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OPA548 -TECHNICAL ISSUE (HEAT PROBLEM....?)

span vinay
Prodigy 30 points
Other Parts Discussed in Thread: OPA548

In our company we are using OPA548 for programmable power supply application purpose.We have used your Digitally-Controlled Programmable Power Supply schematics for our application refer to page number 15 of OPA548 datasheet .Whatever mentioned in that schematics ,used all IC and capacitor except resistor(used 10k and 4.12k instead of 9k,1k respectively for a gain 3.4)  .

                 We are facing problem with OPA548 IC , IC gets very much heated after connecting to the load then it goes to the thermal shutdown within  40 second for both higher current and lower current.Without load there is not much heat in the IC.We provided heat sink to the OPA548 IC ,tried with big heat sink but still same problem.I have attached the readings taken for OPA548 output voltage.Kindly find the attachement.Can you suggest a solution for this problem.....? 

6332.OPA548_OUTPUT.txt 
For DAC, supply voltage is 5v.
Reference voltage to DAC is set to -5v.
For OPA548, supply voltage is 15v.
OPA548 gain is set to 3.42.

*Buffer output and Analog output taken without connecting to the load
*Measured the values in multimeter.s
*Readings taken when current digital value is set to zero and varying the voltage digital value.

S.NO   Digital	Buffer output  	Analog  	
			
1.     4096       4.09/2.92		13.40
2.	 2048	      2.4			 8.35
3.	 1024	      1.2			 4.17
4.	  512	      0.6			 2.08
5.	  256		0.3			 1.03
6.	  128       0.15			 0.58
7.	   64		0.075			 0.53
8.       32       0.037			 0.51
9. 	   16		0.016			 0.50
10.       8		0.011			 0.50
11.       4		0.011			 0.50
12.       2		0.011			 0.50
13.       1		0.011			 0.50
14.       0		0.011			 0.50


*Readings taken when current digital value is set to max and varying the voltage digital value.


S.NO   Digital	Buffer output  	Analog  	
			
1.     4096       4.09/3.6		13.16
2.	 2048	      2.4			 8.35
3.	 1024	      1.2			 4.17
4.	  512	      0.6			 2.08
5.	  256		0.3			 1.03
6.	  128       0.15			 0.62			 
7.	   64		0.075			 0.56
8.       32       0.037			 0.54
9. 	   16		0.016			 0.54
10.       8		0.011			 0.54
11.       4		0.011			 0.54
12.       2		0.011			 0.54
13.       1		0.011			 0.54
14.       0		0.004			 0.54

********************************************************
For 12v ,digital value is 2950
For 11.87v ,digital value is 2900

  • 0
    Guru 21975 points

    Span,

    Please provide information on the power supply voltages, output voltage and output current under the condition when the device overheats. This is to calculate the internal power dissipation during this condition.

    You may find this application note helpful:  

    http://focus.ti.com/general/docs/techdocsabstract.tsp?abstractName=sboa022

    Regards, Bruce.

     

     

     

  • 0 in reply to Bruce Trump
    Prodigy 30 points

    Hi Bruce,

    For DAC, supply voltage is 5v.
    Reference voltage to DAC is set to -5v.
    For OPA548, supply voltage is 15v.
    OPA548 gain is set to 3.42.

    *Buffer output and Analog output taken without connecting to the load
    *Measured the values in multimeter.
    *Readings taken when current digital value is set to zero and varying the voltage digital value.

    S.NO   Digital    Buffer output      Analog     
               
    1.     4096              4.09                  13.40
    2.     2048              2.4                       8.35
    3.     1024              1.2                       4.17
    4.      512               0.6                       2.08
    5.      256               0.3                       1.03
    6.      128               0.15                     0.58
    7.       64                0.075                   0.53
    8.       32                0.037                   0.51
    9.        16               0.016                   0.50
    10.       8                0.011                   0.50
    11.       4                0.011                   0.50
    12.       2                0.011                   0.50
    13.       1                0.011                  0.50
    14.       0                0.011                  0.50


    *Readings taken when current digital value is set to max and varying the voltage digital value.


    S.NO   Digital    Buffer output      Analog     
               
    1.     4096              4.09                 13.16
    2.     2048              2.4                     8.35
    3.     1024              1.2                     4.17
    4.      512               0.6                     2.08
    5.      256               0.3                     1.03
    6.      128               0.15                   0.62             
    7.       64                0.075                 0.56
    8.       32                0.037                 0.54
    9.        16               0.016                 0.54
    10.       8                0.011                 0.54
    11.       4                0.011                 0.54
    12.       2                0.011                 0.54
    13.       1                0.011                 0.54
    14.       0                0.004                 0.54

    ********************************************************

     

    With load ,i connected 8ohm resistor across the output of OPA548 then after some time device got heated very much and went to thermal shutdown for both higher current and lower current.

  • 0 in reply to span vinay
    Guru 21975 points

    It appears that the circuit is operating properly.

    I am assuming that the 8 ohm load is connected from the output of the OPA548 to ground. The maximum power dissipation in the OPA548 will occur when the load voltage is 7.5V, (one half the supply voltage). At this output voltage, the power dissipation in the OPA548 is equal to the power dissipation in the load, approximately 7 watts. You have not described your heat sink but at this power dissipation, it will require a sizable heat sink to adequately cool the power amplifier. I'm only guessing, but a piece of aluminum approximately 10cm x 10cm x 2mm may be a minimum size for room temperature ambient operation. The op amp will still feel quite warm.

    Read the application note for further insight.

    Regards, Bruce.