OPA1622: why thermal pad must connected to VEE(negative supply)

Hi Andy,

The OPA1622, like many of the TI operational amplifier products, use semiconductor processes that incorporate junction isolation to isolate the various transistors and passive components from each other that reside together on the same common silicon substrate. Basically, the component structures are set up to be isolated using reversed biased PN junctions. I show an illustration that I made for a TI blog several years ago showing two separate NPN transistors, electrically isolated from each other by reverse biased PN junctions.

Connecting the die substrate to the most negative supply, Vs- assures that the device to device isolation is maintained, and unexpected parasitic structures such as latches (4-layer devices) are not created.

As a rule we don't usually test out op amps with the thermal pad at any other level besides Vs-. Not biasing the thermal pad as required could lead to unexpected operating conditions in, or even damage to the die. On rare occasions where we have left the thermal pad floating we have observed that some devices continue to operate pretty normally, but we haven't characterized their electrical performances in that case. Connecting the thermal pad to ground when Vs- is connected to a negative supply would forward bias the PN isolation diodes and completely disrupt the op amp operation or even damage the die.

Regards, Thomas

Precision Amplifiers Applications Engineering

Hi Thomas,

thanks  for your feedback, so per my assume , the internal circuit of OPA1622 does not connected thermal pad to VS- together, however  , we can done this by connected the thermal pad and VS- together externally in PCB layout,  which to prevent the reverse biased PN junctions not in forward state , keep the internal transistors isolated from each others , is my understanding correct?