AN3303

Layout considerations

Doc ID 18164 Rev 2

21/27

6

Layout considerations

The IC is designed with two ground pins, SGND and PGND. SGND is used as the ground

reference for all the internal high precision analog blocks. PGND, on the other hand, is the

ground reference for all the digital blocks, as well as the current return for the gate drivers.

Listed below are the main recommendations that should be taken into account when

designing the PCB:

●

Close the output current loop as short as possible by connecting the SR MOSFET

drains as close as possible to the transformer termination.

●

Route the connection between the two MOSFET drains and transformer terminals

symmetrically to each other.

●

Connect the MOSFET sources close to the output capacitor ground terminals.

●

Route the trace that connects MOSFET sources to the SRK2000 PGND pin as short as

possible and separately from the load current return path.

●

Keep the source terminals of both SR MOSFETs as close as possible to one another.

●

Design the PCB as geometrically symmetrical as possible to help make the circuit

operation as electrically symmetrical as possible.

●

The SGND pin must be directly connected to the PGND pin using a path as short as

possible (under the device body).

●

Connect the drain voltage sensing resistor as physically close to the drain terminals as

possible: any stray inductance involved by the load current that is in the drain-to-source

voltage sensing circuit may significantly alter the current reading, leading to a

premature turn-off of the SR MOSFET.

●

Use bypass ceramic capacitors between Vcc and both SGND and PGND. They should

be located as close to the IC pins as possible. Sometimes, a series resistor (in the ten)

between the converter’s output voltage and the Vcc pin, forming an RC filter along with

the bypass capacitor, is useful to get a cleaner Vcc voltage.

Figure 22.

Board layout