TPS82140: TPS82140SILT - Maximum Output Capacitance

Groger,

I do not think a super-capacitor is the solution you are looking for.  All that will do is transfer energy out of your battery into the super-capacitor.  The super capacitor will have a linear discharge function so you will no longer have a regulated 5 V output.  Normally you would use the super-capacitor in place of the battery.  Can you tell me your battery characteristics?  What is is voltage range and Ah capacity?

Hi John,

It is a Saft 33600: 3.6V, 17Ah. (x2 series) The battery is rated to -60, but with very poor performance! It's the -20 to -40 range of concern, and interest, to this application.

With the poor ability of the battery to deliver the peak current needed by the modem at transmit, a 500mA to 600mA peak for about 300ms, I am concerned the battery will not deliver, especially if it's too close to cutoff voltage( about 5.6V) .

So you are thinking its better to use the cap ahead of the regulator? I have searched for information on this, but found also where the super cap is used after the regulator. I have thought it better before, it makes more sense to charge the super cap while there's no current into the node of the regulator, and so long as the regulator can handle the inrush and peak current of the load, it should work better.

Out of curiosity - would this also be the correct approach, even if using a single Li battery of 3.6V, in a buck-boost topology, for example in a TPS63805 device?

Thank you for taking my question.

Gary

Gary,

Even in the best case scenario, that battery will not deliver the required current.  At 7.2 V input, and 5 V out at 600 mA, the battery would be required to deliver 453 mA.  It only gets worse as the battery discharges.  i would probably use something like BQ24620 to charge the  supper-capacitor from the battery, then then generate your regulated 5 V from the super-capacitor, using a buck or boost depending on your super-capacitor voltage.

Gary,

What further questions do you have?

John,

Sorry - did not look a t the PDF. I'll do some reading. It maybe solved the problem!

Thanks!

Gary

Hi John,

Does it need to be that complex?  My initial goal - and sorry if this original question didn't address that - was to use a single cell 3.6V battery. Then, I decided had to use 2x in series to achieve the performance. However, I keep going back to this little app note I found on the web, it uses a "competitors" boost device. BTW - if I need to use 2 batteries in parallel due to poor temperature performance - I will do it. My preference was, and will be to use 1 though.

It's near identical to what I need. Using a low quiescent current buck-boost converter, such as the TPS63805, would this not work? Once the battery charges the cap, it should have very low leakage current and will supply the necessary current when it's needed. Both voltage and current shown in this diagram are right where I need them. The difference - the "boost" device would now be a "buck-boost". Where are the problems or pitfalls with this?

Gary,

Sure TPS63805 should work well for that application if you want to use a very simplistic approach to charging your capacitors,

Hi John:

My last question on the last post: "Where are the problems or pitfalls with this (approach)?"  Are my comments true, regarding leakage current and the input quiescent current of the TPS63805?

Appreciate your feedback, thanks.

Gary

I cannot think of any downside to using a buck boost.  My only concern was regarding the super-capacitor charging.  I am certainly not an expert with regard to battery and super capacitor charging, but the literature I have read deals with constant current/constant voltage charging, ESR monitoring, over current protection, etc.  That circuit is just RC charging with current limited to less than 116 mA by the 30 ohm resistor.

John,

BTW - the diagram shows 700mA for 1 sec, my app is actually 500 to 600mA for not more than 500ms.

Regarding your comments - understood. Is this simply too good to be true, maybe. After the initial charging of the cap, limited by Mr.30ohm, and while the load is very light (1mA in sleep mode for the device) , the input to the regulator would be appreciably small.So the losses would be quiescent current plus input current plus any leakage of the capacitor. Yes? Seems right...

Then - when the load/device is awake, and requires a higher peak demand on transmit, the super cap would supply that as it has the lowest source resistance that's in parallel, with the battery that's in series with the 30 ohm. After the push from the super-capacitor, and the device again in sleep, the battery would then charge the cap to equilibrium again.

My concern - as with you - is that the capacitors should be balanced with resistors. But, it can be avoided by using this: a 5F/6V part from AVX. With a 5V rating even at elevated temperatures, it has adequate headroom to handle the voltage. The datasheet indicates 36uA leakage at 72 hours.

SCMS32H505PRBB0

Thoughts?

Gary