Reducing Voltage for internal RTC

[FelixK]

Hi,

I've been building some projects using the Arm M0+ KL25z micro controller family on the mbed platform. I'm looking to upgrade to STmicro's STM32F401 chipset. I've been using an external real time clock on the KL25z and I've noticed that the STM32F4 family has one that is built in. For this to work I need to connect the STM32F4's Vbat pin to battery so when my regulator is switched off, the RTC still would be powered. The STM32F4's VBAT can be powered with voltage source of 1.65 to 3.6 V, however the LiPo batteries I've been using output a maximum of 3.7V and charging cutoff at 4.2V.

What would be a reliable way to power the RTC? Would a simple voltage divider or diode be sufficient?

Thanks!