A5975AD datasheet(29/50 Pages) STMICROELECTRONICS

29 / 50 page

A5975AD

Application information

Doc ID 018761 Rev 1

29/50

The overall losses are:

Equation 26

The junction temperature of the device is:

Equation 27

Equation 28

8.3.2

Thermal impedance ZthJ-A(t)

The thermal impedance of the system, considered as the device in HSO8 package soldered

on the application board, takes on an important rule when the maximum output power is

limited by the static thermal performance and not by the electrical performance of the

device. Therefore, the embedded power elements could manage an higher current but the

system is already taking away the maximum power generated by the internal losses.

In case the output power increases, the thermal shutdown is triggered because the junction

temperature triggers the designed thermal shutdown threshold.

The RTH is a static parameter of the package; it sets the maximum power loss which can be

generated from the system given the operation conditions.

If we suppose, as an example, TA = 40 °C, 140 °C is the maximum operating temperature

before triggering the thermal shutdown and RTH = 40 °C/W, therefore, the maximum power

loss achievable with the thermal performance of the system is:

Figure 15 represents the estimation of power losses for different output voltages at VIN=5 V

and TAMB=40 °C. The calculations are performed considering the RDS(on) of the power

element equal to 0.4 A.

TOT

DS on

()

OUT

()

⋅

OUT

⋅

⋅⋅

⋅

0.4 2

0.3

12 2 70 10

–

500 10

12510

–

⋅⋅

⋅

⋅⋅

1.38W

≅

thJA

TOT

⋅

1.38 42

128

°C

≅

⋅

MAX DC

ΔT

-----------

J MAX

AMB

–

--------------------------------------

100

----------

2.5W

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