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I want to power STM32F429 Vbat pin with an existing Lithium Ion battery on board. However, the maximum voltage on the discharging curve of this battery is 4.2v which is above the maximum operating voltage of the Vbat pin(3.6v).

My question is can I directly connect the battery to the Vbat pin, or should I use the voltage divider to divide the battery voltage into a suitable range? If I have to use the voltage divider, is there any problem if I simply use two resistors to divide the voltage?

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  • \$\begingroup\$ A diode would be better since it would just lop off a constant 0.7V regardless of current draw. \$\endgroup\$ – DKNguyen May 20 at 3:54
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VBAT pin is for a backup battery for the real-time clock built into STM32F429. RTC backup batteries are usually non-rechargeable Lithium cells. Their voltage doesn't exceed +3V. CR2032 is an example of such battery.

If your design doesn't have a separate RTC backup battery, connect the VBAT pin to Vcc.

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No you shouldn't connect it directly. And you shouldn't use a voltage divider to create the right supplying voltage. When the power draw changes the voltage will also fluctuate creating problems for you STM32.

The best option would be a LDO (Low Dropout Regulator) this has a small dropout voltage making it run longer on battery.

The MIC5504-3.3YM5-TR is a example of a usable LDO. It has a dropout voltage of 160mV meaning that the battery voltage must remain above 3.3V+160mV for it to remain operational. It can supply 300mA which is probably more than enough for your application. It also has a low quiescent current of 38μA. (current during no load)

A diode would also be possible but this will have a constant voltage drop of 0.7V instead of the 160mV the MIC5504 has. Resulting in less on time.

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