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Basic Buck Driver Help

S

shumon

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Hello! I'm trying my hand at designing a simple buck driver, and this is what I have so far: I'm confused/looking for advice on a couple things (or anything you notice I did wrong, I'm still a noob so any advice is appreciated) if anyone would be willing to lend some time. Thank you all in advance!

Is a single capacitor to ground sufficient for output filtering? The datasheet sample application only had a single capacitor on the output for filtering, and when I google output filtering with capacitors it's usually a single capacitor connected to ground. But when I look at other buck drivers it's usually a system of 4-6 capacitors on the output (particularly giannis' open source buck driver with 6 capacitors for output filtering). If it isn't, how would I determine what capacitor values and types to use?

Are the two capacitors shown sufficient for input filtering, and how would I determine the value of C3 connected to GNDA?

Are there particular considerations I should take in capacitor placement (or just general part placement) when making the actual PCB design?

My current schematic:
1734735513113.png

Sample application circuit from datasheet:
1734735647845.png
 





Looks like you have a voltage regulator diagram here. You need a current regulator. The voltage is allowed to swing to keep the current constant.
 
paul1598419 said:
Looks like you have a voltage regulator diagram here. You need a current regulator. The voltage is allowed to swing to keep the current constant.
Click to expand...
The datasheet says that the output current is set by connecting a sensing resistor to the feedback pin. How is the diagram for a voltage regulator? It seems to me that in the application circuit that the current is set through the resistor labeled RD.

Or are you referring to my schematic?
 
Your feedback needs to be above the cathode side of the diode. What is usual is a sense resistor at the anode side. That resistor should be in series with the laser diode and the feedback should come off at the anode of the diode.
 
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What exactly do you mean by 'above the cathode side of the diode'? At the anode?

Also, all the circuit diagrams I can find for this IC does not have the resistor in series with the laser diodes, and instead has a junction where one branch has the resistor going to ground and the other branch goes back into input after the cathodes of the diodes. Should I ignore that and instead put the resistor in series on the anode side and take the feedback from that?

Sorry, I may not be fully understanding what you are saying due to my inexperience.
 
No you don't understand. Your circuit does have the sense resistor in series with the laser diode, it's just in the cathode side of the the circuit. This may work in this circuit. Most of the time and I've done this is was in the anode side. I just don't know and it's not worth my time to look it up.
 
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paul1598419 said:
No you don't understand. Your circuit does have the sense resistor in series with the laser diode, it's just in the cathode side of the the circuit. This may work in this circuit. Most of the time and I've done this is was in the anode side. I just don't know and it's not worth my time to look it up.
Click to expand...
You are absolutely incorrect....
 
Giannis_TDM said:
The shunt is supposed to be exactly like this, It is how low side sensing works, refer to my open source buck design it is this exact IC https://sites.google.com/view/giannislasershack/open-source-drivers
Click to expand...

It's not a shunt. It's a sense resistor. A shunt would be in parallel to something. An example of a shunt resistor would be one that is in parallel to a meter movement in a volt meter.

If anything this is a series resistor. It is always in series with the laser diode.
 
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paul1598419 said:
It's not a shunt. It's a sense resistor. A shunt would be in parallel to something. An example of a shunt resistor would be one that is in parallel to a meter movement in a volt meter.

If anything this is a series resistor. It is always in series with the laser diode.
Click to expand...
A shunt resistor is a resistor used to measure electric current by creating a voltage drop that is proportional to the current flowing through it. It is typically placed in parallel (or "shunt") with the current path in a circuit.

Placing a shunt resistor in parallel with the current path means connecting the resistor across two points in the circuit such that it provides an alternate pathway for the current to flow. The 'Normal' Current path would be the diode cathode directly connected to ground, the 'Parallel' path is the cathode to the shunt to ground.

Why It's Still Called a Shunt Resistor:​

  • The resistor is being used to measure current by creating a proportional voltage drop, which is the primary function of a shunt resistor.
  • The term "shunt" comes from its role in enabling indirect current measurement, regardless of whether it's parallel to an alternative current path or inline (series) with the component.
 
Giannis_TDM said:
A shunt resistor is a resistor used to measure electric current by creating a voltage drop that is proportional to the current flowing through it. It is typically placed in parallel (or "shunt") with the current path in a circuit.

Placing a shunt resistor in parallel with the current path means connecting the resistor across two points in the circuit such that it provides an alternate pathway for the current to flow. The 'Normal' Current path would be the diode cathode directly connected to ground, the 'Parallel' path is the cathode to the shunt to ground.

Why It's Still Called a Shunt Resistor:​

  • The resistor is being used to measure current by creating a proportional voltage drop, which is the primary function of a shunt resistor.
  • The term "shunt" comes from its role in enabling indirect current measurement, regardless of whether it's parallel to an alternative current path or inline (series) with the component.
Click to expand...

You have no idea what you are talking about. The sense resistor isn't a shunt of anything unless you believe that a resistor across two pins of an IC is a shunt. That is nonsense. A shunt is ALWAYS in parallel to another component. It can never be a series resistor like a sense resistor. This isn't open to debate like politics are.
 
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paul1598419 said:
You have no idea what you are talking about. The sense resistor isn't a shunt of anything unless you believe that a resistor across two pins of an IC is a shunt. That is nonsense. A shunt is ALWAYS in parallel to another component. It can never be a series resistor like a sense resistor. This isn't open to debate like politics are.
Click to expand...
Thank you for the correction, English is not my first language, I was confused since my academic level Greek textbook on electrical engineering used the equivalent (translated) terms of shunt and current sense resistors interchangibly in regards to a resistor being used to measure current.
 
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