What's the best way to regulate 12v automotive (11-15v) into a stable 12vdc then drop that to 10.5V dc (c.c.@2A)

 

There is the LM317 that would do it for both.
Max.

 

None, because you cannot regulate both the voltage and the current.

Bob

 

You could use a buck-boost switching regulator which can provide 12V output if the input is above or below that volage.

But why do you want 12V and then 10.5V?

 

to 10.5V dc (c.c.@2A)

Do you want constant 10.5V from 0 to 2A "cv"
or do you really want 2A at any voltage from 0 to 10.5V "cc"?
What are you doing? Driving LEDs?

 

We need to know what the OP really wants, since he has specified a regulated voltage and a constant current which is impossible.

@MaxHeadRoom: how does an LM317 get you 12V from 11, and 2A output?

Bob

 

I need 2A C.C. for driving LEDs, but I need to be as close to 10.5V to be as close to the forward voltage as possible. i can easily do this in two steps, but the end result would be a 2A C.C. and a 10.5 -11v Max.

 

A reasonable guess is that the TS really needs a 10.5 volt source that can provide up to 2 amps. My suggestion is a buck type switching regulator circuit. BUT the selected regulator must be able to handle the higher voltages also found in an automotive system. But now looking at post #7, I can see that really what is needed is a constant CURRENT regulator that can be set to 2.0 amps.

 

but I need to be as close to 10.5V to be as close to the forward voltage as possible.

Why? If you use a cc circuit the LEDs will set their own voltage. LEDs should be current-driven, not voltage-driven.

 

I need 2A C.C. for driving LEDs, but I need to be as close to 10.5V to be as close to the forward voltage as possible. i can easily do this in two steps, but the end result would be a 2A C.C. and a 10.5 -11v Max.

You can do that by putting your unregulated voltage into a constant current driver. The driver sets the voltage to exactly what is required to get 2A.

What you cannot do is have 2A at 10.5V if the LED requires 10.2V or 11.1V to draw 2A. You can regulate either the voltage or the current, but not both. For LEDs, the correct way is to regulate the current.

Bob

 

Your problem would be much easier if the LED voltage was above or below the supply voltage.
A 2A 9V LED would make a simple buck PWM work well.
Here is a example of a cc LED driver. (up to 3A) (AL8822 is a 2A part) Current is set by Rset.
The part will not boost up the voltage. In the case of the LED voltage=11V the part will work when Vin is larger than 11.3V. Below that point is stops regulating and just applies Vin to Vout with a little loss because of Rset.
If you can live with the light dimming below 11 volts, then there are many low cost options.

 

I have no experience with this part. I think it should work.

 

Really, if the LED assembly requires a current at a voltage below the 12 volts, the really simple way is a properly sized series resistor. First the TS needs to know the operating forward drop voltage when the LEDs are at the desired current. Then, if they are to be lit while the engine is running, presume the supply voltage to be 13.4 volts, or maybe 13.2 volts. Subt6ract the forward drop voltage from the battery supply voltage. That is what the voltage drop across the series resistor must be. Use the formula voltage over current = resistance. So if the required drop is 4 volts and the current is 2 amps then the resistor is 2 ohms. For additional protection put a zener diode rated at just a bit more voltage across the string of LEDs.

 

yeah so simply I have a 11.0vdc to 4.6vdc 500A capable power supply ( automotive battery and charging system )
the LEDs are 3.5v @2A each and I was going to run them, 3 in series and 3 in parallel so I need 10.5Vf @2A. I need to be able to cycle the lights on and off at a variable duty cycle and ON-OFF rate.

 

That BIG power supply may not be adequately filtered for running LEDs. I suggest using an accurate voltmeter to measure both the DC and AC output voltages before connecting any LED devices.
Variable duty cycle circuits have been presented and discussed a lot in this forum so you should be able to locate what you need here someplace.

 

okay, what's the best way to filter something like that to a stable voltage?

 

I am guessing that the big power supply has a peak output quite a bit above 11 volts, because 11 volts will not provide a useful charge for a 12 volt battery. So my first suggestion is to get a diode rated for at least 5 amps, and connect that from the power supply positive terminal to the positive terminal of a capacitor rated at least 470MFD at 20 volts or more, with the capacitor negative connected to the supply negative. Then check the voltage across that capacitor. My guess is that the voltage will be higher. That is the voltage that you will need to use for calculating how much resistance you need to provide the correct voltage and current for your LEDs. You can power them using the resistor feeding from the capacitor. If there is any flickering of the LEDs you will need more capacitance in parallel with the first capacitor.

 

Please post the datasheet of your 3.5V/2A LEDs so we can see their range of forward voltage (3.0V to 4.0V?) and how they must be cooled.

 

As mentioned above, forget about the voltage. Focus on a constant current supply. LEDs are current devices, not voltage. The forward voltage is the point at which they start to work.
Think as like a weir. No water flows over the weir until the water height gets above it. But then, a small increase in water height can cause a large water flow change. In a similar way, if you run the LED on a voltage supply, a small increase of voltage can easilly destrpy the LED as it can draw excessive current. The constant current supply controls the current output to keep it at the required level, independent of the supply volts and the load resistance. Of course, that is all within the limits of the operating conditions.
As long as the CC supply can output the current at the lowest supply volts, that will be ok.
You said 3 LEDs in series and 3 in parallel. So, if each string needs 2A, that is 6Amps total.
I would use 3 CC supplies, one for each string. That way you gain redundancy.
A switch mode CC supply is the way to go to reduce the heat.
Also, do not run the LEDs at their max rated current if you want a long life for them.

 

@dendad, I'm going to be running them at an 80% duty cycle to reduce heat, and they won't be on steady. the use for them is to flash not cast light.