Linear Regulator Output Adjustment  - Page 1
Forum section: electronics / general

author: joniengr081
author_rank: Regular Contributor
timestamp: June 10, 2024, 06:31:34 am
content: 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 ?

author: xvr
author_rank: Frequent Contributor
timestamp: June 10, 2024, 10:26:54 am
content: QuoteYou 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 alsoincreaseoutput 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.

author: joniengr081
author_rank: Regular Contributor
timestamp: June 10, 2024, 11:58:57 am
content: 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.

author: xvr
author_rank: Frequent Contributor
timestamp: June 10, 2024, 12:08:52 pm
content: 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.

author: joniengr081
author_rank: Regular Contributor
timestamp: June 10, 2024, 12:13:39 pm
content: 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.

author: xvr
author_rank: Frequent Contributor
timestamp: June 10, 2024, 12:23:13 pm
content: Almost any PMIC. Most of them controlled through I2C

author: xvr
author_rank: Frequent Contributor
timestamp: June 10, 2024, 12:33:21 pm
content: https://www.digikey.ie/en/products/filter/voltage-regulators-dc-dc-switching-regulators/739?s=N4IgjCBcoCwdIDGUBmBDANgZwKYBoQB7KAbRAGYwAGcgdhhAPIE4qwYBWEAXQIAcALlBABlAQCcAlgDsA5iAC%2BBALTMooZJAkBXfEVLgeSkMoBM6pFB17ikMl24KnQAJust 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

author: David Hess
author_rank: Super Contributor
timestamp: June 10, 2024, 04:17:34 pm
content: 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.

author: joniengr081
author_rank: Regular Contributor
timestamp: June 11, 2024, 05:07:27 am
content: 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 VinNon-inverting: The output voltage is Vout = (1 + R1/R2) x VinThe 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 ?

author: joniengr081
author_rank: Regular Contributor
timestamp: June 11, 2024, 05:08:38 am
content: Also back to Digital Potentiometer DS1868B from the first post. In which kind of application we can use Digital Potentiometer DS1868B ?

author: xvr
author_rank: Frequent Contributor
timestamp: June 11, 2024, 10:01:06 am
content: QuoteSomething like this:QuoteIf you want to get negative voltage from opmap you should supply negative power for it.QuoteYes. 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)

author: wraper
author_rank: Unknown
timestamp: June 11, 2024, 10:17:59 am
content: You absolutely don't need digital potentiometer for this. You need either smoothed PWM or DAC.

author: wraper
author_rank: Unknown
timestamp: June 11, 2024, 10:20:58 am
content: Quote from: xvr on June 11, 2024, 10:01:06 amYes and the result is relatively expensive crappy 8-bit DAC.

author: David Hess
author_rank: Super Contributor
timestamp: June 11, 2024, 11:02:55 am
content: Quote from: joniengr081 on June 11, 2024, 05:07:27 amWhether 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.

author: joniengr081
author_rank: Regular Contributor
timestamp: June 11, 2024, 11:55:34 am
content: @David Hess:Thank you very much for your comment. Can you please explain bit more.

author: joniengr081
author_rank: Regular Contributor
timestamp: June 11, 2024, 12:03:41 pm
content: @xvr:I did not know before that some operational amplifier can have higher output current for example 1A and above.

author: xvr
author_rank: Frequent Contributor
timestamp: June 11, 2024, 01:21:46 pm
content: Apex opamp can handle up to 50A and 2400V (not simultaneously of course)

author: David Hess
author_rank: Super Contributor
timestamp: June 11, 2024, 04:24:17 pm
content: Quote from: joniengr081 on June 11, 2024, 11:55:34 amNo 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.

author: wraper
author_rank: Unknown
timestamp: June 11, 2024, 07:15:59 pm
content: ^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.

author: joniengr081
author_rank: Regular Contributor
timestamp: June 12, 2024, 11:49:48 am
content: 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.

author: joniengr081
author_rank: Regular Contributor
timestamp: June 21, 2024, 09:14:21 pm
content: Sorry, I am again on this thread.I am still not getting. Is there any argument against using Digital Potentio-meter DS1868B controlled by FPGA to get a digitally controlled variable voltage using voltage divider at the output of fixed output voltage regulator.The output of the fixed output voltage divider will be feed to the voltage divider, one resistor will be Digital Potentio-meter DS1868B controlled by FPGA.What will be the load current limitations ?

author: xvr
author_rank: Frequent Contributor
timestamp: June 21, 2024, 09:24:56 pm
content: QuoteDS1868B is not designed to be used as single resistor. It shoud be used as potentiometer because it has very huge tolerance for it resistors string (up to 20%).QuoteLoad current will affect divider output. You can only neglect load if its resistance will be at least 10-100 times more than dividers resistor.So, even for minimal DS1868B nominal resistance (5K as I remember) load should be about 50-500K, at least. This is microamper range

author: joniengr081
author_rank: Regular Contributor
timestamp: June 22, 2024, 07:44:13 am
content: Quote from: xvr on June 21, 2024, 09:24:56 pmDS1868B is a digital potentio-meter. I am not getting the tolerance is up to 20 %. Is that stated in datasheet or a general statement about potentio-meters ?

author: xvr
author_rank: Frequent Contributor
timestamp: June 22, 2024, 08:11:08 am
content: Datasheet, p8 : End to end resistor tolerance - -20% to +20%

author: tooki
author_rank: Super Contributor
timestamp: June 22, 2024, 08:24:44 am
content: There are adjustable linear regulators whose voltage set pins do not work via feedback from the output. These often allow setting the voltage down to 0V, and often just use a single resistor to set the output, so a digital pot might work very well for those. For example, look at the LT3045. The SET pin sources a 100uA current, and you set the voltage by connecting it through a resistor to ground. The resistor value is simply the desired voltage x 10000, e.g. 33kohms for 3.3V.(Is there anything I’m not thinking of here?Also, for regulators with traditional FB pins, an option to control output voltage is to inject voltage into the feedback node. TI has a downloadable tool called Power Stage Designer that includes a handy calculator for this, and I’ve successfully used it. You choose some variables (like some of the resistor values, to set your quiescent current through the divider), and then give it the feedback voltage, the output voltage range, and your control voltage range, and it spits out the remaining resistor values. It’s also got the ability to calculate values for digital inputs, so if you happen to have enough unused GPIOs, you can dispense with a DAC altogether.https://www.ti.com/tool/POWERSTAGE-DESIGNER

author: joniengr081
author_rank: Regular Contributor
timestamp: June 23, 2024, 09:11:53 am
content: Yes I found in the datasheet of DS1868B on page 8, "End-to-End Resistor Tolerance" is -20% to +20%. This means it will not be a good solution to go with DS1868B.I will look at the tooki suggestion. If there is a set pin that can be controlled by FPGA/DAC to set the output voltage.