Linear Regulator Output Adjustment

[joniengr081]

Hello,

I am wondering about Digital Potentiometer DS1868B in order to adjust the output voltage of a Linear Regular using voltage divider. Let's suppose I have a Linear Regulator with a fixed output of 3.3 V and the Linear Regulator can deliver or support 1.5 A load current.

If the load requirement I my design is variable between 2.5 V to 3.0 V with current approximately 1 A. Can I use Digital Potentiometer DS1868B in a voltage divider such that I can adjust the output voltage by Digital Potentiometer DS1868B through FPGA ?

How much current will run through the voltage divider resistors ? Do I need to care about power rating of the resistors used in the voltage divider ?

[xvr]

Can I use Digital Potentiometer DS1868B in a voltage divider such that I can adjust the output voltage by Digital Potentiometer DS1868B through FPGA ?

You can't use Digital Potentiometer as direct divider between output of LDO and load because current capacity of DP is quite low (mA ranges).
You can't use DP in feedback/ground circuit of LDO because any divider here can also increase output voltage of LDO, but you need to decrease it.
You can create buffered (by transistor) divider between LDO output and load, but it will be roughly the same as adding another one LDO to already existing one.

[joniengr081]

The third option you mentioned with transistor. Can I also invert the voltage as well ? But this will be a fixed output. I am wondering how can I control the voltage level through FPGA. I also need to make it adjustable.

[xvr]

No, transistor can't invert output - it used as simple follower. For digital control of output can be used various regulators (both liner and switched mode) and even trivial opamp circuit. It depends on requirements for current and stability. There are a set of regulator with direct digital control also exist.

[joniengr081]

Thanks for your comment. Do you mean linear regulator with digital control through FPGA. I am not aware of. Kindly give me some example because this is exactly what I am looking for.

[xvr]

Almost any PMIC. Most of them controlled through I2C

[xvr]

https://www.digikey.ie/en/products/filter/voltage-regulators-dc-dc-switching-regulators/739?s=N4IgjCBcoCwdIDGUBmBDANgZwKYBoQB7KAbRAGYwAGcgdhhAPIE4qwYBWEAXQIAcALlBABlAQCcAlgDsA5iAC%2BBALTMooZJAkBXfEVLgeSkMoBM6pFB17ikMl24KnQA

Just first - https://www.digikey.ie/en/products/detail/microchip-technology/LX7165-01CSP-TR/4385420 (programmable via I2C)

Next - https://www.digikey.ie/en/products/detail/onsemi/FAN49103AUC340X/6009677

[David Hess]

To lower the output voltage of a fixed regulator requires a negative supply, and the voltage divider may still need to be buffered with an operational amplifier or a couple of transistors.

[joniengr081]

It is mentioned that "voltage divider may still need to be buffered with an operational amplifier or a couple of transistors."

The Linear Regulator we have is a fixed output of 3.3 V that can deliver or support 1.5 A load current as maximum.

The load requirement is variable between 2.5 V to 3.0 V with current approximately 1 A. The negative voltage (between 2.5 V to 3.0 V) is also fine. This is not mentioned in the port #1.

I am wondering how can I use operational amplifier to buffer and reduce the voltage of fixed output Linear Regulator.

There are two basic configuration of operational amplifier I can refer here.

Inverting: The output voltage is Vout = -(R1/R2) x Vin

Non-inverting: The output voltage is Vout = (1 + R1/R2) x Vin

The inverting configuration can reduce and invert the polarity. But how about the maximum load current capability ? is this limited by the output current of the Operational Amplifier ? Usually the output current of the Operational Amplifier is around 100 mA. How can I get 1 A ?
 

[joniengr081]

Also back to Digital Potentiometer DS1868B from the first post. In which kind of application we can use Digital Potentiometer DS1868B ?

[xvr]

I am wondering how can I use operational amplifier to buffer and reduce the voltage of fixed output Linear Regulator.

Something like this:

The inverting configuration can reduce and invert the polarity.

If you want to get negative voltage from opmap you should supply negative power for it.

But how about the maximum load current capability ? is this limited by the output current of the Operational Amplifier ?

Yes. You can use opamp with power output stages (as from Apex - but they are VERY expensive). Or use just powerfull enough opamps: Digikey opamps

(OPA567 - just first)

Or use transistor in output cascade (as above)

[wraper]

You absolutely don't need digital potentiometer for this. You need either smoothed PWM or DAC.

[wraper]

I am wondering how can I use operational amplifier to buffer and reduce the voltage of fixed output Linear Regulator.

Something like this:
...

Yes and the result is relatively expensive crappy 8-bit DAC.

[David Hess]

It is mentioned that "voltage divider may still need to be buffered with an operational amplifier or a couple of transistors."

The Linear Regulator we have is a fixed output of 3.3 V that can deliver or support 1.5 A load current as maximum.

The load requirement is variable between 2.5 V to 3.0 V with current approximately 1 A. The negative voltage (between 2.5 V to 3.0 V) is also fine. This is not mentioned in the port #1.

I am wondering how can I use operational amplifier to buffer and reduce the voltage of fixed output Linear Regulator.

Whether the operational amplifier is required depends on the whether the regulator has an adjustment pin or common pin.

An adjustment pin has low input bias current so the voltage divider, with a negative voltage reference, can be directly connected.  If the regulator has a common pin, then this is where the quiescent current of the regulator flows, and then the operational amplifier will be required to buffer the voltage divider, but a negative reference voltage will still be required.

In all cases a negative voltage reference will be required to pull the adjustment or common pin below ground to achieve a lower output voltage.

[joniengr081]

@David Hess:

Thank you very much for your comment. Can you please explain bit more.

[joniengr081]

@xvr:
I did not know before that some operational amplifier can have higher output current for example 1A and above. 

[xvr]

Apex opamp can handle up to 50A and 2400V (not simultaneously of course)

[David Hess]

Thank you very much for your comment. Can you please explain bit more.

No matter how you lower the output voltage of the fixed regulator, it will require a supply voltage which is lower than ground by at least the amount that you want to lower the regulator's output.

[wraper]

^Therefore generally you should use adjustable regulator version if you need voltage adjustment. Usually they output about 1.2V more than voltage present on adjustment pin.

[joniengr081]

Thanks all for your comments. I will continue working on it and if there is still some question, I will put in a new thread.