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LT3094: Input voltage effect on PSSR

Hi,

I'm using LT3094 in my one of the design.

I observed that the dropout voltage is given as 235mV in datasheet. As well as there is a graph which provides the input voltage reference formula as Vin = Vout - 2.3V.

So what is this about ?

Can anyone please explain me this formula significance on the input voltage.

I'm intended to provide -2.5V input and -2V output on this device.

Can anyone please help.

Thanks & Regards,

Nanjunda M

  • Hi,

    Vin = Vout - 2.3V was used with PSRR checking, RMS output noise etc.
    since the maximum input voltage of the chip is -2.3V (@500mA: see page 4 of datasheet).
    This is a condition which most of the electrical characteristics of the chip were tested. See table of elec. characteristics conditions.

    You can still refer on the dropout voltage at page4 of the datasheet 235mA @500mA load.

    regards,

  • Hi ACaldejon,

    Thanks for the response.

    1. The datasheet specifies that the minimum input voltage is -1.8V in 1st page, so this is at what condition?

    2. What is the ideal difference between the input and output voltage for this LDO? Say for example, I want to generate a -8.5V output, so what is the best value of the input can be? Is it -9V or is it -12V ? In which condition the PSRR will be better?

    3. Should we always follow the 

    Vin = Vout - 2.3V

    formula given at the graph for best performance of PSRR ?

    I know that the dropout is just 235mV, but is it good to maintain too much of difference between the input and output voltage?

    Please let me know ASAP.

    Thanks & Regards,

    Nanjunda M

  • Hi Nanjunda,

    1.  -1.8V is the typical value of the minimum input voltage. But the range is -2.3V to -20V. 
          Please see table of electrical characteristics at page4 of the datasheet.

    To answer 2&3,

        There will always be different consideration on what is the better input-to-output voltage difference for an LDO.
        A smaller input-to-output voltage differential will give you better power efficiency. But then, you have to know that     having a bigger dropout/ input-to-output differential will give you a better PSRR performance. There will always    
        be tradeoff's between the two on which you want to prioritize. It is up to your design to balance it and/or which
        aspect you prioritize more.
        *See note10 at page6 of the datasheet.
        *See page12 of the datasheet for Input-to-output voltage differential versus PSRR graph

    regards,
    Ash

  • Hi Ash,

    -1.8V is the typical value

    Do you mean this is absolute maximum value? But in the datasheet maximum value given as 0.3V.

    Regards,

    Nanjunda M

  • Hi Nanjunda,

    -20V to -2.3V is the OPERATING input voltage range of the chip.
    *Please see the table of "Electrical Characteristics" at page4 of the datasheet. 

    Indicated in page 3 of the datasheet, which is -22V and 0.3V are the ABSOLUTE MAXIMUM RATINGS. Going beyond this voltages may or will result in IC breaking down or being damaged.
    It is stated in NOTE1(page6):
    "Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime."

    regards,
    Ash

  • Hi,

    Then what is this range "Wide Input Voltage Range: –1.8V to –20V " mentioned in the 1st page of the datasheet. Can you please clear this statement.

    Regards,

    Nanjunda M

  • Hi Nanjuda,

    -1.8V is the typical max Vin at room temperature.
    While the guaranteed max Vin over the temperature range (-40degC to 125degC) is -2.3V.

    Regarding your initial concern with input to output voltage differential and it's PSRR. 
    Yes it is OK to have -2.5V input  and -2V output.

    I'm intended to provide -2.5V input and -2V output on this device.

    See page12 of the datasheet. PSRR vs. Input to output differential (V).
    *note: PSRR does not change with the output voltage.

    regards,
    Ash