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LM25117: LM25117 Data Sheet Question

Masa_0000
Intellectual 885 points
Part Number: LM25117

Hi everyone !

I have a question about LM25117 Data Sheet.

"Figure 18. External VCC Supply For 14.5 V <V OUT"

The circuit input to VCC is posted from VOUT, via Zener Diode.

-Q1: Is the circuit of Figure 18. necessary? (must  applying the circuit ?)

(In the WEBENCH simulation result, the circuit of Figure 18 was not applied.)

-Q2: What are the advantages of applying the circuit in Figure 18.?

-Q3: Using the circuit in Figure 18,

I think it will collide with the VCC output terminal (internal LDO: 7.6 V typ.),

Should I disable the internal LDO on the VCCDIS pin?

- Q4: Can you suggest a recommended circuit for me?

My design specification:

Device: LM25117

Vin: 36 V

Vout: 24 V

Iout: 11 A

Please let me know if you have any information.

Best Regards

ShingLM25117.pdf

  • 0
    TI__Mastermind 39980 points
    Hello Shing,

    Figure 18 is not absolutely needed. Figure 18 is used should you wish to reduce power dissipation in the device. This is the advantage.

    Should you decide to supply VCC bias from the output of the converter to reduce power dissipation, you will need to implement figure 18 circuit. There is no need to implement through VCCDIS, please follow Figure 18, should you wish to implement?

    Hope this helps?

    David.
  • 0 in reply to David Baba
    Intellectual 885 points

    Hi David,

    Thank you for your help.

    You said

    "to reduce power dissipation" &

    "no need to implement through VCCDIS".

    In this case,I have to stop of VCC output.

    Please let me know why reason about "no need VCCDIS".

    I have additional questions.

    zener diode design:

    Vout:24V - VCCout:7.6V  + Vf:0.7V = Vz:15.7V  is zener voltage

    Is this true or false?

    May I haver your comment?

    My design specification:

    Vin: 36 V

    Vout:24 V

    Iout:11 A

    Best Regards

    Shing

  • 0 in reply to Masa_0000
    TI__Mastermind 39980 points

    Hello Shing.

    Vout-Vz - Vfwd = External VCC voltage.

    Vout = 24V

    VZ = 12V

    Vfwd = ~0.5V

    Where External VCC Voltage>VCC

    External VCC Voltage =11.5V.

    hope this helps?

    David.