TPS543C20A: What is the differences between AGND and GND?

Hello,

I would follow the recommendations in section 11.2 of the datasheet.

AGND is short for analog ground and GND is typically used as a ground return for the power traces of the converter. The reason why we recommend two separate ground pours is to have AGND as a "quiet" ground and have the GND as the "noisy" ground, which is the ground return for the power stage traces. For pins such as VSEL, RT, MODE, and RAMP on the TPS543C20A, the device uses a ~11uA current source to apply a current to each of the resistors and then reads the resulting voltages in order to latch the operating parameters associated with each of these pins. Given the need for accuracy, it is recommended that the ground return for these pins be as quiet as possible, which is why we demonstrate those pins connected to AGND in section 11.2. It is also recommended to have a quiet ground in the form of an AGND pour to provide a quiet ground reference for the device's control circuitry for the most accurate regulation. In order to ensure GND and AGND are at the same potential, we recommend connecting the ground pours at a single point under the thermal pad of the device. 

Regards,

Alec Biesterfeld

Kazuhiko Hirano 

In Power Electronic Circuits, ground typically performs two mutually exclusive functions.

1) An infinite current sink

2) A zero-voltage reference

Due to parasitics in electrical connections, these two functions don't works well together.  A "ground" that is sinking high dynamic currents does not maintain an accurate zero-voltage reference.  In order to provide the two features of "ground" we separate them between "GND" (Ground) or "PGND" (Power Ground) for an infinite current sink and "AGND" (Analog Ground) for a zero-voltage reference.

Separating these grounds and controlling the connections between them helps prevent the high dynamic currents of the switching power-stage from affecting the accuracy of the zero-voltage reference AGND.  They need to be connected by the PCB layout since control signals from the zero-voltage reference AGND need to control power devices on the infinite current sink GND, but separate names allows designers to be selective and careful about those connections.

As Alec Biesterfeld mentions, the programming resistors ILIM, SS, VSEL, RT, MODE, and RAMP should all be terminated to AGND and AGND should be connected to GND at the thermal pad.