Powering a ProMini ( 8MHz / 3.3V) with a Lithium cell

The ProMini is connected to a RFID reader and SD card reader and is powered for 10 secs for every reading. I am planning to power up the setup using a Lithium cell. ( Range 3.7 to 4.2V)

While the ProMini can with stand the Lithium full range, I am afraid the RFID / SD Card reader cannot tolerate beyond 3.3V.

So what options are there to regulate the voltage and which LDO regulator will fit this application.

Incidentally in a simple LDO regulator what happens to the Output when the Input falls below the recommended minimum ? Does it become a pass through element with no regulation or shuts down ?

I do not have knowlage about LDO regulators but i think using a 3.7v cell is not suitable for your project you can get yourself 7.4v battery and use a normal reguator

A 3.7 V (nominal) battery is perfectly suitable for the project and you can use an LDO regulator (buy a regulator with the lowest dropout voltage possible).

When the lithium battery voltage has dropped to 3.3V it is essentially dead. The battery voltage should never be allowed to drop much below that or the cell will be destroyed. Lithium batteries MUST be charged with the proper charging circuit, or you risk a possible fire.

Check the regulator data sheet for the exact behavior, but when the cell voltage reaches 3.3V, the regulator output will usually be about 3.3V - (dropout voltage), so the RFID device should still function.

Why not bring the LiPo in on raw to use the onboard LDO, then use Vcc to power your readers ? Specs call it at 150mA max output with dropout voltages of 17mA at light loads and 165mA at max (150mA). If you are using 100mA for all 3 (you need to check, this figure is probably high), and figuring max dropout voltage, the battery would have to stay above 3.465V to maintain regulation.

Thanks to all those who posted. ( The 7.4V option. I know its easy to use but not the charging part which requires a balanced charging)

Summing up the options to use the 3.7V cell I guess I will proceed as below :

  1. Use the LDO on board the ProMini and power the RFID + nRF24 from the Vcc output.

  2. Bring in code to warn user about a Low Battery situation at about 3.5V ( some good ideas are here : https://www.gammon.com.au/power) In fact I am a big fan of the tutorials on this site !!

Incidentally I am not sure I understand this fully : Specs call it at 150mA max output with dropout voltages of 17mA at light loads and 165mA at max (150mA).

What exactly does the above mean ?

Incidentally I am not sure I understand this fully : Specs call it at 150mA max output with dropout voltages of 17mA at light loads and 165mA at max (150mA).

What exactly does the above mean ?

It means the regulator can output 150mA, but they also need overhead voltage to do the regulating part. In the case of this LDO, the datasheet specs that overhead voltage as 17mV for light loads and 165mV for max (150mA) loads. That is the required input voltage in addition to the output voltage of 3.3V, so 3.3V+.165V(max)= 3.465V min.

I see I typed in mA instead of mV in my first post. That was a mistake on my part. Also, just because the regulator CAN put out 150mA doesn't mean you SHOULD try pulling 150mA.

Mogaraghu: Incidentally in a simple LDO regulator what happens to the Output when the Input falls below the recommended minimum ? Does it become a pass through element with no regulation or shuts down ?

For most of the modern LDOs I have checked, the output voltage will follow the battery down, which is helpful in a way.

Whilst a LiPo might seem to be an ideal battery for the project its depends who is keeping an eye on it and where it is used.

Ideally you want the circuit to shut itself down when the battery voltage falls to circa 3.3V, there is virtually no capacity left beyond this point anyway.

Yes a lot of Lithiums have internal shuts offs, but this is often 2.4V, and if you let a Lithium go that low you really ought not to use it again, especially in a commercial application.

Promini's 150mA regulator will not put out enough current for many SD cards. A regulator capable of 250-300ma is needed.