TJ=TA+Rtℎ,JA⋅PTOT

(32)

where TA is the ambient temperature and PTOT is the sum of the power losses just seen. RthJA is the equivalent

thermal resistance junction-to-ambient of the device.

It can be calculated as the parallel of many paths of heat conduction from the junction to the ambient. For this

device the path through the exposed pad is the one conducting the largest amount of heat. The RthJA, measured

on the demonstration board described in the following paragraph, is about 40 °C/W for the HTTSOP16 package.

6.6

Layout considerations

The PCB layout of the switching DC/DC regulators minimizes 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 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 interference

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 (1 μF or higher) must 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 junction to ambient thermal

resistance; so a wide ground plane enhances the thermal performance of the converter, allowing the high power

conversion.

A7987

Layout considerations

DS12928 - Rev 3

page 22/36