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AN2141

LED array reference design board

STMicroelectronics 

AN2141

Figure 3. General configuration

STP16CPS05

Reducing power dissipation on the chip

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

Ptot = Ic ⋅ Uc – #outputs ⋅ I ⋅ (Vc – VF)

As previously stated, only the LED supply voltage Vc is variable. Thus the correct choice is

an LED supply voltage Vc about 0.5 V higher than VF or n*VF for n LEDs connected in

series.

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

is calculated as:

Equation 4

Ptot = Ic ⋅ Uc – #outputs ⋅ I ⋅ (Vc – VF) ⋅ D

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

Ptot = Ic ⋅ Uc – #output⋅s I ⋅ (Vc – (VF – I ⋅ R))

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