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LM34936: Design Review

Nagendra Prasad
Prodigy 30 points
Part Number: LM34936
Other Parts Discussed in Thread: LM5176,

Tool/software:

Hi Team,

Greeting.

We have used the LM34936R in our design. 

Kindly requesting for the design review. 

Thankyou,

Nagendra Prasad [Engineer - Electronics]

Nash Tech Labs.

  • +1
    TI__Guru 58641 points

    Hi Nagendra,

    sure we can review the schematic. Please share it together with the filled out LM5176 Design Calculator to this thread? This would be the most efficient workflow for us to do a schematic review.
     

    Best regards,

     Stefan

  • +1 in reply to Stefan Schauer
    Prodigy 30 points

    Hi Stefan

    Good day.

    Thank you for quic reply.

    Can we have this review through mail! As it will much easy and efficient way to work.

    Thank you.

    Nagendra Prasad

  • +1 in reply to Nagendra Prasad
    TI__Guru 58641 points

    Hi Nagendra,

    the most efficient way and fastest way is the E2E forum as this gets tracked and maintained daily by the team - so e.g. even if I am out of office you would get a reply

    If you worried about putting your data in the forum, you can send this via private message (hover the mouse over my name and select private message.

    In this case please also drop a note here that you have sent a private message.

    Best regards,

     Stefan

  • +1 in reply to Stefan Schauer
    Prodigy 30 points

    Hi Stefan,

    Please find the Design and Design Calculator. Kindly check and let me know your comments.

    Link - LM34936R Design

    Thankyou.

    Regards,

    Nagendra Prasad

  • 0 in reply to Nagendra Prasad
    TI__Guru 58641 points

    Hi Nagendra,

     

    (In below list items which should be checked again have been marked bold) (Note: other info are left here to give an overview of what has been checked)

    Quickstart Calculator:

    • Slope compensation capacitor is much lower then recommended
    • Full load at lowest VIN will not be possible as the losses are getting very high

      

    Schematic:

    • Series Resistor with 2k at VISNS (required if Vin > 40V but recommended for all conditions as VIN can also go higher during bursts, only LM5176)
    • BIAS connected
      If BIAS is not used connect to GND or VIN (do not keep open) - BIAS max = 40V !
    • MODE: allowed Resistor
      (LM34936 only 93k1 or 200k or Vcc)
    • Cap at VCC
      Cap at VCC large enough - check ratio to Boot Caps (at least 5)
    • Cap at EN/UVLO
      Adding a Cap to the low side resistor of the EN/UVLO can help to avoid ripple in startup esp. of very low startup voltage is selected
    • Snubber on SW1 and SW2  (or Schottky diode )
      Place Footprint for a snubber at SW1 and SW2
       (they can then be populated in case needed (e.g. due to EMI) without layout change)
      Note: Snubber Resistor connected to GND not Inductor for better Thermal performance
    • Snubber connected to PGND not to Sense Resistor
      -> do not connect to Sense resistor to avoid snubber current injection into Sense signal
    • Add Series Resistor into MOSFET Gate signals lines
      (they can then be replaced in case needed (e.g. due to EMI) without layout change,
      additional option: add a diode in parallel for slow on and fast off.
    • Gate resistors are <=5 Ohm
    • Voltage rating of MOSFET
      (Have a margin of 30% is recommended)
    • Use Kelvin connection for Current Sense (CS/CSG) to LM5176
    • UVLO setting relative to lowest input voltage 
    • Select the suggested capacitor for slope compensation. Preferable next lower instead of higher value (= more slope compensation)
    • Please check phase margin over the full range (recommend > 60 Degree)

     

    Layout checklist (for you reference only )

    (In below list items which should be checked again have been marked bold)

    • Vcc Cap close to pins
    • AGND and PGND connected at Thermal pad - yes
    • Via's in Thermal pad: Thermal Pad could get more via's to better distribute power loss of the LM5176
    • AGND island
        -> a AGND power area help to avoid disturbances injected into the AGND related components
    • Kelvin connection to CS/CSG sense resistor -> CSG should have dedicated wire to sense resistor not via ground plane
    • Kelvin connection to ISNS+/- sense resistor (shorted to GND if not used)
    • Small current loop for HDRV1/SW1 (area enclose by this wires should be as small as possible - best to have both on top of each other)
    • Small current loop for HDRV2/SW2 (area enclose by this wires should be as small as possible - best to have both on top of each other)
    • Input caps close to MOSFET
    • Output caps close to MOSFET
    • FB divider close to LM517x

     

     

    PDF link:
    HTML links:
    Additional info on layout can be found here :

    (1) Four-switch buck-boost layout tip No. 1: identifying the critical parts for layout

    (2) Four-switch buck-boost layout tip No. 2: optimizing hot loops in the power stage

    (3) Four-switch buck-boost layout tip No. 3: separating differential sense lines from power planes

    (4) Four-switch buck-boost layout tip No. 4: routing gate-drive and return paths

     

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 30 points

    Hi Stefan,

    Thank you for the your comments. 

    let me check and comeback. If any hiccups will let you know.

    Regards,

    Nagendra Prasad

  • 0 in reply to Stefan Schauer
    Prodigy 30 points

    Hi Stefan,

    Greeting.

    We have implemented all the hardware modification shared previously, also now there is update in current requirement.

    Can you kindly confirm whether the design is compatible?

    New requirements:

    Input Voltage – 12V to 24V

    Input Current – 28A @ 24V & 56A @ 12V

     

    Required Output Voltage 12V

    Required Output Current – 24A (Typ) & 48A (Max - only in certain load condition).

    Please find the design calculation sheet - LM5176 Buck-Boost Quickstart Tool Calculations R01.xlsm

    Note: As per the calculation sheet we have considered the efficiency to be 85%.

    Thankyou.

    Regards,

    Nagendra Prasad

  • 0 in reply to Nagendra Prasad
    TI__Guru 58641 points

    Hi Nagendra,

    as you see from the calculation sheet the recommended sense resistor is 1.7mOhm while you selected 3mOhm. So controller might detect overcurrent already to early.

    I am also concerned about the losses. This show already losses of > 10 Watt for higher currents which would be a temp increase of 200 Degree.

    For lower input voltages even more.

    Same for the inductor loss.

    For that output power level you either would need active cooling or a multiphase design.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 30 points

    Hi Stefan,

     

    This 48A current is not continuous current, It will react this stage only for couple of seconds(<1min).

    The steady current is only 24A, while checking in this condition the losses are minimal (around 2W), PFA -LM5176 Buck-Boost Quickstart Tool Calculations R02.xlsm

    For thermal design, the IC and its respective components (Inductor & Mosfet) are placed close to fan.

    Kindly suggest on this update.

    Thankyou.

    Regards,

    Nagendra Prasad

     

  • 0 in reply to Nagendra Prasad
    TI__Guru 58641 points

    Hi Nagendra,

    thanks for this update.
    I suggest to check carefully on the real hardware.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 30 points

    Hi Stefan

    Please find the schematics - LM3496 Circuit.JPG

    Kindly provide your feedback.

    Thankyou.

    Regards,

    Nagendra Prasad

  • 0 in reply to Nagendra Prasad
    TI__Guru 58641 points

    Hi Nagendra,

    can you please list what changes you have done since the first review.

    And esp. address what you have done/changed based on the firs review.

    Thanks,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 30 points

    Hi Stefan.

    Please find the below table:

    Query:

    1. Will the controller able to support for 48A for couple of seconds(<1min)??

    2. Our design Typical Current is 24A, where as IC max capacity is 25A. are there any critical points which needs to be considered??

     

    Thankyou.

    Regards,

    Nagendra Prasad

  • 0 in reply to Nagendra Prasad
    TI__Guru 58641 points

    Hi Nagendra,

    FYI, The schematic can not be accessed anymore - link expired

    From the change list and the last recommendation following items seems not to be reflected:

    • Snubber connected to PGND not to Sense Resistor
      -> do not connect to Sense resistor to avoid snubber current injection into Sense signal
    • Slope compensation capacitor is much lower then recommended

    To your questions:

    1. output power: this is not limited by the controller but by the external components, e.g. inductor, MOSFETs,.... and also the thermal conduction of the PCB.

    Best regards,

     Stefan