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LMS3655: Adding backup power to the output

Taroimo
Intellectual 680 points
Part Number: LMS3655

Dear TI team

 

I would like to use "LMS3655" under the condition of "24VIN/5VOUT".

Also, this time, I would like to add a 5V backup circuit using a supercapacitor to the output side of the IC.

(I checked the application note below.)

https://www.ti.com/lit/an/slva676/slva676.pdf?ts=1683011067011&ref_url=https%253A%252F%252Fwww.google.com%252F

 

I have a question about turning off the power supply to the "LMS3655" and setting VIN to 0V.

At this time, if the supercapacitor is charged, "VIN < VOUT" may occur, and the current may flow backward through the parasitic diode.

Is this perception correct? Also, if there are any other possible problems, please let me know.

 

Also, I'm thinking of placing a diode for reverse current prevention at the position shown in the attached figure.

I realize that by putting a diode between the FB resistor and the inductor, I can keep the output voltage at 5V without worrying about the voltage drop due to the VF of the diode.

Is this perception correct? Please let me know if I've overlooked anything.

 

Best Regards,

Taroimo

  • +1
    TI__Guru 50120 points

    Hello Taroimo,

    That is correct that when the input supply is disconnected/off , the output voltage is high due to the supercapacitors and will cause a reverse current to flow from VOUT to VIN through the IC. In that case there may be damage to the internal circuitry/ FETs, more details and solutions are explained in this app note (SLUA962). 

    Alternatively, putting a schottky diode between VOUT and VIN similar to what is shown in this E2E blog (https://e2e.ti.com/blogs_/b/powerhouse/posts/ldo-basics-preventing-reverse-current-in-ldos)

    Regards,

    Jimmy

  • 0 in reply to Jimmy Hua
    Intellectual 680 points

    Hi jimmy,

     

    Thanks for the answer.

    Sorry for the late confirmation.

     

    I found that the method of adding a Schottky diode between VIN and VOUT of IC is effective for IC internal circuit/FET protection. ,

    On the other hand, I think that the 5V backup circuit at VIN = 0V (when power is off) that I want to realize this time will not function.

    (Because the supercapacitor discharges via the Schottky diode when VIN=0V)

    Therefore, as I showed last time, I think that a diode should be placed in front of the FB resistor to prevent reverse current.

    Please let me know if there is anything I should consider.

     

    Best Regards,

    Taroimo

  • +1 in reply to Taroimo
    TI__Guru 50120 points

    Hi Taroimo,

    Yes you can place the diode in the original proposed configuration as well. However in this configuration you will have increase power dissipation since the diode is always conducting in the forward direction. You would need to select the appropriate diode with power rating and low Vf to reduce power dissipation.

    Regards,

    Jimmy