AN2846 datasheet(19/35 Pages) STMICROELECTRONICS

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AN2846

Thermal dissipation calculation

Doc ID 15130 Rev 1

19/35

Thermal dissipation calculation

2.1

Forward inputs polarity case

In reference to the application schematic defined in Figure 31, the dissipated power into the

SCLT3-8 PSCLT can be calculated as following:

Consider the worst case where all inputs are connected to 30 V.

PSCLT = P1 - P2

where:

P1 is the total power delivered by the supplies

P2 is the total power dissipated by the external components

P1 = VCC · (IC + IDD + 8 · ILIM)

External components are: RIN, RC, LEDs, regulator load

Power dissipated by input resistors = 8 · RIN ·ILIM

Power dissipated by supply resistor = RC · (IC + IDD)2

Power dissipated by LEDs = 8 · VLED · ILED

Power supplied by the VDD linear regulator = VREG · IDD

The PSCLT = 560 mW

Assuming:

VCC = 30 V, ILIM = 2.35 mA, RIN = 2.2 kΩ, RC = 1.0 kΩ, VLED = 2 V, VDD = 5 V, IDD = 7MA.

Note:

The current flowing through the LED is almost the same as ILIMIT. The difference is about

125 µA used to bias the input circuit device.

With the above mentioned conditions and using copper area of 1cm2 as heat sink, the

SCLT3-8 junction temperature will be around 120 °C with an ambient temperature of 85 °C.

Figure 28 below shows the RTH_JA variations versus the heat sink area on a 35 µm FR4

epoxy single side board.

Figure 28.

RTH_JA versus copper area

HTSSOP38

100

120

100

120

140

160

180

(°C/W)

th(j-a)

SCLT

(mm²)

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