Application informations

L5980

26/37

5.5

Thermal considerations

The thermal design is important to prevent the thermal shutdown of device if junction

temperature goes above 150°C. The three different sources of losses within the device are:

a)

equal to:

Equation 26

higher to compensate the losses of the regulator. So the conduction losses increases

compared with the ideal case.

b)

switching losses due to power MOSFET turn ON and OFF; these can be

calculated as:

Equation 27

and the current flowing into it during turn ON and turn OFF phases, as shown in Figure 16.

switching time is 50ns.

c)

Quiescent current losses, calculated as:

Equation 28

Equation 29

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

P

ON

R

DSON

I

OUT

()

2

D

⋅⋅

=

P

SW

V

IN

I

OUT

T

RISE

T

FALL

+

()

2

------------------------------------------- Fsw

⋅⋅

⋅

V

IN

I

OUT

T

SW

F

SW

⋅⋅

⋅

==

P

Q

V

IN

I

Q

⋅

=

T

J

T

A

Rth

JA

P

TOT

⋅

+

=