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The battery charger L6924U has two status pins, ST1 and ST2, to tell what is happening inside the charger: charge in progress, charging done, stand-by mode, etc.

In the datasheet STM always writes that these pins are open-collector pins, but in the schematic I see two MOSFETs.

Two open collector outputs are available for diagnostic purpose (status pins ST1 and ST2).

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How can I test which it is, and is it important to know? It is connected to LEDS and a uC. Can I just generally use pull-up resistors?

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How can I test which it is and is it important to know?

It's a common "mistake" in technical papers to refer to outputs as open-collector when they are open-drain and, luckily, in all cases that I've come across, it makes no difference to functionality or, how you use/wire the pin.

So, it's unimportant to know and, it's pointless to test whether the internal devices are BJTs or MOSFETs. They can be either and it will work the same.

Can I just generally use Pullup resistors?

Yes, you need pull-up resistors and the value of the pull-up resistor will be the same whether they are open-collector outputs or open-drain.

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  • \$\begingroup\$ I'll add that a MOSFET drain is capable (depending on drain current) of getting closer to zero volts than a BJT collector, which may be handy. \$\endgroup\$
    – Simon Fitch
    Commented Feb 28, 2023 at 13:56
  • \$\begingroup\$ That argument is usually incorrect when the drain/collector current is quite high (hint IGBTs). But, given this is a low-power logic output, whether it drops as low as 0 volts or only as low as 0.5 volts, is of no consequence @SimonFitch \$\endgroup\$
    – Andy aka
    Commented Feb 28, 2023 at 14:07
  • \$\begingroup\$ another question to this topic: if I do a pullup resistor to 3V3 instead of Vin = 5V (USB) is there a difference? Meaning will there be a current flowing into the SD pins when Vin = 0V (USB not connected) but the pullup is still to 3V3 ? (should I add this to my question above? not that fluent in stackoverflowish) \$\endgroup\$
    – Lyoner
    Commented Feb 28, 2023 at 15:33
  • \$\begingroup\$ @Lyoner unlikely there will be any problem to worry about here. Either a bjt or a mosfet will prevent back current. \$\endgroup\$
    – Andy aka
    Commented Feb 28, 2023 at 17:31

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