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LM317M: HOW TO UNDERSTAND FIGURE 6 & 10 IN THE MANUAL?

Part Number: LM317M

Hi,
Could you please explain the other conditions to understand Figure 6 & 10 below?

I calculated these data tables but I think this is not matched. how do I understand Figure 6 & 10?

Tj_ Max

125

 

Ambient

25

 

Thermal Resistance (RθJA)

60.2

℃/W

 

Output Voltage

5.6

V

 

 

 

 

 

Input Voltage

Vi-Vo

Max Output Current

28

22.4

74.16

mA

26

20.4

81.43

mA

24

18.4

90.28

mA

22

16.4

101.29

mA

  • Hi HJ, 

    For Figure 6, the value is for the current limit, and when at the current limit, the junction temperature could go beyond 125C. 

    For Figure 10, the differential value here is equivalent to dropout at different current loads, and it may be interpreted as the minimum headroom between Vin and Vout for the LDO to stay in regulation. 

    Regards, 
    Jason Song

  • Hi Jason,

    Thank you for your reply.

    When I use as below condition, I got over 100 degrees. 

    Vin = 28V, Vout = 5.6V, Iout = 0.12A


    But according to Figure 6, the condition that I use should be okay.

  • Hi HJ, 

    With a Rja= 60C/W, the temperature rise for your application is 160C and it will push the junction temperature well beyond 125C. Figure 6 is for the current limit at different temperatures with different Vin-Vout, it's meant to tell when the internal current limit protection will kick in to clamp the voltage and protect the IC from damage due to overheating. The current in Figure 6 is the maximum current the LDO can output, but it's not the maximum amount of current the part can support. 

    For example, your car's maximum speed is 200KM/H, but the speed limit is only 80KM/H, the 200KM/H is similar to the LDO's current limit, but the speed limit of 80KM/H is the number you calculated using Rja. Does it make sense? 

    Regards, 
    Jason

  • Hi Jason,

    Thank you for your prompt answers. Have a nice day!

    Regards,

    HJ Jang