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LM317A-337N-EVM: Formula VOUT+ = VREF (1 + R3/R1) + (IADJ × R3)

Sadok Abbaki
Prodigy 200 points
Part Number: LM317A-337N-EVM
Other Parts Discussed in Thread: LM317A, LM317-N

Tool/software:

Hello,

Could you please help me for understand the use of the fromula VOUT+ = VREF (1 + R3/R1) + (IADJ × R3). Indeed this formula is not related with VIN?

In other terms : I want to use this channel with VBAT+ under 12V to have in the outuput 10V +/-5%. The output voltage regulation stayed within 0.1%

I have tested the initila conf of the EVAL board "LM317A-337N-EVM" with 8V input and the meadured output is 5V (R1 = 240 Ohms and R3 = 715 Ohms)

I want to keep R1 under 240 Ohms. So what is the formula to apply with VIN (+12V) to have in the output 10V? for define the R3 output

The R3 changed with 7K15 and when I apply 12V Input, I obtain 11,06V Output. 

Note: the needed output current is low (under 100mA)

Thank you for your help

  • 0
    TI__Genius 13240 points

    Hi Sadok,

    A 7.15kΩ R3 would result in an ideal Vout of 1.25 * (1 + 7150/240)  + 7150 * 100e-6 = 39.205V. Since Vin is below this voltage, the LDO will operate in dropout, meaning the output will be ~1.5V below the input. 

    To obtain a 10V output, we can rewrite the formula for Vout to solve for R3 as follows: 

    R3 = (Vout - Vref) / (Iadj + (Vref / R1))

    Plugging in values from the datasheet and EVM: 

    R3 = (10V - 1.25V) / (100µA + (1.25 / 240)) = 1648.4Ω. The closest 1% standard resistance is 1.65kΩ. Using this value, we can calculate the expected Vout to be 10.008V. 

    Best Regards,

    Alex Davis

  • 0 in reply to Alex Davis
    Prodigy 200 points

    Hello Alex,

    Thank you soo much for your answer. 

    Three questions to more understand : 

    1) Why this formula not use the VIN?

    2) Why we use 100µA for IADJ?

    3) This formula, is applied whatever the value of the load current? I need to supply pressure sensor under 10V (current must be < 10mA)

    Yes, I plan to change the proposed resistor value (1K65) and then perform the tests

  • 0 in reply to Sadok Abbaki
    TI__Genius 13240 points

    Hi Sadok,

    1. Unless the LDO is operating in dropout, where Vin is too low for the LDO to provide the desired output voltage, Vin's effect on Vout is captured in either the Line Regulation spec (for DC variation) or Power Supply Rejection Ratio (for AC). For the LM317A, Line Regulation is specified as 0.02%/V max. This means that if the LDO is configured for 10V output, a 1V change in Vin will produce a 2mV change in output voltage. 

    2.This is my mistake - I used the 100µA worst-case spec for the LM317-N instead of the 5µA worst-case value for the LM317A. Redoing the calculations with the lower Iadj gives slightly different values. I'd suggest either 1.67kΩ or 1.69kΩ for R3, resulting in an output voltage of ~9.96V or ~10.06V respectively.

    3. Load current will have an impact on Vout - see the Load Regulation specification in the datasheet. The LM317A requires a 10mA minimum load current to properly regulate. In your design, the divider resistance will result in 5.2mA of output current, I'd recommend adding a provision for an additional ~2kΩ resistive load to ensure the minimum output current requirement is met regardless of load current consumption.

    Best Regards,

    Alex Davis