Electrolytic capacitor on battery powered circuit

[Pha5e]

I'm not quite sure what size electrolytic smoothing capacitor to use for a battery powered circuit after the 10V precision reference regulator?

I was thinking 470µF but I only have 50V ones which are a bit large for the small circuit.

Can I use 100µF (16V) or 47µF (25V) instead?

How do you calculate the correct value?

Thanks.

 

[D.A.(Tony)Stewart]

The size of the cap supports the surge current which can exceed steady state current limit.
The series resistance or ESR of the Cap , regulator and load all affect the ripple.
What ripple and ESR values can you provide?

 

[Pha5e]

The size of the cap supports the surge current which can exceed steady state current limit.
The series resistance or ESR of the Cap , regulator and load all affect the ripple.
What ripple and ESR values can you provide?

The load is approximately 35mA. There is no ripple as the circuit is battery powered. I don't know the ESR of the capacitor as I haven't chosen the value yet (as mentioned above).

The micropower precision reference is an LM4040.

Thanks.

 

[D.A.(Tony)Stewart]

If there is no dynamic load and no dynamic charger source voltage and hence no ripple then you do NOT need a cap because the battery dielectric is your capacitor.

This is only needed if you have ripple noise. then cap with the impedance required to reduce the ripple is chosen where Z=1/(2pi*fC)+ ESR and often more than one value in parallel is needed.
e.g. 33 pF // 1000 pF for microwave.... or 0.1 /100uF or SMPS with low ESR caps.

 

[Pha5e]

I read somewhere that even with battery powered circuits, it is still good practice to use an electrolytic reservoir capacitor to allow for surge demand from the circuit and to prevent dips in the supply?

 

[D.A.(Tony)Stewart]

surge demand is the same as voltage drop or ripple. which was my previous question about ESR of your dynamic load.

If your load is constant. then there is no need for a cap.

 

[Pha5e]

If there is no dynamic load and no dynamic charger source voltage and hence no ripple then you do NOT need a cap because the battery dielectric is your capacitor.

I have summarised below an extract from a TI datasheet for an LDO regulator:

A 1-μF (tantalum, ceramic, or aluminum) electrolytic capacitor should be placed locally at the input of the regulator if there is, or will be, significant impedance between the ac filter capacitor and the input; for example,
if a battery is used as the input or if the ac filter capacitor is located more than 10 in away. There are no ESR
requirements for this capacitor, and the capacitance can be increased without limit.

This seems to imply that an electrolytic capacitor should be used even when operating from a battery - regardless of the load? I have also seen other battery powered circuits and websites which recommend using an electrolytic capacitor as good practice.

 

[D.A.(Tony)Stewart]

This is true in general when you do not know what the load step current is. In general it is a good practise to add a cap if the source is > 10" away meaning if the inductance or ESL is such that it is more than the equivalent of >10" ..... regardless of whether it is a cap source or a battery source.

My point was that if your load was steady 60mA battery was equivalent to a capacitor that no additional cap is needed but if there is a long wire implying high impedance from ESL to a transient load that an additional cap near the chip is recommended in the general case to suppress transient ripple ( which you did not observe)

What I neglected to say was that " in the general case" add a cap close to the chip if the battery is far away.

The test in any design is the load regulation step response. If a reasonable step load say 25% to 75% is representative to worst case conditions shows no ripple on the input battery or output during this condition, the cap is redundant. Step Loads such as LEDs, Motors, relays, wifi Tx might be sufficient to warrant an additional cap.
Otherwise if close it may be redundant.

For example pulsed an LED was driven from a uC that was too much load to sustain from a 3v CR3042 coin cell, I would definitely add a cap to prevent a transient.

When I started life as a design Engineer, I read everything I could about the imperfections and advantages of each type of capacitor, and how magnificent and complex these lowly parts can be.

This is a start.

Sorry my wording is not crystal clear tonight. Here are some more info that may be useful. https://www.analog.com/library/analogDialogue/archives/45-01/bypass_capacitors.html

 

[Pha5e]

When I started life as a design Engineer, I read everything I could about the imperfections and advantages of each type of capacitor, and how magnificent and complex these lowly parts can be.

This is a start.

Sorry my wording is not crystal clear tonight. Here are some more info that may be useful. https://www.analog.com/library/analogDialogue/archives/45-01/bypass_capacitors.html

Great, thanks for that link! I've been looking for a good guide on bypass capacitor types and a comparison between each.