AN2141

Reducing power dissipation on the chip

7/18

Figure 3.

General configuration

When only one LED is connected to each output (the most common configuration used in

LED array reference boards) the equation can be simplified as follows:

Equation 3

series.

If a PWM signal controls the brightness of the LEDs, the total power dissipation on the chip

is calculated as:

Equation 4

where D is the duty cycle.

If there is no possibility to obtain the proper value of the LED supply voltage in an

application, an external resistor R in series with each LED must be connected to limit the

power dissipation on the chip as shown in Figure 4. The power dissipation on the chip is

consequently decreased as follows:

Equation 5

STP16CPS05

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P

tot

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c

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–

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–

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IR

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–

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#outputs