AN2141 데이터 시트보기 (PDF) - STMicroelectronics

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AN2141

LED array reference design board

STMicroelectronics 

Reducing power dissipation on the chip

3

Reducing power dissipation on the chip

AN2141

One of the most important considerations in this application is the calculation of the

maximum power dissipation on the driver chip. The maximum power consumption can be

calculated with the ambient temperature and the thermal resistance of the chip. The thermal

resistance depends on the type of package and can be found together with the maximum

allowed junction temperature in the datasheet for the device. The maximum allowable power

consumption of this device is calculated as follows:

Equation 1

Pdmax

=

T----j--m----a---x----–----T----a-

Rthja

● Pdmax - maximum power dissipation [W]

● Ta - ambient temperature [°C]

● Tjmax - maximum junction temperature [°C]

● Rthja - thermal resistance junction to ambient [°C/W]

The maximum forward current for each type of LED is provided in the datasheet and must

not be exceeded. Each output channel of the driver operates as a linear current sink. As the

sink current for each output of LED driver is set as constant by an external resistor, the

power dissipation of the chip depends on the value of LED supply voltage (Vc) minus the

forward voltage drop of the LEDs. To optimize the power dissipation of the chip, it is

recommended to use the lowest possible supply voltage for the LEDs. An example of how to

calculate the power dissipation of the chip is shown in Figure 3. The equation for this basic

connection is:

Equation 2

#outputs

∑ Ptot = Ic ⋅ Uc + I (Vc – niVF)

i=1

● Ptot - power dissipation of chip [W]

● Ic - supply current for driver [A]

● Uc - supply voltage for driver [V]

● I - constant LED current set by external resistor [A]

● #outputs - number of outputs

● Vc - LED supply voltage [V]

● ni - number of serial connected LED for each output

● VF - LED forward voltage [V]

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