Driver Ideas. DC/DC stepup AND CCregulation

yeah but you have to trick the vreg and that uses voltage unless you use a high side current sense amp which pretty much eliminates it. i want to try to modify this schematic by woop to run with that amp, he said hes going to do it but im impatient. anyone want to help?

If you read the Datasheet it is pretty much self explanitory

The Vout range of 1.8 to 5.5V is referred to ground, not to the FB input. If the reference voltage is 500mV (on the FB input) the max allowable output voltage across the load would have to be 5V.

phenol said:

The Vout range of 1.8 to 5.5V is referred to ground, not to the FB input. If the reference voltage is 500mV (on the FB input) the max allowable output voltage across the load would have to be 5V.

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thats true, i should have changed the output voltage label

I just cant put together how the CC works. Only sort of. Anyone care to explain it more. I read the datasheet but it only explains voltage regulation and I barely understand that, let alone understand the logic of how it works to apply it for current..... Sorry....

you can measure current using a resistor and measuring the voltage across it.

because V=IR.

so a switching chip can measure current by measuring the voltage across a resistor.

getting it yet? read on

the switching chip is programmed to increase its output until it gets the right voltage on its feedback pin. thats how it would normally regulate voltage.
so if we connect the feedback pin to a resistor in series with the load, it will increase its output until the voltage across the resistor is right, this will happen when the current is right, because v=IR. so effectively it is regulating current.

Oh thank you so much I completely get it now. All I needed to hear was

the switching chip is programmed to increase its output until it gets the right voltage on its feedback pin.

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But I thought of another questions while I was trying to sleep last night. Supposedly, you can use these buck boost drivers to drive a blueray, or a red. Lets assume blue is 5v required and red is 2.5v. If the chip regulates voltage, to get the correct current, then it must always be the same voltage for the same current. So to sort of solve my own question, since v=ir then for a bluray diode we must increase the ..resistance?.. Or maybe... another solution to my question... the chip just steps it up more so that the voltage supplied to the blu diode is the blue required, + .5v (tps chip). So if the resistance total was 10 ohms then that means we would be setting the current at 20 ma. And it would automatically supply the blu diode with the right amount of voltage to leave just .5 left for fb pin. It would increase it increase it increase it until it started getting .1, .2, .3 v on feedback til it reached 5.?..? Can anyone confirm this or explain it in their own way. If you get what im getting at. I just am having trouble understanding how different voltages can be produced for the diodes, while keeping the same current.( that is, without changeing the resistance) (if the resistance is changed, is it increased or decreased) can someone do a sample problem with real numbers>..> Thanks guys this is definately helping I'm learning so much more about electronics that before!

Ok, i think i understand what you want.. Yes, for any given current, there is only one voltage, if the resistance doesn't change.

If LDs had an unchanging resistance, you could drive them safelly with constant voltage, and also get constant current.
But it doesn't work that way..

First of all, every diode has a different Vf. Even diodes from the same batch. If you tried to power several diodes from a constant voltage, the current would be slightly different for each of them, and you would have to tweak the series resistor, untill you adjusted it.

With constant current, the current will always be the same, regardless of small differences in Vf or even with a completelly different diode (BR or red - with CC, current will be the same).


The other thing is, that diodes don't have a fixed resistance. Their internal resistance changes with the temperature. With heat, the resistance drops. This means, that even if you select the right resistor to achieve 100mA with that particular diode, the diode will start warming up, and the current will slowly climb. More current causes more heat, more heat more current, and if you went too high, the diode can commit suicide (thermal runaway). So if you do this, you need to limit the current enough, that it will still be lower than max, even when the diode is at it's hottest.

With constant current, current is always the same, as long as the input voltage is within the drivers requirements. You can change the diode, the diode can change it's resistance, the current will stay the same.


Many people don't understand the importance of CC, and think, that they can just boost to a certain voltage, and adjust the current with a resistor. But it is actually more complicated to do safely (lots of tweaking involved) and in the end, you have to run the diode at a lower power. With CC, you can run the diode at close to it's limits, and rest assured, that it won't cross over (and start eating other diode's brains).


With constant voltage you wouldn't have to worry about the 0.5V for the FB pin. You just set the voltage with the correct resistor. But there are so many disadvantages, that it does not make sense.

If a chip can do CC, use CC. If it can't, don't use it. It's as simple as that.

Oh IgorT... I know that I'm not dumb! I am confused as to how, when in constant current, you can drive a blu ray diode (5V) and a red (2.5V). What is deciding the voltage for the diode. How come when you attach the blu, to a red setup, it doesn't feed it 2.5v, or does it? Does it just figure the voltage out like I mentioned in my previous thread, increasing the output until it reaches that .5v? I keep sitting around my house thinking "how can you drive a blue and a red with the same circuit, they are different voltages" How does it know what voltage to give it; I know it just uses constant current and the voltage figures it out or something. I don't know anymore... Do you get where I'm coming from? Thanks for such an in-depth explanation of CC IgorT!

airy52 said:

Oh IgorT... I know that I'm not dumb! I am confused as to how, when in constant current, you can drive a blu ray diode (5V) and a red (2.5V). What is deciding the voltage for the diode. How come when you attach the blu, to a red setup, it doesn't feed it 2.5v, or does it? Does it just figure the voltage out like I mentioned in my previous thread, increasing the output until it reaches that .5v? I keep sitting around my house thinking "how can you drive a blue and a red with the same circuit, they are different voltages" How does it know what voltage to give it; I know it just uses constant current and the voltage figures it out or something. I don't know anymore... Do you get where I'm coming from? Thanks for such an in-depth explanation of CC IgorT!

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E=IxR The resistance offered by a BR is different than that of a red.

Also the diode pulls what it needs almost like the LM317, the diode pull the voltage nee.ded you just need to set the current

like magic..... its just weird because i think of a power supply, like ones i have, that i set the voltage, and that is what comes out, but with these, you can set it to step up and it goes to the right voltage. I just misunderstand how you can have different voltages, for the same current. ... HOW!?

different voltages across the diode... same current... w t f


edit: you use a smaller resistor or pot since the 5v diode is a higher resistance, which leads to a larger voltage drop across the diode, and a smaller one across the added resistors. which leads to a larger voltage drop across the diode <--- THATS it isn't it!!! :-? :-?

ok imagine you have a 12V light globe and a 100V light globe, they both have the same current, say 1Amp.
and you are sitting there with your adjustable power supply. you have an ammeter in your supply too.
you plug in the 100V globe and increase the voltage from zero until you get 1amp. this happens at 100V.
then you plug in the 12V globe and increase the voltage from zero until you get 1 amp. this happens at 12V.

essentially you are controlling the voltage to achieve a constant current. for different loads, the right current will happen at different voltages

wow, you are pro. I get it now, and partially from what I thought. The 100v globe also has a much higher resistance then.makes sense. Were you like this when you first started learning..??

toked323 said:

Also the diode pulls what it needs almost like the LM317, the diode pull the voltage nee.ded you just need to set the current

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Actually, a diode doesn't pull a voltage.. On a constant current driver, you set the current, and the driver gives the diode the exact voltage it needs, for it to pull the current you set.

With a different diode, the same driver with the same resistor will again give it the exact voltage it needs, to pull that same current (as long as the driver input voltage is what the driver needs to do so).. Since the diode is now different, the voltage required for that current will be different. And that's what the driver will give it, if it can.


It all starts with a voltage. Only then comes the current. Constant current drivers achieve constant current by constantly adjusting the voltage, so that it is always just enough for the desired current to flow through the load...



And Airy, i was not implying, that you're dumb. I was just trying to explain. A while ago, i did not understand much of it myself. But my own misunderstandings now help me understand, what others misunderstand..

i love you guys!!!!!!!!!!!!!!!!