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AN3981

Common application information

Doc ID 022217 Rev 1

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Figure 46.

Maximum power dissipation vs. ambient temperature for 24-bit LED

drivers (TQFP-48)

Example 1: Applications with a 16-bit LED driver (e.g. traffic signs), one amber LED (60 mA,

2 V-2.2 V forward voltage, depending on current and temperature) per channel.

Assuming an LED voltage rail of 3.3 V, the voltage drop across current generators can be

considered around 1.2 V.

The power dissipation, including only the main contribution of the current generators, can be

calculated as:

Equation 15

Once the power dissipation has been estimated, the ambient conditions must be taken into

account. A large LED panel can produce significant heat and can contribute to a

considerable increase of the temperature inside the system.

Figure 47 shows, once the maximum power dissipation is fixed, which ambient temperature

range can be covered according to the package.

The TSSOP-24 and the QSOP-24 are surely the least suitable for this application. So the

choice is oriented towards the HTSSOP-24 (the best, thanks to the exposed pad) or to the

SO-24 (if a lower maximum ambient temperature can be accepted).

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DISS

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