PowerWise® Design University

Audio Amplifier Thermal Considerations

Research Assignement Answers

   
Q:

Using the application information from AN-1192, locate and use the equations for maximum power dissipation (PDMAX), calculate (PDMAX) for a mono single ended amplifier if VDD=5V and RL=8 Ohms (nearest mW).

A:

(VDD)2/(2pi2RL)=
                          =(52)/(2pi28)
                          =25/158
                          =158mW
See equation in section 4.2 of AN 1192, page 2.

   
Q:

Using the same conditions in question 1 determine the PDMAX for a mono BTL-mode amplifier.

A:

(4)(VDD)2/(2pi2RL)=
                              =(4)(52)/(2pi28)
                              =100/158
                              =633mW
See equation in section 4.2 of AN 1192, page 3.

   
Q:

Why is it important to know the maximum power dissipation when trying to reduce thermal resistance?

A:

The maximum power dissipation value is required to determine the proper heat sink for an application. See LM3886 datasheet, page 14.

   
Q:

Determine the minimum heat sink area required for the following LM4890 application:

Conditions: VDD=3.3V
                  BTL Connection
                  RL = 8Ohms
                 10 pin LD package
                 TA = 120 degrees C

A:

120mm2
See graphs in LM4890 datasheet, pages 13 and 14.

Explanation:  Power derating curves are used to ensure that the device's maximum junction temperature (TJMAX) is not exceeded (See Graph 1).  The curves usually require knowledge of two out of three variables:  PDMAX, Maximum Ambient Temperature, and heat sink size.  The unknown variable is determined using the two known variables. 

Note:  The measured device dissipation (PDMAX) is slightly higher than the calculated value because of quiescent supply current.  This can be illustrated by comparing a calculated PDMAX with the PDMAX shown in a datasheet graph.

For example:
You want to determine the maximum ambient operating temperature for our LM4890, 5V, BTL amplifier.

Conditions:  VDD = 5V
                     BTL connection
                     RL = 8 Ohms
                     Heat sink = 120mm2
                     Package = 10pin LD
PDMAX:
                    Calculated = 633mW
                    From Graph = 670mW (See Graph 2)

Answer: As graph 3 illustrates, the maximum ambient temperature = 80 degrees C


  • Why is it important to know the maximum power dissipation when trying to reduce thermal resistance?        
  • Determine the minimum heat sink area required for the following LM4890 application: 
    • Conditions: VDD=3.3V
                        BTL Connection
                        RL = 8Ohms
                       10 pin LD package
                       TA = 120 degrees C