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I am implementing my own 3-cells battery management system charger based on the BQ76920 but I have a doubt about the REGOUT pin, in the datasheet it says that pin is the output of LDO (Low-Dropout Regulator according to what I read). enter image description here

In the typical circuit application of the datasheet in the same way, this pin is connected directly to the VCC terminal of a microcontroller MCU. So that makes me think that this pin regulates the voltage between Pack+ with respect to VSS (GND) to supply voltage to the microcontroller? doesn't it? enter image description here

I am also considering for reference a github project schematic which implements a BMS with the same integrated circuit (IC) but it uses different devices connected to the REGOUT terminal (Note that it connects the REGOUT terminal to SDA and SCL pins with 10 kΩ resistors R2 and R1, and a bridged solder jumper, and also the capacitor C1 to GND in the same way as the above circuit):

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Another connection of REGOUT is to a LED D23 as indicator:

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REGOUT pin is also connected to ATMEGA328 microcontroller VCC pin (exactly as the typical application of the datasheet) and to the PC6 pin of the ATMEGA with a 10 kΩ resistor R14, what is the purpose of that?

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And finally, REGOUT pin is connected to a voltage regulator (I guess), and a CH340G USB to serial converter which is working in 3.3 V according to the configuration mentioned in datasheet. So basically, the REGOUT pin supplies voltage?

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So my final question is: Do I really need the voltage regulator to feed the microcontroller? Or could I only connect the REGOUT terminal to the VCC terminal of my microcontroller? My schematic is this: (I would connect REGOUT to a female connector to connect a cable to another PCB where the microcontroller is.)

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The last question would be: Do I really need the REGOUT pin connected to the SDA and SCL terminals? (Why is needed? if the datasheet does not show it?) Thank you so much!

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    \$\begingroup\$ You're right, REGOUT supplies 3.3V to the circuit. The PC6 is obviously a RESET pin of the MCU, and it resets the MCU when it goes low (to GND). The 10k resistor is a pull-up to keep it high during normal operation. The T1 is a little unclear to me, is there a model number for it? Finally, you do need the 10k resistors on the SDA and SCL communication lines (they're I2C lines), they serve as pull-ups. \$\endgroup\$ Mar 22, 2023 at 17:26

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The functional block diagram of the BQ76920 shows an LDO regulator built into the chip that supplies the REGOUT pin:

Functional Block Diagram of BQ76920

The datasheet specifically calls out its purpose:

8.3.1.3.5 Output LDO
An adjustable output voltage regulator LDO is provided as a simple way to provide power to additional components in the battery pack, such as the host microcontroller or LEDs

This LDO outputs either 2.5 or 3.3V depending on the particular part number of your IC, which is detailed in the device comparison table.

For all intents and purposes, you should treat the output of the REGOUT pin like you would treat an LDO external to the chip. The regulator can only supply between 30 and 45mA per the I_EXTLDO_LIMIT spec in the electrical characteristics section, so as long as whatever you're connecting to the REGOUT pin draws less than 30mA, you should be fine to power your 3.3V (or 2.5V, depending on part number) circuits from this pin instead of an external regulator. Be mindful that the REGOUT net also powers internal circuitry, so even if you don't use it to power external circuits, it still requires an external capacitor.

In some of the schematics you show, the I2C lines (SCL and SDA) are pulled up to the output of the REGOUT pin because I2C lines are open-drain and require pull-ups. You should have pull-ups on your SDA and SCL lines, but whether you put them on the output of REGOUT or another logic-level supply voltage in your design is largely up to you.

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