voltage while the DAC range is at -10V. 

    Please note that the load is approximatively 470mA for the screenshots below. FSW is the default (800KHz). PH voltage is at zero volt since it is routed to a separate plane and not used. Lastly , the schematic of the actual configuration of the LMZ34002 that is driving the 4 DACs. 

    I will try in the meantime to add an input to output capacitor to see if its makes the ripple "smoother" 

     

    Thanks

    Thanks 

  • 0 in reply to Carl Trudeau
    TI__Mastermind 31550 points

    Thanks, I hope to follow up with you on the review, Monday.

  • 0 in reply to Marshall_Beck
    TI__Mastermind 31550 points

    Hello Carl,

    Can you confirm your measurement approach? Assuming your operating in CCM (I assume you should at least be constant frequency with your quoted loading condition), the device ripple should be sub 10mV I would think.

    Output ripple measurement can be quite difficult sometimes. Here is a good resource (below). I would like to see the measurement with a time base so I can see the periodic switching cycles and captured with low-inductance measurement. probe I often utilize 50ohm resistor in series with signal trace connect to SMA cable, with measurement done at output cap.

    Here is resource

    www.ti.com/.../slvaf30.pdf

  • 0 in reply to Marshall_Beck
    Prodigy 60 points

    Hi Marshall,

    thanks for your help, I managed to solve my issue. Seems like the 10uF tantalum capacitor ESR was too high and induced ripple. I changed it for a low ESR 22uF ceramic and the ripple is quite better, about 70mV. 

    Regards

  • 0 in reply to Carl Trudeau
    TI__Mastermind 31550 points

    Nice, good to hear Carl.

    Let me know if you run into any additional difficulties.