Recommended circuit for RTC Battery backup

[MartinHo]

Hi

I'm planing of using the LPC5526 MCU for a new project. The Project needs a Battery backup RTC. As I understand the LPC55xx family don't has a dedicated RTC Power-domain ( there is no VBAT_RTC pin only a VBAT_PMU ). So I think that I have to power the entire MCU from my Battery backup circuit. This is not a big problem by it self (the circuit i normally use has a schottky Diode to 3.3V with up to 70mA), but gets critical when the external power (3.3V) gets switched off.

I have to detect the lost of the 3.3V supply and switch the MCU in Deep power-down mode, my problem is that the my simple Battery backup circuit has a rather large serial resistor ( typically 620R ) so the battery can not deliver the current for the running MCU.

Is there any example how to use a RTC Backup battery with the LPC55xx family?

best regards

Martin

[rjm]

To be precise: one would need two diodes: where both cathodes be connected to VBAT_PMU, and one cathode to the battery and the other to external supply which also connects to VBAT_DCDC. So my conclusion is: it is suboptimal to not have a dedicated VBAT_RTC pin as is found quite common.

As a sidenote: in datasheet there is no such thing as VBAT_DCDC_CORE. Actually there may be two pins (qfp 49 and 50) that are both vbat_dcdc.

[xiangjun_rong · Accepted solution]

Hi, Martin,

As you said that there is NOT dedicated VBAT_RTC pin which can power only RTC module for LPC552x family. As you know that there are VBAT_PMU, VBTAT_DCDC, VDD_PMU and VDD, VDDA power supply pin. The VBAT_PMU is connected to internal ALWAYS-ON LDO, the ALWAYS-ON LDO provide power for the on-chip RTC module, so you can connect the back-up battery to the VBAT_PMU pin via a schottky Diode.

Connect the 3.3V external power supply to VBAT_PMU, VBTAT_DCDC, and VDD, VDDA power supply pins, for the VDD_PMU pin, you can use external power supply or internal power supply, it is dependent on you.

Pls refer to the 10.3 Power consumption in data sheet of LPC552x, the total current is about 6mA when the chip running at 150MHz, if your power supply has large serial resistor, because of the low current, it is not a big issue.

Hope it can help you

BR

Xiangjun Rong

The RTC is powered by the VDD_PD_AO power domain for LPC55xx family.

[MartinHo]

Hi Rong,

Thank You for the excellent and fast answer. The second image is very interesting (a shame it was not included in the UM).

Is the document from where it comes accessible?

best regards

Martin