two circuits, one battery

[tokoivun]

Feeling a bit silly but here goes the question.

My intention is to drive two separate circuits from a single power source (battery, 2x 3.7V Li-Ions in series, getting 7.4V out).

The schematic goes like this:



So, I'd basically have 2x LD1117V33C voltage regulators after the battery, one before each circuit. The first circuit operates an AVR plus other minor stuff, taking 3.3V and about 200mA max. The second circuit operates on 3.3V as well, normally taking 20mA-200mA, but there are very short spikes of 2A draw (bluntly put, it'll be a SIM908 or similar), hence the 220uF capacitor in the schema. according to the docs a 100uF capacitor should be enough so 220uF might be a bit of an overkill.

And now to the question itself...

Do I need to put in any additional protection to the first circuit or will it be unaffected by the short current spikes that circuit 2 is pulling periodically?

Another question as well. Do I actually need two regulators or would one suffice if I threw in a capacitor for circuit 1 as well?

Also, ignore the led icons in the schematic Couldn't find a proper abstraction to describe a circuit

 

[ads-ee]

Unless you are using some AVR module (which includes the bypass caps), you are going to need some bypass capacitors for the AVR and other minor stuff (if that stuff is digital logic like the AVR). Look at the datasheet/user guides for the AVR you've selected and check what the bypass requirements are.

You might also want to look into getting a dual DC-DC switching regulator like this one from Linear or this one from TI.

 

[BradtheRad]

If your load #1 is light, then you might get by with this arrangement. A resistor serves to isolate it from noisy load #2. The smoothing capacitor has an easier job at load #1.

My modification, with two components added, in black:



The resistor must not be too large an ohm value. It must allow sufficient current and voltage to reach your load #1.

It will take experimentation to tell whether this simple method can work for your purposes.

 

[KlausST]

Hi,

To avoid ringing at the voltage regulator caused by the 2A pulses you could additionally use RC or LC filtering on both channels.

Klaus