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WEBENCH® Tools/LM5088: No Output/Low Output 1.8V

GOVINDARAJ KRISHNAN
Prodigy 130 points
Part Number: LM5088

Tool/software: WEBENCH® Design Tools

I am designing a Buck converter with following specs

Input Voltage min 44V

Input max 52V

Output voltage 12V

Switching frequency 818KHz

Here are the design values 

Rt = 6.2K

Inductor 4.7uH

Rsense 8mOhm

Cramp 220pF

Rramp 158K

Cout 180uF

Input cap 10uF

Vcc cap 1uF

softstart cap 1uF

Rfb1 = 1.62K

Rfb2 = 14.7K

Crestart cap = 0.02uF

Rcomp = 100K

Ccomp = 560pF

Chf = 6.8pF

Problem

When switched ON, gives output voltage of 1.8V. I have no clue why its happening. Same design worked in Webench. 

Measured voltage at VCC 7.7V

Voltage at Rt 1.93V

Enable 2.2V

Output waveform

 Signal at mosfet gate

  • 0
    TI__Mastermind 26590 points
    Hello Govindara,

    What is your required output current? Please can you share your schematic? Thank you.

    David.
  • 0 in reply to David Baba
    Prodigy 130 points

    Dear David

    Thanks for your comment. Output current designed for 5A. The scope i shared above is at no load. I tried connecting 470 Ohm at output, but response remains same.

  • 0 in reply to GOVINDARAJ KRISHNAN
    Prodigy 130 points
    Dear David

    Few more observation that might help to solve the problem

    VCC 7.7V
    Changed SS cap to 0.02uF
    Changed Compensation loop based on excel calculator Rcomp 52k , Ccomp 1200pF , Chf 22pF
    Changed boot cap 0.02uF and 0.47uF,

    Above changes doesn't help, response is still same.

    LM 5088 not generating gate pulse (based on above scope shows only ~6KHz, it should be around 800KHz)

    Thanks
  • 0 in reply to GOVINDARAJ KRISHNAN
    TI__Mastermind 26590 points
    Can you change Rcomp to 20k and Ccomp to 3.3nF and CHF to 100pF.

    If this does not help, please can you take a scope capture of Vswitch and inductor ripple current?

    Also it might be a good idea to get the part numbers for the following.

    1. Indcutor?
    2.Sync Rect Diode?


    Also it maybe a good idea to check the layout of your design?

    Thanks.
  • 0 in reply to David Baba
    Prodigy 130 points

    Dear David,

    Thanks a lot for the reply. I will share the scope soon.

    1) Inductor part number  Digikey: SRP1245A-4R7MCT-ND

    2) Diode Part number Digikey: V40PW10C-M3/IGICT-ND

    I have Three buck converter on a board like 48 to 5V, 48 to 12V, 48 to 24V. All the buck converters shows same responses. Did some debugging today results are as follows

    1) Added bulk cap at input - No improvement

    2) Changed Cboot to 0.01uF - Ouptut increased to 6V (48V input) and 8V (at 24V input)

    3) Changed Cramp to 50K, 100k, 190K - No improvement

    4) Increased/Decreased VCC cap 2.2uF/0.1uF - No improvement

    5) Changed inductance to 6uH - No improvement

    6) Changed Compensation network Rcomp 50K, Ccomp 1200pF, Chf 22pF - Helped a little output improved to 8V. Noticed Mosfet gate switching at 790KHz.

    Please find below the layout it comprises of 48V to 24V Conversion and 48V to 12V conversion.

    Thanks 

  • 0 in reply to GOVINDARAJ KRISHNAN
    TI__Mastermind 26590 points

    lm5088 Flour Plan.pdfThanks you for your response GOVINDARAJ,

    I am having trouble locating components on the layout.  Can you please refer to attached and confirm you have followed the layout guideline there in?    Please pay close attached to the Cin cap and the power FET/Diode.  Also please see additional comment relating to current sense.

    If the board is not laid as as described, it is likely the cause of the issue, having ruled out Stability, install errors etc.

    Hope this helps?

    David.

  • 0 in reply to David Baba
    Prodigy 130 points

    Dear David

    Thanks for your support.

    I have changed the compensation loop as suggested. Rcomp 20.5K, Ccomp 3.3nF, Chf  120pF, Response remains same. Output voltage 4.6V

    I have validated the component layout. Here are my observation

    1) Cin cap is just below the Mosfet and Diode (On bottom layer)

    2) Gate signal is accompanied with board copper pour.

    3) CS signal is again accompanied with ground plane.

    I am not sure weather it will affect the functionality itself. Please correct me if  i am wrong. The chances of install errors would be minimal because i tried soldering multiple buck converters.

    One more thing i have notices is that whenever i connect 100 Ohm load output goes OFF. 

    Please find below the scope data

    Output voltage at 48Vin. 500uS timebase

    Scope at Mosfet's source 5uS timebase

    Scope at Mosfet Gate 5us timebase

    Please suggest way to proceed.

    Thanks

  • 0 in reply to GOVINDARAJ KRISHNAN
    Prodigy 130 points
    Dear David

    I have tried debugging. Removing VCC Cap helps. Now all the buck conversions are happening at no load. Whenever i tried to connect 100 Ohm Load (Varied load from 20 to 1K Ohm) even at 1K voltage drops (2V). Output drops down to few volts around 2 to 4V in all the buck converters.

    I am not sure why its happening?

    Thanks
  • 0 in reply to GOVINDARAJ KRISHNAN
    Prodigy 130 points

    Dear David

    Sample scope date for 48V to 12V Converter, but its output voltage is 14.4V. (All other converters are showing similar data)

    Output Voltage Scope

    Scope at Mosfet's source

    Scope at Gate

    Problem remains same. Its not taking the load. Please suggest the potential possible cause for this issue. 

    Thanks

  • 0 in reply to GOVINDARAJ KRISHNAN
    TI__Mastermind 26590 points
    The Waveforms do not look right to me.

    Looking at the layout more closely Assuming that pin one is top right on the bttom layer view) I believe this is the cause of your issues due to the following.

    1. Can you confirm on your layout that you have a diff pair routed from CS to rsense + and CSG to Rsense -? CSG should not be Grounded at the IC, but Grounded at Rsense - (minus).
    2. CVCC (C38)should not be removed! Need to make sure VCC is close to its respective pin. I see you have a Via to VCC?
    3. Cboot (C37) need to be placed nect to Vsw and Boot pin. Not done in layout.
    4. Hgate (gate drive) and Vsw from the device to the Gate and Source of the FET need to be routed differentially.
    5. Vsw sits right on top of the device on the other side of the board, which will make the device susceptible to noise
    6. FB Resistors and FB trace to pin 8 routed far away.

    I am sorry to say, there is a large difference between the recomneded layout and what has been implemented here in.

    This will need to change.

    Hope this helps?

    David.