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LM5117EVAL: Chip damaged only for load connecting with output voltage change

Gu-Do Lee
Prodigy 20 points
Part Number: LM5117EVAL
Other Parts Discussed in Thread: LM5117

Hello,

I am attempting to modify LM5117EVAL board to generate 20V output with 9A current capacity.

Using the LM5117 Quick Start Calculator, I found that the only R16 (357Ohm) should be changed to 208Ohm resistor.

However, the resulting circuit gives strange behavior as follows

1) Without connecting any load, the board gives 20V output as expected,

2) But as soon as connecting any load (such as 1KOhm resistor), the output drops to a few mili volts and chip may be damaged

3) Once damaged, the board does not give any output voltage even without any load.

Before modifying the value of R16, I tested the original board of 12V/9A and the board operates well as expected.

(The only change is the value of R16 based on the quick start calculator)

Since the board does not operated as soon as load connected, I cannot find any clue to fix the problem.

Pleas give me some advice or comment on this strange board behavior.

Thanks!

  • +1
    TI__Mastermind 49647 points

    Hi Gu-Do,

    Please check your design with the LM5117 quickstart calculator. In particular, note that the abs max rating of VCC is 15V, so it should not be driven from the output. Also, the output electrolytic and ceramics caps are all only rated for 16V, so you need to change to 25V caps.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 20 points

    Thank you, Timothy!

    I disconnected the VCC path driven from the output and the circuit operates well just after input power is applied.

    Also the capacitors are already replaced with proper voltage ratings.

    But after tens of seconds, the output voltage fluctuates and eventually drops to nearly zero voltage.

    According to your comments, I rechecked the quick start calculator and all components values.

    Can the un-soldered bottom pad (to GND) cause this kind of problems?

    Currently due to the chip damage, I replaced the chip without bottom soldering. 

  • +1 in reply to Gu-Do Lee
    TI__Mastermind 49647 points

    Yes, it's recommended to solder the DAP to GND for optimal thermal performance. Also, check the FETs as they are likely rated for 20V gate drive and may have had an overvoltage condition when the high VCC voltage was applied.

    Regards,

    Tim