Booster 4.2V output?!

[Jensb]

Hi all.
I have a question to you guys.
I like the TI Booster chips because they give a permanent and stable 5V Output.
Now since i use my own Attys i dont have a longer use for 5V but want to have a permanent and stable 4.2V output on these.
I dont want to put a resistor after the the chip, because they just burn the energy.
Anyone a idea how to get a stable output?

Jens

 

[SurvivorMcGyver]

This has commonly been accomplished by placing a diode on the Line out to the atty. Although I'm sure there are many to choose from, you might take a look at the N5822 as an example. Extrapolating from the specs, you might get a foward votage drop in the neighborhood your looking at.

Hope this helps

{edit} I'm sure you can get closer to the 4.2 if you do some searching -- Look for the forward Voltage spec and go from there (perhaps 2 in line with a Fv of .4?) Temperature and amperages as well as voltages and frequency of pulse from the regulator WILL affect their performance so pay attention to these factors in your final design - like what the ohmage of your atties will be etc, etc

 

[asdaq]

You can drop the voltage using a 3A barrel diode, but this decreases the efficiency too. It seems to me that to have the stable voltage it would be better to set the Ohms you like when making the coil. It would make sense to have 5v atties and ones for 3.7v. Especially when you've got so many nicely cast ones like you do.

 

[Java_Az]

adding a 100k resistor between the adj and v out is supposed to work. Source of info Fistpack document https://docs.google.com/Doc?docid=0AXdaMZo4FKMVZGhqdnE4YnNfNDNjN2Q0cHBoZg&hl=en

 

[asdaq]

That increases the voltage, they go from 5-15v.

 

[Java_Az]

That increases the voltage, they go from 5-15v.

From the document it says is decreases the voltage. Adding resistors to the adj and ground increase voltage. They say to add a 100k to the adj and v out pin to reduce it.

 

[asdaq]

With the fistpack, there are resistors and a slide switch to raise the voltage at increments, but a diode to drop (I believe each setting) to get variability below 5v. The booster by itself does 5v naked, and up to 15v depending on the resistor used.

 

[Java_Az]

Directly from the fist pack document.

Alternate Lower Voltage Build

For those that want the slider to output between 4.0 and 4.5v there is a really easy trick that RjG shared with me. If you add a 100k resistor between the Adj pin and and vOut pin on the booster the 4 voltages will be changed to:

4.0v
4.15v
4.3v
4.5v

The diode in the build is still required.

These are perfect voltages for those that don't want a really heavy hit but still desire a reliable solid hit similar to what you get with a large fully charged battery. The best part is, the booster is only generating 4.6v so it also adds some battery life to each cycle.

Thanks again to RjG again for sharing.

Warning: Do not reduce the vOut below this level as the booster needs headroom between the source and output voltage. Doing so could harm your booster.

 

[perlionsmitnick]

Directly from the fist pack document.

This is true.

 

[WillyB]

From the document it says is decreases the voltage. Adding resistors to the adj and ground increase voltage. They say to add a 100k to the ADJ and Vout pin to reduce it.

Yup in Nuck's example that drops the entire group by .35V. Another value may drop the volts even more. That 100k value may have been chosen to not let the volts drop below 4V.

@ Jensb

You could try this with a few values and see how it works. The resistor (1/8W) in this application is not dissipating watts but simply controlling the output (I think).

But in the final wash a 3A silicon diode and a simple 3A on/off switch will be easy and still give you dual voltage.

 

[Java_Az]

This is true.

I don't see why they would lie about it. But no i have not personally Hooked up my booster chip like that or built a fist pack. But i think it is safe to say it is more then likey true

 

[Nuck]

You can use a 62k resistor between the adjust and the voltage out pins and it will fool the booster to generating only 4.4v as its base without wasting any energy at all. I highly recommend you don't go below this value using this method as the boost needs room between the voltage in and voltage out.

 

[asdaq]

Cool, I didn't know that. It would be much nicer to add a resistor than one of those diodes.

 

[Nuck]

Cool, I didn't know that. It would be much nicer to add a resistor than one of those diodes.

The best part is that with the diode and this method combined the output under load is about 3.78v which is damn near perfection for a 1.5 ohm atty. If you want regulated power for LR without bothering with variable voltage this is a great way to get there.

 

[Jensb]

thanks for all the input guys. Sound very interessting and i have to give it a try tomorrow

Jens

 

[SurvivorMcGyver]

And There you have it JensB -- Let us know your findings.

@Nuck

And who'd have thunk one could fool this regulator as you have {chuckling} good finding!! -- my calcs show a negative ohmage requirement to get below 5V and a divide by zero scenario for the 5 volt setting (according to TI's formula for the chip) - go figure -- I'd have never gone there ..... but Nuck, you have, and it is quite intriguing -- thanks for chiming in on this and reiterating that factoid. I recall seeing this this in your writtings quite awhile back but overlooked it as it seemed "unconventional" at the time - I'll learn....... (to not set aside a therum until proven)

 

[roadrash]

As suggested by The Nuck, I have been using a 100k resistor between adjust and V out for months, I get a constant 4.6 volts.
Which is good for my 510 cartos.:>)

 

[Gummy Bear]

Do you still need the diode if your using a 100k resistor between adjust and V out?
And if so which one? (Specks or PN ???? )

 

[roadrash]

If you put a 100k resistor between v out and adjust it tricks the booster into putting out 4.6v.
Diodes are used to lower the voltage even further. this diode will drop the voltage about .7 v I think?

Digi-Key - SR208-TPCT-ND (Manufacturer - SR208-TP)

 

[Gummy Bear]

Thanks a bunch, So would more resistance = less voltage or the other way around?