Should a regulator be added to a circuit where the power source voltage matches the ICs?

Question

I am trying to design a very simple circuit consisting of an accelerometer IC and a Bluetooth IC, all rated at 1.8 up to 4.something volts and thinking to use a single CR2032 cell. Seems like these usually come at 2.8 - 3V and about 250mAh current, depending on manufacturer and other factors.

I have already planned to add required capacitors around each IC, but I am wondering if I should still use a voltage regulator with this circuit?

So far the only good reason I could think of would be to prevent frying the ICs in situations where the user would manage to somehow cram up 2 batteries instead of one in the enclosure, aka making the circuit idiot-proof.

Answer 1

Adding a voltage regulator would dissipate more power (although not much, if it's a good switching regulator), which isn't desirable in a battery-operated device. If those chips are meant to run over that full range, then I would take advantage of that. They were probably designed that way to address battery-powered applications.

If you want over-voltage protection, put a zener diode across the battery input. That would also give you reverse-battery protection and won't draw additional power (except possibly for a little leakage). Select one whose zener voltage is greater than one battery and is less than two batteries. The current carrying capability should be greater than the current capability of the battery.

Answer 2

From your description, you do not need a voltage regulator on a single 3.0V lithium primary cell to prevent damage to your two devices.

Note that there are still some reasons you may consider regulating the cell down to a lower voltage:

• The quiescent current of the circuit may be lower at a lower V_DD (though you have to add the regulator current)
• The circuit may change performance as V_BATT changes (due to state of charge, or large pulse currents)
• The circuit outputs an analog voltage proportional to V_DD
• You want a lower transmit power from your Bluetooth

That being said, you probably don't need to include a regulator for your application.

Answer 3

Most available linear regulators have a "drop out" voltage by which the output will be less than the input. This can range from under 100 mV for good examples lightly loaded, to well over a volt for others. So putting one in means you reduce the available voltage, which may not be something you want if it is already in range.

If all of your parts can tolerate the open-circuit voltage of a new CR2032, then you probably do not want a linear regulator.

There are, however, some tiny switching regulators designed to save power when running devices that can operate in the sub-2v range, from a coin cell by efficiently reducing the voltage to make them consume less energy. Good low power design of course is an art that extends beyond the supply - you need to make sure never to leave a voltage across a pulling resistor, and to sleep your processor and its main clock the overwhelming majority of the time.

More importantly, if you ever think of using a different chemistry, such as lithium ion or lithium polymer, then the voltage of a fresh cell may be around 4.2v and so likely too much for some of your components. In that case you might try using a very low dropout 2.8v or 3.0v linear regulator, or else looking at the switching solutions.