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LP3856: Digitally Programmable Output Voltage on LP3856-ADJ

Periyasamy Balasubramanian
Prodigy 215 points
Part Number: LP3856
Other Parts Discussed in Thread: , LP2954

Hi,

I'm planning to use LP3856 in one of my design. Here is the requirement,

1)Vout= 2.4V to 6.0V

2)Iout= up to 2A., typ (1.75A)

3) LDO output should be programmable through serial interface (I2C) - may be using Digi potentiometer (or) DAC

I think DAC would be better option considering accuracy (please let me know if different opinion),

a) So, does LP3856 is suitable to operate with DAC for make it digitally programmable output?

b) which one would be good option for LP3856, Digital potentiometer (or) DAC?

c) Please help to share if you have any app note/ reference material for LDO output adjustment with Digi POT or DAC

Thank you very much for your support.

Thanks,

Bala.

  • 0
    TI__Genius 10380 points

    Hi Bala,

    a) Yes, the LP3856 output can be programmed using either a DAC or a digital potentiometer.

    b) The best choice will depend on the cost and accuracy requirements of your application. You are correct in thinking that the DAC is the more accurate solution. I would recommend this if output accuracy is critical for your application. Digital potentiometers are limited in accuracy because integrated Silicon resistors will have a typical tolerance of around 20% and a maximum tolerance of around 50%. However, because of this limitation, programming with a digital potentiometer is the cheaper solution. It is also the slightly simpler solution, as DAC programming requires an additional resistor connected to the feedback network. 

    c) You can learn how to use a DAC to program an LDO's output voltage with Matthew's blog post here:

     

    You can also find TI's portfolio of DACs and digital potentiometers with the following links to assist in your decision:

    Thanks,

    Gerard

  • 0 in reply to Gerard Copeland
    Prodigy 215 points

    Hi Gerard,

    Thanks for your clarification & details.

    could you please help me to share Thermal Resistance ThetaJC (θjc) & θCH/ Theta CA (θca) for LP3856-ADJ TO-263(DDPAK) package for my thermal calculation?

    Thanks,

    Bala.

  • 0 in reply to Periyasamy Balasubramanian
    TI__Genius 10380 points

    Hi Bala,

    θja (junction to ambient) is a combination of θjc (junction to case) and θca (case to ambient). θja is always given in datasheets and is sufficient to ensure that the LP3856-ADJ does not enter thermal shutdown in your application. θja,max can be calculated based on the maximum expected ambient temperature and power dissipation. The maximum recommended junction temperature for this device is 125 C. 

    θja,max = (Tj,max - Ta,max) / Pd = (125 - Ta,max) / Pd

    Figure 27 in the datasheet can then be used to determine how much additional copper is needed on the PCB to achieve this θja,max. Note that an additional heat sink may be necessary if θja,max for your application is less than about 35 C/W. 

    Thanks,

    Gerard

  • 0 in reply to Gerard Copeland
    Prodigy 215 points

    Hi Gerard,

    Thanks for your clarification. Please note, the LDO's power handling is more, So Heat Sink is required based on your thermal data/ datasheet info,

    1)Vout= 2.4V to 6.0V

    2)Iout= up to 2A., typ (1.75A)

    So, I'm looking for addition thermal parameters of LDO, like Thermal Resistance junction to Case ThetaJC (θjc) & Thermal Resistance Case to Thermal Heatsink PAD of LDO θCH/ Theta CA (θca) for LP3856-ADJ TO-263(DDPAK) package..

    Could you please help to provide the requested parameters..

    Thanks,

    Bala.

  • 0 in reply to Periyasamy Balasubramanian
    TI__Genius 10380 points

    Hi Bala,

    The LP2954 has similar thermal performance to the LP3856-ADJ in the TO-263 and TO-220 packages. From the datasheet, θjc(bot) is 1.0 C/W and typical θch values will depend on the interface material:

    The heat sink will need a θha such that:

    θha = θja,max - θjc(bot) - θch

    Make sure to choose a heat sink with a θha at least a few C/W smaller than the result from calculation above. This will allow some margin to account for any differences in thermal performance between the LP2954 and the LP3856-ADJ. 

    Thanks,

    Gerard