EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: law_freak on May 08, 2017, 06:07:08 pm
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Im interested in using the PCF2127 in my project over the DS3231, but do not want to change the power supply of the design.
The PCF2127 is much cheaper and has more flexible IO options.
Is it suitable to power the PCF2127 from a series diode to the +5V rail? The device accepts up to +4.2V.
What variety of diode should I choose? Any diode with a Vf of 0.8V or more?
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Any normal diode will do. But 4.2v is the absolute max voltage , so 2 diodes in series will do the trick.
What ever diode you choose ,check the datasheet of the diode for the plot graph of Forward Voltage to be sure.
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The absolute maximum voltage is actually 6.5V, however the device is not specified above 4.2V. So it shouldn't be harmed by running it at 5V, but it may not perform to its specifications.
The PCF2127 claims to have a typical Idd as low as 470nA, so even two jelly bean diodes in series may not be adequate to drop the 0.8V required. A separate regulator (even a zener as a shunt regulator) could be a better bet.
Also note that you'll need to make sure that logic levels are compatible among the devices in your system.
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If you worry about the minimum current through the series diodes, then add a suitable resistor from Vdd to Vss to pull a mA or so through the diodes at all times. Again, see the I-V curves for the P-N diodes, including at any extreme temperatures you may expect.
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If you are using a signal diode like 1N914, or, 1N4148, you must use at least 2 since at low current, the voltage drop will only be 0.4v, especially if the temperature goes up.
Or, if you are using 2ma or less, and you don't mind a slightly larger voltage drop, you can try a red led.
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I will try the series pair diode suggestion. After seeing that +5V is within the absolute maximum range, I will also see how the device performs from full +5V.
I am not a fan of the cost of the DS3231Z, but would like an integrated-crystal RTC solution. I'll spin up a cheap breakout board and see how the PCF2127 performs.
Thanks for the suggestions all.
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Or if your board have a white or blue power indicator LED, tap off there like a Zener shunt. Only white or blue LEDs have enough voltage drop across them for this.
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My board has three indicator LEDs. One happens to be blue. For curiosity's sake, I will try that too.
My first test is to try running the PCF2127 at +5V and see how it fairs.