L7986TA datasheet(29/43 Pages) STMICROELECTRONICS

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L7986TA

Application information

Doc ID 022098 Rev 3

29/43

Figure 15.

Switching losses

6.6

Layout considerations

The PC board layout of the switching DC/DC regulator is very important to minimize the

noise injected in high impedance nodes and interference generated by the high switching

current loops.

In a step-down converter the input loop (including the input capacitor, the power MOSFET

and the freewheeling diode) is the most critical one. This is due to the fact that the high

value pulsed currents are flowing through it. In order to minimize the EMI, this loop must be

as short as possible.

The feedback pin (FB) connection to the external resistor divider is a high impedance node,

so the interferences can be minimized by placing the routing of the feedback node as far as

possible from the high current paths. To reduce the pick-up noise, the resistor divider must

be placed very close to the device.

To filter the high frequency noise, a small bypass capacitor (220 nF - 1 µF) can be added as

close as possible to the input voltage pin of the device.

Thanks to the exposed pad of the device, the ground plane helps to reduce the thermal

resistance junction to ambient; so a large ground plane enhances the thermal performance

of the converter allowing high power conversion.

In Figure 16 a layout example is shown.

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