AN4149 datasheet(37/43 Pages) STMICROELECTRONICS

37 / 43 page

DocID023523 Rev 2

37/43

AN4149

Layout hints

The layout of any converter is a very important phase in the design process which is

sometimes neglected by the designers. Even if the layout phase sometimes looks to be

time-consuming, a good layout undoubtedly saves time during the functional debugging and

the qualification phases. Additionally, a power supply circuit with a correct layout needs

smaller EMI filters or less filter stages, allowing consistent cost saving.The L4984D does not

need any special attention to the layout, just requiring that the general layout rules for any

power converter be carefully applied. Basic rules are listed here below, using the EVL4984-

350W PCB layout as a reference (Figure 23).

–

Keep the power and signal RTN separated. Connect the return pins of the

components carrying high current such as the input filter, sense resistors or output

capacitors as close as possible. This point is the RTN star point. A downstream

converter must be connected to this return point.

–

Minimize the length of the traces relevant to the boost inductor, MOSFET's drain,

boost rectifier and the output capacitor.

–

Keep signal components as close as possible to each relevant pin of the L4984D.

Specifically, components and traces relevant to the error amplifier have to be

placed far from traces and connections carrying signals with high dV/dt, such as

the MOSFET's drain.

–

Connect heatsinks to power GND.

–

Add an external shield to the boost inductor and connect it to power GND.

–

Please connect the RTN of the signal components including the feedback,

PFC_OK and MULT dividers close to pin 8 (GND) of the L4984D.

–

Connect a ceramic capacitor (100÷470 nF) close to pin #10 (VCC) and to pin #8

(GND) of the L4984D. Connect this point to the RTN star point (see rule 1).

Figure 22. EVL4984-350W PCB layout (SMT side view)

AM13320v1

AN4149 Datasheet(PDF) 37 Page - STMicroelectronics