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LM5175: LM5175 SS not stable in boost mode and load

Thomas Joseph1
Prodigy 20 points
Part Number: LM5175

Hi,

My design spec. is

TI Part : LM5175

Vin range : 8V - 27V

Vout = 24V / 6Amax

I am facing problems with output regulation in buck-boost and boost mode

#1. At Vin = 24V, Vout is 24V with light load

#2. At Vin = 24V, Vout is not regulating to 24V 1A load  (Refer. Fig 1)

#3. As Vin reduces from 24V and with light load, Vout is not reaching 24V (Refer Fig 2)

#3. The SS pin shows a sawtooth waveform when Vout is not regulating to 24V  (Refer Fig 4)

#4. COMP pin waveform is pulse like and is not settling to a stable level. (Refer Fig. 5 and Fig 6)

I used "LM5175 Buck-Boost Quickstart Tool"    for the design and the screen shot is attached. (Refer Fig 3)

Kindly help me to resolve the issue.

Thanks,

Thomas Joseph

  • 0
    TI__Guru 60235 points

    Hello Thomas,

    Why do you limit the OCP to 3A, if you want to get 6A output current OCP should not trigger below 7A.

    Please as well check the waveform from CS to CSG.

  • 0
    Prodigy 50 points

    Joseph,

    I am currently working with the LM5175 as well, with these specifications:

    Input: 14-32V

    Output: 24V/10A max

    I ran into some issues with the output voltage becoming unstable at various input voltage ranges, particularly when the regulator was in boost mode. I found the most beneficial change was to change the slope capacitor. I am curious to see if by changing your slope capacitor value (most likely lowering it) if it would stabilize your output voltage. If you do get a fix to your problem I would be very interested in hearing how you fixed your issue!

    Best of luck,

    Quinn 

  • 0 in reply to Quinn Cook
    TI__Guru 60235 points

    Hello Quinn,

    You are right, lowering the SLOPE capacitor helps the device in boost and buck-boost mode for better stability.

  • +1
    Prodigy 20 points

    Hello Brigitte and Quinn

    Thanks for suggestions.

    We are using the same design for multiple products and this is for 1.5A load. Hence we put OCP as 3A.

    With a closer look, we observed VCC was not going higher than 3.5V. Hence we replaced the IC then it started working.

    We did not try changing the SLOPE cap as the device failed to work in the entire input range.

    Thanks,

    Thomas Joseph