LM5116: IC heating and Vcc fails

Hi Pradeep Would you please check these items on-by-one ? 

• Please check how the GND node and the GNDPWR node are connected each other.  
• Please check how the EP is connected to ground
• At which input voltage the problem happens ? 
• Please check the voltage rating of D5 and let me know the part number 

Eric Lee (Applications Engineer)

Hi Eric, Thanks for your timely reply.

• GND and GNDPWR are connected by a net tie on the bottom copper plane ( around 1mm to 1.5mm copper track)
• I'm afraid EP is not connected to ground , please can you let me know how this affects thermally or electrically too?
• The input voltage we tested was 18V to 20V .
• It's 100V ,part number is 771-BAS16J135 .

Hi Pradeep

EP should be connected to ground. The device temperature will be higher than expected if the EP is not connected to a large ground plain.  It also can cause a malfunction when switching noise exists. 

• Any other device is damaged together ? 

Eric Lee (Applications Engineer) 

Okay in the new design I'll try to include this,

No other device was damaged.

Hi Eric

Is there a way to confirm that if an issue is from components used or with PCB?

And how to confirm if an issue is caused by instability ?

Thanks

Hi Pradeep 

Because I haven't heard about any VCC pin failure which is caused the device itself until now and the device has been in production since 2007, I think the issue might be caused by an error (either by a wrong component selection or a wrong connection on PCB). 
Please check these items. 

• Excessive leak current of D5 can over charge the VCC capacitor and damage the VCC pin. Please replace it with a low leak diode.    
• VCC can be unstable if the performance of C18 capacitor is poor. Please try to replace it
• if VCCX is overcharged by accident, the VCC pin can be damaged. Please ground the VCCX pin.  

Thanks
- Eric Lee (Applications Engineering)

Thanks Eric,

We used the same category diode suggested by WEBENCH .

And may I know how to find if an issue is caused by stability or something else?

Hello Pradeep

Since both  VCC and  VCCX are not connected to VOUT, I think there is no possibility that the loop instability causes the VCC pin damage. Just to make sure, I suggest you to populated the unit (whose VCC is 9V) on other working board and  double check if the VCC voltage is still 9V or not. 

 Thanks
- Eric Lee (Applications Engineering)

Hi, I'm afraid we ran out of all IC which we had used during testing but  our order is in transit. We will make this test once we get our components with us. Moreover we are designing new board only for the buck converter. Once the design is done, Can I share the file with you so that you can review and suggest some changes if something exits?

Thanks 

Hello Eric !  I have concern about point 1 .

Leak current of D5 over charge VCC capacitor means diode reverse recovery current causes SW OVERSHOOT and VCC capacitor overcharge ?

Hello Pradeep

No, the reverse recovery current of D5 doesn't cause  SW overshoot or VCC capacitor over charge. If an excessive reverse leakage current of D5 can overcharge the VCC capacitor. 

Thanks,

-Eric Lee (Application Engineer)