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I am trying to calculate how much current some components will draw. However, it is not clear to me from the datasheets. I also have several other questions.

Firstly, the NL17SZ08 AND gate

MAXIMUM RATINGS

  • DC Input Diode Current -50mA
  • DC Output Diode Current -50mA
  • DC Output Source/Sink Current ±50mA
  • DC Supply Current per Supply Pin or Ground Pin ±100mA
  • Latchup Performance ±100mA

DC ELECTRICAL CHARACTERISTICS

  • Input Leakage Current ±1.0μA
  • Quiescent Supply Current 10μA

DC Supply Current sounds relevant but I don't understand the ± or the per Supply Pin or Ground Pin. 100mA seems like a lot of current for an AND gate, which I would expect to be simple and low power.

Secondly, the 6N137 optocoupler

ABSOLUTE MAXIMUM RATINGS

  • Average forward current 20mA
  • Enable input current 5mA
  • Surge Current 200mA
  • Output current 50mA

RECOMMENDED OPERATING CONDITIONS

  • Input current low level 250μA
  • Input current high level 15mA

ELECTRICAL CHARACTERISTICS

  • Reverse current 10μA
  • High level supply current 7mA
  • Low level supply current 7mA
  • High level output current 1μA
  • Input threshold current 5mA
  • High level enable current -1.6mA
  • Low level enable current -1.6mA

If the max average forward current is 20mA, then will the max forward current is 40mA? What is the High level supply current. That sounds reasonable for an LED + some other stuff

Lastly, the 74HC165 shift register

  • Absolute maximum supply current 50mA - is this only relevant if this chip was connected in series with something which draws current? When connected "normally" in parallel with some circuit, would this ever be an issue?
  • supply current 160 μA + additional supply current approx 2500μA = 2.7mA - is it really this low?
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  • \$\begingroup\$ Absolute maximum ratings are the values for which they can't guarantee the device will survive - they are device damage values, not vales that it would consume during normal operation. \$\endgroup\$ Commented Oct 12, 2020 at 23:59

2 Answers 2

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Under normal circumstances, devices are not operated beyond rated conditiond. Under fault conditions, those may be exceeded but immediate damage is avoided if conditions are limited below absolute maximum ratings.

AND gate: consumption is 10uA. More current will flow on supply pins if the chip is driving a load or clamping voltage, and max current it tolerates is 100mA.

Optocoupler: no max current won't be 40mA. It tolerates a surge of 200mA for a short 100us period defined in the datasheet, but long term average absolute max is 20mA. Recommended rated current is max 15mA which means under normal conditions. High level supply max current means how much the supply current is consumed by the coupler at most when data output is high, not driving a load, and it has nothing to do with the LED.

Shift register: When the chip is abused under abnormal conditions again like overvoltage or shortcircuit or too much load, it does not get permanent damage if supply pin current stays below 50mA. The chip really itself consumes max 160 uA under extreme conditions, and will consume the extra current for each input that is held at indeterminate voltage level between logic high and low, which again is not valid input to the chip and should never happen.

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  • \$\begingroup\$ The AND gate will be controlling an input (a transistor base / microcontroller or chip input) Can I assume the current draw is negligible for when considering overall power consumption? Re the optocoupler, you say it tolerates a surge. I expected it meant that it might draw this amount. If you are able to say, why is this information in the datasheet? How much current it can tolerant in 100us seems very specific Re the shift register, cool thanks. \$\endgroup\$ Commented Oct 12, 2020 at 23:26
  • \$\begingroup\$ Transistor base will need some DC drive current so that is not negligible, whether that is enough to matter in your case I don't know. Driving another chip does not take DC current, but switching takes AC current. The datasheets tells all kinds of information that may be useful in some applications, even if they are not important to your application. They guarantee that it can take that pulse without breaking, due to current, or power, or momentary heating so it can cool down between pulses, etc. But if you turn the LED on indefinitely, your circuit must limit the LED current to below 15mA. \$\endgroup\$
    – Justme
    Commented Oct 13, 2020 at 8:17
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This is a complicated topic. A schematic and usage parameters are required for a complete analysis.

The absolute ratings are meaningless.

I always calculate the dissipated power (not always the same as an IC Vcc multiplied by the current), since that can also be used for the thermal analysis. If an IC is driving a lot of current out, it is dissipated at the destination.

For passive or simple semiconductors use the simple power equation. For the optocoupler, break it into two parts.

Pd = V * I

For the ICs, the quiescent current is very low, the power is highly dependent on the frequency, you need to use the power dissipation capacitance to calculate the power. The spec should list the Cpd.

Pd = Cpd * VCC^2 * fin + ICC * VCC

Power is rarely constant in a working system, you often need to factor in the duty cycle of the circuits.

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  • \$\begingroup\$ Thank you. This is more detailed than I think I need, but I will do all the *ing and +ing tomorrow. \$\endgroup\$ Commented Oct 12, 2020 at 22:49

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