L7981 datasheet(29/44 Pages) STMICROELECTRONICS

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L7981

Application informations

Doc ID 15182 Rev 3

29/44

of heat. The RthJA measured on the demonstration board described in the following

paragraph is about 60°C/W for the VFQFPN package and about 40°C/W for the HSOP

package.

Figure 16.

Switching losses

6.6

Layout considerations

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

injected in high impedance nodes and interferences generated by the high switching current

loops.

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

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

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

as short as possible.

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

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

from the high current paths. To reduce the pick up noise the resistor divider has to 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 17 a layout example is shown.

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