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LMR51635: LMR51635 overheating at 48 V input, AP2114 LDO also heating

sabith ch
Prodigy 60 points
Part Number: LMR51635
Other Parts Discussed in Thread: LM65635, LM76005

Tool/software:

Hello,

I am using LMR51635XFDDCR in a buck converter design. The circuit is intended to convert 48 V down to 5 V, and then I use an AP2114HA-3.3TRG1 LDO to generate 3.3 V.

At VIN = 24 V, the circuit works normally and provides 5 V at the output.
But when I apply 48 V input, the LMR51635 IC becomes very hot within seconds (even with no external load). The AP2114 LDO also heats up when regulating 5 V to 3.3 V, despite no load being connected.

Measurements / Observations

  • VIN (IC pin): 48 V

  • EN: tied to VIN (~48 V)

  • VOUT: ~5 V regulated

  • IC package temperature: rises quickly (too hot to touch)

  • LDO input = 5 V, output = 3.3 V, but LDO heats up at no load

Circuit Details (schematic attached below):

4370.PCB - Power Board.pdf

Questions for TI team:

  1. Is there any known issue or special consideration when operating the LMR51635 at VIN = 48 V? (datasheet says up to 65 V).

  2. Could my inductor or capacitor selection cause this excessive heating?

  3. Are there layout requirements (bootstrap cap placement, CIN placement, GND routing) that could lead to heating if not followed closely?

  4. Why would the AP2114 LDO heat up at no load when supplied from 5 V?

Any advice on debugging or confirming whether this is a component selection issue, layout issue, or IC damage would be very helpful.

Thanks,

  • 0
    TI__Mastermind 31165 points

    Hi Sabith,

    Are you saying that when VIN = 48V, the device is still regulating at 5Vout, or is there no regulation? 

    At no-load, FPWM trim, the device is still conducting as if it is in CCM, so that is why you may still notice the IC heating up at no load. If the AP2114 LDO is close to the LMR51635, the heat may conduct over to the LDO as well. 

    The inductor or capacitor usually is not the culprit of this heating. You may want to add ground vias that fan out from the ePAD to dissipate heat to other parts of the board. 

    I think what you're seeing is just the typical heating of the IC. Per the LMR51635 calculator, at 48V to 5Vout at 400kHz, we should see the case temperature around 96C, assuming a 4-layer board. If it is using fewer layers, the case temperature can be higher. 

    A resolution would be to use a heat sink or alternative forms of cooling.

    Thanks,

    Richard  

  • 0 in reply to Richard Fu22
    Prodigy 60 points

    Hi Richard,

    Thank you for the clarification.

    Yes—when VIN = 48 V, the device is still regulating to 5 Vout, but the IC package temperature is rising significantly.

    You mentioned that in a 4-layer board, the calculator estimates around 96°C case temperature at 48 V → 5 V @ 400 kHz. Since our board is only 2 layers, the thermal performance will be worse, and we will be facing even higher case temperatures.

    Also, in our design the LMR51635 and AP2114 LDO are placed about 5 cm apart, so we don’t believe the heating of the LDO is purely due to thermal conduction. The temperature rise we are observing seems too high to be acceptable for our application.

    Could you please suggest an alternative buck IC that can step down 48 V to 5 V but operates with better thermal performance under no-load/light-load conditions? Ideally, one that is more efficient and runs cooler in this VIN/VOUT range.

    Thanks again for your support.

    Best regards,
    Sabith

  • +1 in reply to sabith ch
    TI__Mastermind 31165 points

    Hi Sabith,

    You may try looking at the LM65635 or LM76005. Please also examine for PFM light-load operation as well, as that will improve efficiency at light load. The thermal performance is better on this device, though it is not p2p compatible with the LMR51635. 

    However, the reality is that even if you switch the IC, you may still see higher temperature rise with a two layer board. We strongly recommend using a 4-layer PCB for power circuit designs in order to meet thermal concerns. 

    Thank you,

    Richard 

  • +1 in reply to Richard Fu22
    Prodigy 60 points

    Hello Richard,

    Thank you for your guidance and for suggesting the alternative parts. That clarifies my concern, and my issue is resolved now.

    Best regards,
    Sabith