On the data sheet of SN74HC164 there are two statements that I apparently don't understand:
- Cover page says: ±4-mA Output Drive at 5V
- Page 3 says: Continuous output current Io at Vo between 0 and Vcc = ±25mA
So which one is what?
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2 Answers
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Page 3 in the Absolute Max Ratings chart, shows +-25mA.
This is the maximum ratings for that device, do not allow it to sink or source more than this, with all it's outputs on.
Cover page is referring to each output pin, being able to sink/source up to 4mA each. If you had all 8 outputs at maximum current sink, you would be exeeding the max ratings by (32-25 =) 7mA. The device will probably overheat and become damaged.
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\$\begingroup\$ This is what I assumed as well. I was mislead by the singular form used in the definition of Continuous Output Current - "This is the maximum output source or sink current that can flow safely into an output terminal of the device at voltages within the normal operating range." See this article: ti.com/lit/an/szza036b/szza036b.pdf \$\endgroup\$ Commented Mar 26, 2015 at 22:20
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\$\begingroup\$ I would read the Asolute Max Rating current as applying to each output pin, not to the whole chip. There is a line for Continuous current through Vcc or Gnd, of 50 mA. \$\endgroup\$ Commented Mar 26, 2015 at 23:42
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\$\begingroup\$ @PeterBennett hm... that's true, I saw that too and was wondering. The output driver stage must only be able to handle 25mA, but if it can only drive +-4mA then how can it really even achieve that? \$\endgroup\$– KyranFCommented Mar 26, 2015 at 23:55
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\$\begingroup\$ @PeterBennett Ah I read in the document linked by Evgeni, the max rating for continuous output current shown is for a single output pin terminal. But remember if the pin is sinking current it can sink far more than this without current limiting resistors I suppose. It would damage a single output pin's sink/source driver stage transistor/s. The 50mA max through VCC or GND is the total max for the package, because the drivers could collectively be sinking some external signals to GND, and exceed the max rating quite easily. A lot of these devices can sink current far better than sourcing it \$\endgroup\$– KyranFCommented Mar 27, 2015 at 0:06
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\$\begingroup\$ @EvgeniNikolov see above, does that make sense? The +-25mA is a single pin's max rating, but the total max for the VCC (souring current) or GND (sinking it) for the device is +-50mA. \$\endgroup\$– KyranFCommented Mar 27, 2015 at 0:07
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Page 3 is MAX specs; i.e. do not exceed!!! so in a short circuit situation you could see 25ma from the output. The recommended conditions is to use 4ma as the typical value. the table on page 4 shows the Ioh and Iol under varying load conditions but only up to 5.2ma.
