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Question about lm317KCT

Luca Agrippino
Intellectual 390 points
Other Parts Discussed in Thread: LM317

Hello to all!

I need a trivial answer about the lm317KCT component. How much is the dropout voltage?

I am not an expert and I do not find this value on the datasheet.

  • 0
    TI__Mastermind 24975 points

    Hello Luca,

    Welcome to the forums!

    Please refer to Section 7.3 Recommended Operating Conditions:

    The minimum recommended Input to Output differential is 3 V.

    Best,
    Michael

  • 0 in reply to Michael J Schultis
    Intellectual 390 points

    Hello Michael, thank you so much for the reply!
    You refer to "Input-to-output differential voltage" parameter right? In this case, this value (3V) it refers to the max output current delivery from LM317, right?
    If i have an application where the max load current is about 360 mAh, this value is reduced right?
    My application is to build a simple power supply for the microcontroller and a few sensors, having a lipo battery (2s, 7.4V and 1500mAh) to feed all components.
    The lipo battery voltage is between 8.4 and 6.4 V, in your opinion it's correct to use a LM317 to power supply this circuit?

  • 0 in reply to Luca Agrippino
    TI__Mastermind 24975 points

    Hello Luca,

    Yes, the 3V differential is recommended to have maximum current draw. The dropout may be able to be lower under lower current conditions, but I can't speak to how much lower the voltage dropout could go.

    You say current is 360mAh, can you clarify? mAh is a capacity and not a current draw. If I assume a constant current over an hour for the given 360mAh, then the current draw would be about 0.1mA, is this correct? If this is the case you also must consider the minimum load current to maintain regulation of 3.5mA typical. If the current draw is below this, then the regulator may not regulate properly and the output voltage will begin to increase above its setpoint.

    What output voltage are you trying to achieve? You may be able to use the LM317, but I also am not sure if you are hoping for a higher efficiency in which case you may want to consider a step down switching regulator.

    Best,
    Michael

  • 0 in reply to Michael J Schultis
    Intellectual 390 points

    Hello Michael!

    I'm sorry, but i have a refuse, the max load current is about 360 mA e not 360 mAh!

    My load is an microcontroller (200mA) + an MPU9150(estimate about 40 mA over all the work conditions) and 6 HC-SR04 (max current of  20mA, 20mA x 6 = 120 mA) so in total i have a max load current of 360 mA. The work voltage for these components is of 5V.

    Under this conditions, the LM317 operate correctly if the input voltage is between 8.4 and 7.4 V?

    Otherwise can you give me some tips about some component that i can use to supply correctly my circuit? 

    PS

    I am not interested the efficiency of the component

    Best Regards

  • 0 in reply to Luca Agrippino
    TI__Mastermind 24975 points

    Luca,

    Thank you for clarifying.

    If the input voltage is 7.4V and you want to get 5V, the LM317 will not be your best option. The LM317 may be able to do it, but I don't have any data to back that up, so I would just go with the datasheet limits and say that it can't since the dropout is 3V.

    In order to guarantee that you get your 5V output over the full range of 8.4V down to 7.4V, I would suggest using the UA7805.

    This way you don't even need external resistors to set the adjustable voltage output and you have a fixed voltage regulator that just needs input and output capacitors.

    I also don't know if you are looking for a specific package, but the TO-220 is pretty standard.

    Typically thermals is also something you want to consider with LDOs. Under worst case conditions you will have a 3.4V dropout with 500mA conservatively so that will be about 1.7W, and the TO-220 (KCS In section 7.4 Thermal Information) has Theta JA of 19 so will heat up about 35 degrees or so, and this is fine for this device since 60C is acceptable.

    As for your microcontroller, have you considered our MSP430 line. We have a number of Launchpads for easy prototyping and development : http://www.ti.com/ww/en/launchpad/launchpads-msp430.html#tabs

    Always happy to help!
    Best,
    Michael

  • 0 in reply to Michael J Schultis
    Intellectual 390 points
    Thank you so much Michael, your reply is very appreciate!
    Now i look at the datasheet of the UA7805, and i think to consider to use it in my circuit. Only one thing about the UA7805, in the figure 2 at page 12 of the UA7805 datasheet, what kind of capacitor i should use?

    For the thermal problem you're rigth! I have a mistake to ignore it!!
    At this moment, my project is a 90% completed prototype and now i can't change my microcontroller, but in the future i consider to use the MSP430 microcontrollers.

    Best regard
    Luca
  • 0 in reply to Luca Agrippino
    TI__Mastermind 24975 points

    Hello Luca,

    Again, happy to help.

    Some regulators require specific capacitor types due to potential instability from ESR, but for this regulator any type of capacitor should be fine as this device is very stable.

    Some references:

    https://e2e.ti.com/support/power_management/linear_regulators/f/321/p/38166/133505#133505

    https://e2e.ti.com/support/power_management/linear_regulators/f/321/p/300811/1048169#1048169

    http://www.ti.com/lit/an/snva167a/snva167a.pdf

    Best,
    Michael

  • 0 in reply to Michael J Schultis
    Intellectual 390 points
    Hello again Michael!
    you helped me a lot!! :)

    Thank you so much!!

    PS
    one more thing about LDO and capacitor. The reference and what you told me apply also when using the component TLV1117-33?
  • 0 in reply to Luca Agrippino
    TI__Mastermind 24975 points
    Luca,

    The TLV1117-33 is a little more complicated.
    If you refer to Section 8.1 Overview, it mentions that it is designed to be stable with specific capacitors.
    There is additional information under section 9.1.1 Design Requirements about specific capacitors.

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Intellectual 390 points
    Ok, thank you so much Michael, your post are very useful for me!
    Best Regards
    Luca
  • 0 in reply to Luca Agrippino
    TI__Mastermind 24975 points
    Happy to help!

    Best,
    Michael