Objective:

Understand device dissipation and properly determine heat sinking requirements.

Reading Assignment:

To develop a better general understanding of thermal conditions, tests, and characteristics read AN-1192 section 4.0.

Additional Resources:

• For more information on heat sinking and power dissipation refer to Application note in LM3886 page 12-18.    
• For another reference on power dissipation refer to the application note in LM4890 page 18.

Research Assignment:

• Using the application information from AN-1192, locate and use the equations for maximum power dissipation (P_DMAX), calculate (P_DMAX) for a mono single ended amplifier if V_DD=5V and R_L=8 Ohms (nearest mW). check answer   
• Using the same conditions in question 1 determine the P_DMAX for a mono BTL-mode amplifier. check answer

Graph 1  

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

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

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

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

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

Graph 2		Graph 3

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

Program Coordinator/Professor

Kevin Hoskins has been a Staff Applications engineer since 1984. He wields his soldering iron at National Semiconductor Corporation's Audio Products Group in Santa Clara, California.  He has also performed application duties with the Data Acquisition Products Group at National Semiconductor Corporation also in Santa Clara, California.

Quiz:

Take a quiz to check your understanding of this subject. Click here.
Answer all questions correctly to receive a certificate from Dean Bob Pease.