AN3031 datasheet(21/28 Pages) STMICROELECTRONICS

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AN3031

Thermal management

Doc ID 16144 Rev 1

21/28

Considering the above application structure and the following conditions:

●

VCC = 30 V

●

VI_MAX = 30 V

●

IIN_MAX = 2.7 mA

●

VIN_MAX = 24 V

●

fCLK = 1.3 MHz

●

IC = 6 mA

●

VC = 24 V

●

VF = 1.9 V

●

VDD = 5 V

●

ID = 4.5 mA

●

RC = 1 kΩ

●

RI = 2.2 kΩ

the overall chip power dissipation is:

Equation 8

PD = 8 x [(VIN – VF) x IIN] + VC x IC – VDD x IDD = 8 x [(24V – 1.9V) x 2.7mA] + 24V x 6mA – 5V x 4.5 A = 599mW

In case of a maximum allowed ambient temperature equal to 100 °C, the thermal resistance

must be lower than:

Equation 9

Since the application RTH value of the HTSSOP38 is 80 °C/W, the SCLT3-8BT8 works

correctly at this ambient temperature.

In the final application we must consider other cases also. One example is spurious

connection of one channel to negative voltage.

In this negative biasing of the input, the current (IREV) flows through the protection diode.

The voltage (VREV) is then equal to the forward voltage of the protection diode, which is

approximately 0.7 V at 16 mA.

All of the other inputs work correctly.

The total power dissipation in this case can be calculated by with the equation:

Equation 10

THJAMAX

ΔT

–

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

150

° C 100° C

–

0.599W

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

° CW

⁄

REV

–

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

0.7V

–

2200

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

13mA

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