So everyone know famous LM317 can be configured as Current Regulator like this:

However the dropout voltage of LM317 is pretty high and so I am looking for some LDO chip to do the same thing. Here I got some RT9018 chip to do this:

http://www.richtek.com/Products/Linear Regulator/Single Output Linear Regulator/RT9018ART9018B.aspx

I have successfully configured RT9018 as Voltage Regulator like what it supposed to be. However I can't get it working as a current regulator. I connect the load exactly like what I do for LM317 but it just don't working. Is it not all the voltage regulator can regulate current or is there something I am missing? Thanks for helping.

 

Your first post. Welcome to AAC!

Post a schematic for the circuit that isn't working and give current and load requirements.

 

So everyone know famous LM317 can be configured as Current Regulator like this:

However the dropout voltage of LM317 is pretty high and so I am looking for some LDO chip to do the same thing. Here I got some RT9018 chip to do this:

http://www.richtek.com/Products/Linear Regulator/Single Output Linear Regulator/RT9018ART9018B.aspx

I have successfully configured RT9018 as Voltage Regulator like what it supposed to be. However I can't get it working as a current regulator. I connect the load exactly like what I do for LM317 but it just don't working. Is it not all the voltage regulator can regulate current or is there something I am missing? Thanks for helping.

Are you connecting Vin to the EN pin? The EN pin is "enable" so the regulator can be turned on/off. It myt have a voltage applied to make the chip work.

 

Your first post. Welcome to AAC!

Post a schematic for the circuit that isn't working and give current and load requirements.

Thanks Here is How I connect the LDO:

 

Are you connecting Vin to the EN pin? The EN pin is "enable" so the regulator can be turned on/off. It myt have a voltage applied to make the chip work.

According to the datesheet:
"For RT9018A, the EN pin function pulls high level internally. So the regulator will be turn on when EN pin is floating."

I have successfully configure it as a voltage regulator so the problem is changing it from voltage regulator to current regulator. I am not sure if all the voltage regulator can be configured as current regulator.

 

Here is How I connect the LDO

Show connections for all pins and give current and load requirements.

 

According to the datesheet:
"For RT9018A, the EN pin function pulls high level internally. So the regulator will be turn on when EN pin is floating."

I have successfully configure it as a voltage regulator so the problem is changing it from voltage regulator to current regulator. I am not sure if all the voltage regulator can be configured as current regulator.

It is possible that the adj needs a ground reference (directly or through a resistor), so it would not be able to work as a current source.

 

It is possible that the adj needs a ground reference (directly or through a resistor), so it would not be able to work as a current source.

You maybe right. I included the block diagram for you to determine the problem better:



If you are right. I think I have to look at another chip to do the job. How could I know whether the adj need a ground reference? Can it be determined by looking at the block diagram?

Thanks

 

The regulator you're using doesn't work like LM317. What are your current and load requirements?

 

Hi Dennis,

I am using this LDO as a LED drive (LED Vf about 3.3v). I am planning to use single li-ion cell and power source (3.7-4.2v), I want to get the maximum current at about 1A. Thanks

 

What is the amp hour rating of the battery?

 

What is the amp hour rating of the battery?

It is a 3400mAh 18650 cell.

 

You don't have enough headroom for a current source @ 3.7V.

 

You don't have enough headroom for a current source @ 3.7V.

Not the problem. I am using the 5V power supply when I test the circuit. I think it it like what GopherT say.

 

You don't have enough headroom for even a simple current source. Even at full battery voltage of 4.2V, a medium power transistor in a current source would be near saturation @ 1A. Not enough voltage to consider a MOSFET.

 

Hi Dennis,

As I said before. I am using a 5V power supply which provide enough headroom when I test the circuit. I don't really care about the input voltage as I can use 2 cell. I just want to get the LDO working as a current regulator now. Thanks

 

As I said before. I am using a 5V power supply which provide enough headroom when I test the circuit.

It wasn't clear to me that this was just an exercise to use the LDO as a current source.

I don't really care about the input voltage as I can use 2 cell. I just want to get the LDO working as a current regulator now.

It won't work. The regulator you've chosen just uses the resistor divider to feedback a portion of the output voltage to the control logic. It doesn't work like an LM317.

There are ways to build a current source that don't require a voltage regulator. That's where I'm trying to guide you. But you don't have a high enough voltage for any I can think of.

 

It wasn't clear to me that this was just an exercise to use the LDO as a current source.
It won't work. The regulator you've chosen just uses the resistor divider to feedback a portion of the output voltage to the control logic. It doesn't work like an LM317.

How can I know whether the regulator I choose can limit the current like LM317?

There are ways to build a current source that don't require a voltage regulator. That's where I'm trying to guide you. But you don't have a high enough voltage for any I can think of.

I don't understand . LDO only have around 0.2-0.3v drop at 1A. With a fully charged cell (4.2v) why it cannot provide enough headroom for 3.3Vf LED? (4.2v-0.3v=3.9v which > 3.3v)

 

How can I know whether the regulator I choose can limit the current like LM317?

Read the datasheet.

I don't understand . LDO only have around 0.2-0.3v drop at 1A. With a fully charged cell (4.2v) why it cannot provide enough headroom for 3.3Vf LED? (4.2v-0.3v=3.9v which > 3.3v)

The LDO you selected doesn't have a constant current mode like LM317, so the fact that it only drops 0.3V@1A isn't significant. It regulates voltage, not current.

 

I think a basic 3-terminal LDO regulator such as the LT1086 with a 1V drop should operate similarly to the LM317 as a current regulator.
But note that the minimum operating voltage drop would still be about 1V plus the 1.25V reference voltage or 2.25V.
If you want a lower dropout than that, you may have to go to an op amp/transistor type of constant-current circuit which can reduce the dropout to well less than a volt.