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LM5176: Droop dropout with 35% current draw

THOMAS CARNEY
Prodigy 10 points
Part Number: LM5176

  • Number: CS0733924
    Contact: THOMAS CARNEY
    First name: THOMAS
    Last name: CARNEY
    Short description: Droop dropout with 35% current draw
    Email: thomasc@charontech.com
    Company: CTL-Systemware
    Company phone:
    Language: English
    State: Open
    Provide case details or comments:

    We have been using a design for a +17.5V 7A max buck-boost regulator with a 9-36 V input.   So far it has been working.   We have an additional requirement now with MIL STD 1275E compatibility.   Before we get to the regulator, we have a Surge Protection Board that handles all the high voltage inputs associated with MIL STD 1275E and keeps the input voltage to this regulator at +34V maximum.   At no point whatsoever do we exceed +36V input.   So far we're handling issues with MIL STD 1275E with the Surge Protection Board and this regulator with ONE exception:   the power droop during starter surges.

    First let's get the overall picture of the regulator:

    Voltage input          +9 - +36 VDC

    Output                   +17.5 V at 7 A maximum

    Temp                     +65 deg C maximum

    CCM & Hiccup        ON

    Dither                   OFF

    Soft Start              2 msec

    Switching freq       340.229 kHz

    Used Selected FETS

    The ONLY difference in the parts specified in the design used are Cboot1 and Cboot2.    The initial TI design specifies the capacitors to be 100nF 1mOhm ESR.   The caps used are 100nF X7R with between 10 and 40 mOhms ESR.    Is that 1 mOhm a typo?   That's very low for a 0.1uF cap.

    That's the background.   The problem we're having is with a droop that is specified in MIL STD 1275E Section 5.1.2.2 and Figure 6 Lower curve.   The initial drop from +20V to +12V is knocking out the regulator for up to 90 msec.   

    What I have attached are files where I have dropped the input voltage from +20V to +12V and in some cases from +16V to +10V so we are mostly in the boost mode.   I have tested where I had it at +32V and dropped it to +20V in 1 msec with a 2A current draw and there was no problem.   Attached are scans off the scope.

     

Journal type:
20V_to_12V_2_1A.jpg
207.9 KB

  • CS0733924 Created

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Case details

Number
CS0733924
State
Work in progress
Short description of your case
Droop dropout with 35% current draw
Updated

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Subject
Droop dropout with 35% current draw
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Attachments

  • GREATER_1A_LOAD.jpg (747 KB)

  • 20V_to_12V_2_1A.jpg (207.9 KB)

  • 16_5V_TO_12V_2_2A.jpg (223.6 KB)

  • Curve.png (97.5 KB)

  • 0
    TI__Guru* 78060 points

    Hi Thomas,

    Thank you for designing with the LM5176.  However, I cannot open your attached files.  Neiither the pdf nor the png file can be opened.   From your description, it sounds like your circuit hit the peak current limit.  Please double check that.  The LM5176 is a buck and boost controller, by theory it should maintain the output voltage over your entire input voltage range unless the current limit is tripped.  Once current limit is hit, the duty cycle would be truncated, and the output voltage would reduce.  If this is the case, you may need to reduce the sense resistor, and adjust the loop compensation accordingly.  Please refer to our design calculator to readjust the loop compensation.

    https://www.ti.com/lit/zip/snvc208

    Thanks,

    Youhao Xi, Applications Engineering

  • 0 in reply to Youhao Xi
    Prodigy 10 points

    I tried several times to upload files and your site would not let me.   I will check to see if we are exceeding current limit.

  • 0 in reply to THOMAS CARNEY
    TI__Guru* 78060 points

    Sorry to hear.  It may be related to some online safety concerns.  Please check the current limit, and let us know if the issue can be resolved by reducing the sense resistance.