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Theoretical understanding of XL 1509

anso-engineer

New Member
Hello,
I am trying to wire this IC to my circtuit for reducing voltages from 12V, so good to understand the purposes of those elements. And the nominals for them choosed, because for 3,3V option there is bigger Cout capactior and more less L1.

Also I am not sure about needance of D1, is it for reverse current protection?

1737538377836.png
 
Hello,
I am trying to wire this IC to my circtuit for reducing voltages from 12V, so good to understand the purposes of those elements. And the nominals for them choosed, because for 3,3V option there is bigger Cout capactior and more less L1.

Also I am not sure about needance of D1, is it for reverse current protection?

View attachment 148548

Hi,

D1 is sometimes called a "catch diode" in that it conducts when the voltage on the left side of that inductor tries to go negative and thus still allows the inductor to provide a positive output during that time. This is the time then the internal transistor turns off.
This is typical for the usual buck circuit. It does have to be a high speed diode like a Schottky though.
 
D1 is needed to provide a path for the current when the switching device in the XL1509 switche off. During the time that the switching element is off the stored energy in the inductor powers the load. I suggest that you google "how switching regulators work."
Les.
 
D1 is necessary for the proper functioning of the XL1509. It is not for reverse current protection but is instead part of the switching regulator's operation. Omitting it will prevent the circuit from functioning as expected because the inductor requires a path to maintain current during the OFF phase of the switching cycle.
 
And the nominals for them choosed, because for 3,3V option there is bigger Cout capactior and more less L1.
Note that the XL1509-5.0 is designed for 5.0V output. If you want 3.3V out, use the XL1509-3.3 version. There are also -ADJ and -12 variants.
 
I think it's something similar to the flyback diode for protection from reverse current. And according to symbols, there is Schottky diode.

Hello there and welcome here.

That's a very interesting reply. First, you asked what the diode was for, then I told you want it was for, then you said you still think it is for some other reason. It appears that you were not prepared to accept any answers.

Myself and many others have worked with this circuits for years and years, so it is more than likely that we know what that diode is for. I realize though that these circuits are a lot more complicated than the simpler linear type regulators, so it's going to take more effort to understand them.

When the transistor internal to the IC chip turns on, it pumps current through the inductor. Some of that current charges the capacitor, some goes right to the load. That puts some voltage at the output to power the load.
When the internal transistor turns off, the inductor wants to keep pumping that current to the output. To do that, the voltage across the inductor flips so that the left side becomes negative. To keep the current flowing, the diode allows the current to keep flowing from ground through the inductor to the output. That helps to maintain the output voltage and keep it somewhat smoothed out.
This also partially 'resets' the inductor getting it ready for the next 'on' period of the internal transistor. Once the transistor turns on again, the cycle repeats.
If the diode was not there the inductor would not have a discharge path when the transistor turns off, so the circuit would not be able to function properly.

You can simulate this in a circuit simulator. There is a free one they call "LT Spice" but I think the actual name is "Switchercad". It was made by Linear Tech and you can download and use it for free.
 
Hello there and welcome here.

That's a very interesting reply. First, you asked what the diode was for, then I told you want it was for, then you said you still think it is for some other reason. It appears that you were not prepared to accept any answers.

Myself and many others have worked with this circuits for years and years, so it is more than likely that we know what that diode is for. I realize though that these circuits are a lot more complicated than the simpler linear type regulators, so it's going to take more effort to understand them.

When the transistor internal to the IC chip turns on, it pumps current through the inductor. Some of that current charges the capacitor, some goes right to the load. That puts some voltage at the output to power the load.
When the internal transistor turns off, the inductor wants to keep pumping that current to the output. To do that, the voltage across the inductor flips so that the left side becomes negative. To keep the current flowing, the diode allows the current to keep flowing from ground through the inductor to the output. That helps to maintain the output voltage and keep it somewhat smoothed out.
This also partially 'resets' the inductor getting it ready for the next 'on' period of the internal transistor. Once the transistor turns on again, the cycle repeats.
If the diode was not there the inductor would not have a discharge path when the transistor turns off, so the circuit would not be able to function properly.

You can simulate this in a circuit simulator. There is a free one they call "LT Spice" but I think the actual name is "Switchercad". It was made by Linear Tech and you can download and use it for free.
Some people learn better by failing for themselves. He's not making a life-sustaining device. Let him build without the diode and see how well it doesn't work.
 
That's a very interesting reply. First, you asked what the diode was for, then I told you want it was for, then you said you still think it is for some other reason. It appears that you were not prepared to accept any answers.
I don't familiar with all possible diode applications so I referenced it to what I know. Wrongly. And I didn't reject your answer, I just messed it due to limiting knowledge.
After you last explanation a lot goes cleared, it's great.

Some people learn better by failing for themselves.
probably this is the crucial step among the many steps to clear understanding :)

You can simulate this in a circuit simulator. There is a free one they call "LT Spice" but I think the actual name is "Switchercad".
I remeber once I tried to simulate it in the KiCAD (not successfull), which propably has SPICE under the hood, but I should definetly goes deeper to the simulations.
 
I don't familiar with all possible diode applications so I referenced it to what I know. Wrongly. And I didn't reject your answer, I just messed it due to limiting knowledge.
After you last explanation a lot goes cleared, it's great.

I remeber once I tried to simulate it in the KiCAD (not successfull), which propably has SPICE under the hood, but I should definetly goes deeper to the simulations.

Hello again,

That's interesting too. I have to highly recommend downloading the LT Spice simulator because many people use it and share schematics. Once you draw your test circuit, if you have any questions you can upload the file and then other people can run it and see if they can figure out what it happening too. This means several people can run it and then provide their own opinion about what is working or not working. This can help you a lot.
There is a little bit of learning to use the LT Spice simulator but it's not that much and it is well worth it. You can even ask about issues with learning about it on forums like this.
 

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