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Altera_Forum's avatar
Altera_Forum
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9 years ago

MAX7000 max output current question

The data sheet says 25mA per IO current. But it does not seem to say if driving a higher load would result in damage. Sometimes its 25mA max, then classed as a short circuit, where 25mA would be the max current anyway. Though if the pin was shorted to 0v, would that actually damage the device ?

My actual issue is I am driving a large capacitance of about 600pF.. The data sheet doesn't seem to say what the max capacitance drive capability is. For example, if I stuck a 10nF capacitance on a IO pin, the initial surge current required could be 100mA or more, though does the max output just cope with 25mA and doesn't care about the additional load ?

Some IC's just current limit to 25mA and don't really care about anything else, of course the IC would get hot, so thermal considerations come into play, but the MAX7000 datasheet doesn't say anything about max capacitance drive from what I can see.

From a simulation my total load capacitance is needs 60mA to drive it. 25mA current limited would mean it would just take longer to drive the capacitance, and the IC doesn't really care about it. But I just wanted to check on this as if I am pulling 60mA for a few nanoseconds, is it going to kill the IO pin.. The datasheet doesn't even say what peak current is safe to use either.

Maybe I missed something somewhere, but can anyone confirm what 25mA max really means ?

1 Reply

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    9 years ago

    Absolute Maximum Ratings is a well defined term in electronics design, in which regard it might be unclear? In addition, the MAX7000 datasheet provides a diagram Output Drive Characteristics which clarifies that the output current of a pin shorted to ground can be as high as 150 mA. At least long-standing current of this magnitude can be safely expected to damage the device.

    Now about capacitive load. It's normal operation to get higher peak currents than 25 mA when driving capacitive loads. Unfortunately there's no exact specification in terms of stored energy that allows to calculate the maximal acceptable load capacitance, similar to a SOA characteristic of a power transistor. MAX7000 is legacy technology with relative large transistors, I guess they can handle the 8 nJ stored in a 600 pF capacitor at 5 V. But the high peak current involves a lot of ground bounce, so if ever possible I would consider a series resistor that limits the peak current to e.g. 30 or 50 mA and absorbs most of the stored energy.