LM1085T Low Voltage Dropout Regulator questions

[fritz.fx]

Hi Guys

I'm trying to get 12volt 1.5amps from my cars dirty 10~15 volt supply.
After a little Googling I'm guessing I'm going to be using the lm1085t.
I found two schematics on the data sheet but i'm wondering which one would be the best to use.
The first is this:
Obviously it has 4.75V in and 3.3V out, so what would I need to change to this to get 12V out?
**broken link removed**


Secondly, there's this:
**broken link removed**
I'm pretty certain that this is the one I need, but, I'm a little screwed on the math >> Vout = 1.25(1+ R2/R1) ??
.. and what type of Cap is C1?

Could anybody help me out here?

Thanks
Fritz

 

[ericgibbs]

hi,
That Vreg has a 1.5V drop out voltage, which means for 12V out, there must be at least 13.5V on the input.

 

[fritz.fx]

Ah bugger. I completely overlooked that.
Although, I think I should be good there, I'm really only going to be using this while the car is running, plus I have dual 100amp batteries.
But, on the off chance that there's a better solution, what would you suggest?

 

[ericgibbs]

Ah bugger. I completely overlooked that.
Although, I think I should be good there, I'm really only going to be using this while the car is running, plus I have dual 100amp batteries.
But, on the off chance that there's a better solution, what would you suggest?

You need a switched mode power supply, say 9v thru 15v at 12Vout rated at 1.5A

 

[fritz.fx]

ok thanks, I'll look into them

 

[ronsimpson]

The first figures of this is close to what you want.

 

[crutschow]

Here's a 2A, 12V, LDO linear regulator that will operate with less than a half volt drop-out differential.

 

[ericgibbs]

Here's a 2A, 12V, LDO linear regulator that will operate with less than a half volt drop-out differential.

The OP has asked for " I'm trying to get 12volt 1.5amps from my cars dirty 10~15 volt supply.

So that device will not be suitable

 

[bountyhunter]

Hi Guys

I'm trying to get 12volt 1.5amps from my cars dirty 10~15 volt supply.
/////so what would I need to change to this to get 12V out?

The laws of physics. That is a linear regulator, it can't boost voltage. It won't put out 12v with 10-15V input.

The kind of switcher that does that is called a SEPIC.

 

[crutschow]

As Eric noted, a linear regulator will not boost voltage, so it will obviously not work for voltages less then 12V plus the dropout voltage of the regulator. But normal battery voltage is 12.6V with the engine off, and >13V with the engine running, so it should work for normal battery conditions in a car.

Not sure why the OP wanted to go down to 10V which is essentially a dead battery(?).

 

[bountyhunter]

A car battery drops to about 9.5V under starter crank. Some specs for auto electronics spec a low min voltage for that reason.

 

[crutschow]

A car battery drops to about 9.5V under starter crank. Some specs for auto electronics spec a low min voltage for that reason.

OK. I wasn't thinking that the circuit he is powering would need to continue working when he is starting the engine.

 

[bountyhunter]

OK. I wasn't thinking that the circuit he is powering would need to continue working when he is starting the engine.

May not, but it's a generic requirement that gets thrown on auto electronics.