# Adding different voltage battery to same circuit

I have 9 volt battery to get 5 volt using step down converter. I want to enable the step down using a timer IC max input voltage 6 volt. I will use 3.7 v battery for this.

So my question is: Can I join the ground or negative pole of batteries in the same PCB or circuit, I will have problem with signal voltage to enable step down and power off the timer from the MCU 5 volts ????????

• We really need links to the manufacturer's datasheets for these components. Oct 7, 2019 at 12:03
• tpl5110, mcu some 5 volts basic, lm2596 Oct 7, 2019 at 18:23

You should have no problem interconnecting all the grounds. As a matter of fact since you want a common "off" signal from the uC connecting the grounds would be required anyway.

Yes, you can join the ground of different voltage supplies together. There's no problem with the circuit as drawn.

Remember that voltages are always relative. Tying the negative poles of the two batteries together only ensures that those poles are at the same voltage, which I'm going to call "0V", just because I can. You can call it whatever you want, but I'm going to use zero for simplicity. The "3.7V" only indicates that the positive pole of the battery is 3.7V above the negative one, but doesn't say anything beyond that.

So, the timer will still see 3.7V across its power supply pin and ground pin, the step-down circuit will see 9V across its pins, and the MCU will see 5V across its pins.

• Great. But what is the voltage will see the timer of signal off from MCU, 5 volts also ? Oct 5, 2019 at 21:05
• @asterix It really depends on the MCU. Most of them represent a logic high by just connecting their supply to the output pin, so, yes, it would see 5V above the common ground. Oct 5, 2019 at 21:43
• thanks you very much friend Oct 5, 2019 at 22:09

You need a self-approving reply relying on incomplete schematics? Try, it might work.

You need a detailed proper answer? Depends on the EN input of the regulator. Some of them are using an open drain solution, with voltage capabilities between supply of the buck and some logic level. In that case, provide you are in the specsheet boundaries, is OK connecting the ground.

BUT different circuits than an open drain requires analysis: does the EN input is a logic input with well determined maximum AND minimum LOGIC levels? Are there bypass diodes in the chip? You might discover are incompatible logic signalling and you need to put something more than just a wire to connect the EN input, ranging from a logic level converter down to just a resistor. But for this, we need to know the architecture of the EN input with respect the whole buck regulator. And with that schematic, we don't.

• I studied the datasheet and input voltage level of logic before to ask. I dont think any problems in schematic because I am using typical circuit from datasheet. I had only ground connectiong doubt Oct 7, 2019 at 18:18