LM27761: lm27761 multple device failure

Hi Leo,

Could you please share the schematic and layout for me to review?

Regards,

Febin

Dear Febin,

I have another issue with LM27761. Below is the original link with the schematic and layout.

https://e2e.ti.com/support/power-management/f/196/t/933418?LM27761-lm27761-Load-issue

Thank you.

Leo

Dear Febin,

In addition to my question, may I know if what are the possible issues why LM27761 is giving low voltage(0.14V) when in fact the input voltage is stable at 5V? I am sure the load issue is less likely to happen because I have only around 20-30mA load current from the sensor. Actually, I have experimented with the thermal pad using a good one and found out that when I place 1.5k ohms between the thermal pad and Ground of LM27761 then the output becomes  0.14V which is the same level as the faulty one. Do you have any explanation for that observation? Is it because of the low pass filter wherein the device is more likely sensitive that prevents from operating normally at lower current? Or is it because of the layout or external component values?. Appreciate your technical explanation from the device perspective. We actually have 3 out of 10 failures on the LM27761 regulator.

Thank you.

Leo

Hi Leo,

-Referring to the schematic and layout in the previous post; I mentioned already that this device needs all the external capacitors to function properly including Cin.

-Please share some measurement plots for VIN, Vout, CPOUT, Cfly, Iout. Please keep the test conditions same at all times. Please measure close to the LM27761 IC (across capacitors).

-Looking at the Layout, I would recommend to double check the Vin of the IC. It is not a proper layouting technique. If the trace or routing is longer then parasitics are added leading to unstable & lower Vin. This could be just voltage spikes/dips causing the IC to fail. If you can share a complete schematic and layout, I will have a better understanding.

-Is there any specific reason to use such low value resistors?

Regards,

Febin

Dear Febin,

-Nothing special with the schematic and the layout. It is just a four-layer pcb wherein LM27761 is placed on the bottom side. Please check below. 

-To answer your comment. Vin (pin1) of the device is actually connected to the TOP layer through VIA as indicated in the layout.

(-1.5k ohms) is just to simulate the effect of impedance when in any chance the device PAD does not follow the recommended layout connection to signal GND.  

- For the low-value resistors if you mean R1 and R2 combination then can you recommend some range of values that we can use to get -5V output?

- I am not able to get a plot from the oscilloscope but with DMM I can measure Vin=5V, Vout=0.14V or lower, CPOUT= low, Cfly =0.9V, Iout= 0.01mA

-Enable pin connected to GPIO (always high)

Thank you.

Leo

Hi Leo,

Have you used an input capacitor for LM27761? I cannot spot this in the layout/ schematic.

I require all the oscilloscope plots to understand the behavior of the device. Simply measuring using a DMM is not helpful to investigate and find the root cause.

Regards,

Febin

Hi Leo,

Do you have some updates to share?

Regards,

Febin