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Can I get 4.1V form this 3.3V regulator

user6202586
Intellectual 915 points
Other Parts Discussed in Thread: MSP430FR2433, TPS560430, TLV700, TPS7A19

Hello 

The linear voltage regulator that I'm using is TLV70033DSET. The link is: https://www.digikey.com/en/products/detail/TLV70033DSET/296-25215-1-ND/2172702?itemSeq=341807830

I'm making my own prototype board using MSP430FR2433. I wrote the firmware on the launch pad and it's time to try out on my own board. The active components on my board are as follows:

msp430fr2433 needs 3.3V 

LCD screen needs 5V (Minimum 4.5V)

LCD backlight needs 4.1V (Max 4.3V)

My supply voltage is 12V or 24V, so the regulator used on the launch pad is a no go. I'll Look for a regulator that can take an input up to 25V and bring it down to 5V. This can power the LCB screen. Is there a way to bring the output of this regulator to 4.1V and 3.3V?

Here's what I'm thinking: 

2 Zener diodes of 4.1V and 3.3V.
Two Voltage divider to get 4.1 and 3.3V. 
3.3 voltage regulator and one voltage divider
3.3 voltage divider and Diode to drop the voltage. 

Which one is the best approach? Is there another way that is not listed above?

I know the voltage divider is not ideal as the voltage might not be as stable as the rest. But I say this because we already regulated the voltage to 5V using a regulator.  

Appreciate your help

Varun R

  • 0
    TI__Guru 50025 points

    Hi Varun,

    The methods you listed have some issues. Zeners are used as references and not as power supplies, so you would not get the behavior you expect if you tried that. Using voltage dividers would be very inefficient because you are burning power in the resistors and the resistors limit the amount of current you can get. 

    I think the solution you are looking for when you need an atypical voltage like 4.1V is an adjustable-output regulator. What is your expected load for each of the rails? How are you regulating to the 5V? I can find a part recommendation for you with this information.

    In general, you can have a series of regulators for all of your rails, but there's a catch. Say, for example, that you have 5V->4.1V->3.3V. The 4.1V and 3.3V regulators and everything they power become a load for the 5V regulator, and the 3.3V and everything it powers becomes a load for the 4.1V. So you just have to be careful if you're going to do that that the regulators can all handle the loads you put on it. 

    Best regards,

    Nick

  • 0 in reply to Nick Butts
    Intellectual 915 points

    Hello 

    The loads are as below:

    LCD 1mA at 5V 
    Backlight 60mA at 4.1V
    msp430fr2433

    I'm not sure how much the micro will pull. I'm not operating it in any sort of power saver mode though. 

    I initially plan on using the UA7805 regulator for 5V. It's fixed 5V. Any recommendations?  

    Varun R

  • 0 in reply to user6202586
    Intellectual 915 points

    Hello 

    Is there another way to implement this instead of using 3 different voltage regulators for 3 different voltages? I mean there is not much creativity in this? 

    Varun R

  • 0 in reply to user6202586
    Intellectual 915 points

    Hello 

    I have been looking for a 5V regulator for my application and found the following: TPS560430. The simulation is as follows: 

    https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=EF79C029756E5278

    The above simulation will give me 5V. I got the values of the resistors from datasheet table 1. THe voltage across the lower resistor is 1V and the upper is 4V. How Can I modify the circuit so that I can get 5V and 3.3V as well?

    I initially thought I could increase the resistance of the lower resistor and increase the drop to 3.3V and get my 3.3V here. but this would affect the 5V output of the regulator. 

    I could use this: https://www.digikey.com/en/products/detail/TLV70033DSET/296-25215-1-ND/2172702?itemSeq=341807830 

  • 0 in reply to user6202586
    TI__Guru 50025 points

    Hi Varun,

    Sorry for the delayed response. 

    The TPS560430 is a switched-mode regulator, and since it uses a switcher it inherently generates more noise than an LDO, which is not desirable for digital components. To answer one of your questions, there is not much creativity involved when you need multiple rails; a rule of thumb is that you need a regulator for each rail unless you have a dual-output part. There are a number of reasons for this:

    1) As I explained before, using a resistor divider is inefficient and inaccurate as is using a zener for a supply. Furthermore, the concept of needing a buffer applies here because if you use a resistor divider to get a voltage and the part pulls any current, the voltage will not be what you thought it would. 

    2) Most LDOs have built-in protection circuitry for thermal protection for when the part gets too hot and current limiting protection for the scenario where the part tries drawing too much current. LDOs also have the benefits of power supply rejection (PSR) expressed as power supply rejection ratio (PSRR), which is filtering from the input to the output for a cleaner signal. 

    3) Many LDOs have other functionalities that can be tuned for more specific applications like a delayed startup or enabling at a precise voltage. 

    The TLV700 may work for the 3.3V rail, but you will not be able to get the 4.1V with it. 

    Check out the TPS7A19. It will work for all of your rails and the datasheet explains how to size the resistors.

    Best regards,

    Nick

  • 0 in reply to Nick Butts
    Intellectual 915 points

    Hello 

    Are you suggesting I use 3*TPS7A19 to get those voltages? That's a lot of space on the board and the BOM will be 4.8$. 

    I'm not a power guy so excuse my skepticism. I have been doing a lot of reading and read multiple blogs about which regulator to use (Linear or switching) and it said that if I plan on dropping a higher voltage, then it's better to go with the Switching regulator as its more efficient. I will be dropping from 12V to 5V best-case scenario. It could also be 24V to 5V. That's a lot of voltage drop and might lead to heating issues. I might also have to incorporate a heat sink as well. 

    But if I use the switching regulator for 5V then it's ideal for higher voltage drop and more efficient. Then it has a smothering capacitor for better DC, then it's fed to a Linear regulator which should reduce the noise and power the msp430fr2433 with 3.3V. 

    I'm using a voltage divider (22 and 122) for 4.1V but the unused resistor is very small, hence very small power dissipation. Also, This is for the backlight on the LED, which won't be needed every time. I have a GPIO from my modem, so I'll turn this ON only when it's past 7 PM and when somebody presses a key.  Essentially, the backlight will be on for very little time, say 5-10 minutes on an odd day when somebody works after hours.