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Hello Ghazali,
Please provide the following information in order for me to better assist you with this issue:
Best regards,
Ian Williams
Linear Applications Engineer
Precision Analog - Op Amps
Hi lan,
Thanks you for assist.
Below information as per request. Part problem used for U30, which part voltage dropping 2.83V and top surface hot temp.
1. Input = +5VDC and Expected Output = 10VDC (Pin 11 shorted to Pin 12).
2. Failure rate about 9.77% ( 13/133).
Hello Ghazali,
I see you have a 0.1uF capacitor on your PGA205 output. This is likely causing stability issues which could explain the overheating and incorrect DC voltage at the output. Please try removing C61 from your application circuit and test a "bad" part to see if the issue goes away.
Best regards,
Ian Williams
Hi Ian,
For C61, designation cap for filtering the noise. This is a normal design approach and we suppose to not make any changes as this is our Customer design.
Can you explain from the design perspective how this cap can cause IC U30 instability issue and overheating.
Removing the cap still does not resolve the problem. All that we can do now is to change the IC with different date code to make it passed. Please explain what is the difference between these 2 date codes that inhibit different result?
Hello Ghazali,
I understand the desire to filter output noise. However, the 100nF load capacitor violates the maximum stable load capacitance specification from the PGA205 data sheet. The maximum is 1000pF, or 1nF. You have connected 100x that amount.
From a design perspective, this capacitor causes instability by interacting with the output impedance of the output amplifier to create a pole in the PGA's closed-loop response. This pole, along with the dominant pole of the PGA's output amplifier, generates approximately 180° of phase shift, reducing phase margin to nearly 0° and resulting in positive feedback. Positive feedback prevents the amplifier from correcting changes at its inputs, which gives you oscillation, high current draw, and overheating as the output transistors try to drive the oscillation to the load.
For more information on how capacitive loads cause instability, I recommend watching our TI Precision Labs video series on stability, available here.
If you need to meet a certain noise requirement, I can recommend an output filter with enough isolation to prevent stability issues. This usually only involves adding a series resistor at the device output pin. If you do not follow this recommendation, it's very likely that even devices with "good" date codes may have undesirable behavior over temperature or other conditions.
Since you have removed the load capacitor and still see issues, there are several possibilities:
I recommend the following debug steps:
Best regards,
Ian Williams