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LMV331: Limitation of Sink Current on the Output

Matthias Lamp
Prodigy 10 points
Part Number: LMV331

Hello Everybody;

The Output of the LMV331 is connected to a capacitor (app 1.5µF @2V): If the LMV331 gets low, the capacitor will be discharged over the output pin of the LMV331. There is no additional resistor.

(The charging of the capacitor is done with a pull up).

In the datasheet there are values of the maximum sink current is between 10..84mA

. 

Does this mean the sink current is limited by the LMV331 (which would be fine for me) or do I have to take care that the current does not exceed these values?

Thank you 

Matthias

  • 0
    Guru 265250 points

    The electrical characteristics table describes how the device behaves.

    If you want to know how to avoid damaging the device, look into the absolute maximum ratings. They allow shorting the output if the ambient temperature is not too high; if you need to run at higher temperatures, calculate the power dissipation.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    As I describe, I want to recharge a capacitor with the output pin. The absolute maximum ratings describe the behavior when shorted to ground

    and in Note 4 some additional Infos when shorted to Vcc

    Is the sink current limited by device parameters of the LMV331 or not. Is the limiting factor the increase of the temperature?

    Best regards

    Matthias

  • +1
    Guru 140200 points

    Hi Matthias,

    please note the test condition of your specification: It says that an output low voltage of Vo = 1.5V can be observed with an output low current (sink current) of typically 80mA and that some devices can result in an output low voltage of Vo = 1.5V even already at a much lower current of 10mA. This specification tells you how strong the output of LMV331 is during sinking. But it doesn't say that the sink current is limited to 80mA or anything like that.

    Short circuiting a huge 1.5µF cap by the low going output of LMV331 is no good idea. It may survive this torture one or a few times during its whole life time. But you will risk a damage when periodically and repeatedly short circuiting the cap with the LMV331.

    I would recommend to insert a current limting resistor to limit the discharge current to about 10...20mA. See section 8.2.1 of datasheet. So, a 100R resistor would do or better a 220R resistor. This would be good design practise. And if your design allows even a further going current limiting may make sense.

    A current limiting resistor will also minimize the danger of instability of circuit. Otherwise the huge short circuit current may result in ground bounce and ground noise issues.

    Kai

  • 0
    TI__Guru 52116 points

    Hello Matthias,

    Yes and ideally yes...

    The output will go into current limit, of somewhere between 10 to 85mA. Current limit is not a "controlled" or trimmed parameter, so there will be wide variations. Your discharge time could vary widely.

    We do recommend that you limit the current when discharging a capacitor. Capacitors are capable of dumping amps of peak current, and, as Kai alludes to, bad things can happen. As he recommended, place a 220 to 330 ohm resistor between the output and the capacitor. Then subtract the 330 ohms from your pull-up value to make the time constant similar. This will also make the discharge more consistent.

    So if limited to 10mA, worse case 3.3V * 10mA = 33mW, which is 252 * 33mW = 8.3°C temp rise (and that is continuous DC).

    Shorting to VCC when output low will eventually cause overheating. These devices do NOT have temperature limiting.

    I'll join the others to recommend a limiting resistor.