Hi, I have ultra-low power (in my standarts) RTC chip PCF8563. All outputs are open drain. It has a lot of advantages but it is not good for low power - while push-pull output consumes energy only when switching open drain consumes considerable energy for the whole time it is LOW - to “see the signal” you need a pull up resistor on the pin. Is there a way to convert the open drain output to push-pull with minimal current consumption?
to "see the signal" you need a pull up resistor on the pin. Is there a way to convert the open drain output to push-pull with minimal current consumption?
To have a high/low going signal you will need a resistor, preferably a high value one like 1 M ohm for example.
You do not say what you want to drive with the output, usually you would arrange this so you do not need 'push-pull' drive for it to operate.
Currently I was only driving my DMM. 100k pull up was too weak, I needed to use 10k. It increased current consumption of the chip from hundreds of nA to hundreds of uA. As a second thought problem is probably on large capacitance of the DMM. Driving CMOS gate will probably need much weaker pull-up. But push-pull output is much more immune to noise than open drain/weak pull-up resistor. Also 1M resistor with 2V supply and 50% duty drains 1uA, much more than the whole chip - great shame IMHO. But I think there is no way to make strong push-pull from open drain without either greatly increased current drain or greatly reduced noise resistance.
1M resistor with 2V supply and 50% duty drains 1uA, much more than the whole chip
The question remains... what do you want to drive with this output ?
What current will that take when it is switched on ?
If you wanted to turn on a PNP high-side switch, the off state could be very low current.
Most RTC chips are not designed to be micropower when the supply is present, only when the circuit is unpowered and the RTC runs just from a coin-cell. There may be a better device for your requirements.
Thank for your input. In fact I have no concrete project in mind. I like to play with low power. It makes sense that when only RTC is powered open drain output is surely great advantage. When something else is powered and driven by the output it is likely the something will eat a lot more current and current needed for pull-up is neligible.
I think the advantage of open drain is that it works with any logic supply voltage, irrespective of the backup battery voltage, and handles the main circuit being powered down without issue. But you're right it is a problem for micropower operation in this context.
Yes, and even then, if you drive a LED there is nothing wrong with doing it low side / draining current / pulling.
And even in low power pull is more energy friendly then push-pull. You don’t need the line to be at any potential only if it’s on. If you want to check it with, for example a micro, you only need to enable pull ups IF you poll it