Proper sizing of PCF2129 Vdd RC network

[tike64]

Hi,

We have had oscillator stopping issues with the PCF2129 RTC most probably because of our own design issues. I am now selecting proper component values for the circuit illustrated in AN11186 Fig 5 on page 13.

The AN says that there may be 1ms from Vdd falling below 2.5V to battery switchover and Vdd should not fall below 1.8V during that time. The AN goes on to say that 330ohm and 6.8uF assures proper operation. But calculating the time from 2.5V to 1.8V with those values gives 740us. This seems not to be adequate but we observed that the monitoring interval seems to be actually on the order of 750us. So those values might work but leave no margin at all. We measured with similar time constant (220ohm, 10uF, 720us) that the voltage on BBS falls really close 1.8V if not below.

So how should we proceed? Can we trust the AN after all or should we choose to go far on the safe side: 330ohm, 20uF, 2.2ms. If we go that far, should we expect some other problems?

[JozefKozon]

Hi Timo,

you can use higher capacitor value. There is no issue expected. Only the turn on time will be longer. There is also a possibility to use Direct switching mode instead of Standard switching mode. Please refer to the section 7.5.1 in the PCF2129 datasheet.

If the Standard switching mode is enabled, the power source is switched to battery if the VDD < VBAT AND VDD < Vth(sw)bat. 

In Direct switching mode, the power source is switched to Vbat as soon as the VDD is lower, than Vbat. 

So the power source doesn't need to drop to Vth(sw)bat=2.5V.

With Best Regards,

Jozef

[tike64]

Hi Jozef,

Thanks for the answer!

About direct switching mode: We are reluctant to use it because it doesn't work with Vbat being higher than Vdd. We measured some of our batteries and they were at 3.23V. Vdd being at 3.3V, that is quite a small safety margin.

What bothers me, is the 330ohm+6.8uF guideline in the AN. With those values Vdd will go below 1.8V in 1ms, in theory it will reach 1.6V which shuts down the power management (if I understand correctly). Is this an oversight on NXP's part or does it depend on the supply turn off speed (or slowness) or did the writer know about some additional margins in thresholds or timings or are my calculations awry.

Regards Timo

[JozefKozon · Accepted solution]

Hi Timo,

you calculation is correct. I have used a calculator in this link. The time at which the voltage at the capacitor reaches the 1.8V from the 2.5V initial voltage with the R=330Ohm and C=6.8uF is 737us. To reach the time of 1ms, the capacitor must be 9.3uF. Or if I have left the capacitor C=6.8uF, but I have increased the resistor to 450Ohm. The serial resistance is recommended not to be higher, than 1kOhm. But every resistance increase will increase the current consumption.

Your calculation looks ok. I am not sure why the application engineer recommended these values. You can increase the capacitor value to reach the 1ms.

With Best Regards,

Jozef

[tike64]

Hi,

Thanks for the reaffirmation. We will try to replace a bigger capacitor.

Regards Timo