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LM675: Supply voltage and output swing clarifications

Nikhil Kulkarni87
Prodigy 40 points

Part Number: LM675

Hi Experts,

The datasheet of LM675 says 'Absolute Max Supply Voltage' as +/-30V. Does this mean that in single supply operations, I can use a supply of maximum 60V?

Also, the output voltage swing is mentioned as typical +/-21V. Does this mean that in single supply operations, I can go up to 42V output if my previous question is true?

Thanks

Nikhil

  • 0
    TI__Guru* 93105 points
    Hi Nikhil,

    Regarding your questions:

    The datasheet of LM675 says 'Absolute Max Supply Voltage' as +/-30V. Does this mean that in single supply operations, I can use a supply of maximum 60V?

    Yes.

    Also, the output voltage swing is mentioned as typical +/-21V. Does this mean that in single supply operations, I can go up to 42V output if my previous question is true?

    The +/-21 V output swing is listed for +/-25 V supplies (RL = 8 Ω). If the supply is changed to a single +60 V source, the +/-21 V swing is referenced to a mid supply voltage of +30 V. The output swing would be +51 V on the high end, and +9 V on the low end with the load connected to +30 V.

    I suggest keeping the LM675 supply voltage as low as possible to minimize power dissipation.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to Thomas Kuehl
    Prodigy 40 points

    Hi Thomas,

    Thanks for the clarifications.

    My application needs a single power supply and output voltage of 30V DC. So I am planning to use a supply of +34V that I have. My application will draw a max of 0.8A current from output.

    Is there any quick calculator to check whether the power dissipation in LM675 will be within safe limits? How much heat sinking will be required and other thermal calculations?

    Thanks

    Nikhil

  • 0 in reply to Nikhil Kulkarni87
    TI__Guru* 93105 points

    Hello Mikhil,

    Unfortunately, the LM675 datasheet does not include safe operating area (SOA) curves. When available they make determining safe operating conditions more easily determinable. However, the datasheet Absolute Maximum Ratings provides a maximum Power Dissipation of 30W (assumes TA equal to 70°C. For operation at higher tab temperatures, the LM675 must be derated based on a maximum junction temperature of 150°C). That at least bounds the power dissipation.

    Since you are using a single +34 V supply I suspect that the load will actually be connected from the output to ground. In the case of an output voltage of +30 V the voltage across the LM675 output transistor that will be sourcing the 0.7 Amp current, there will be 4 V across the transistor. Thus, the power dissipated by the transistor will be 4 V x 0.7 A, or 2.8 Watts. That is well below the LM675 30 W maximum.

    Even if the worst case dc power dissipation is applied where Vo is one-half the supply voltage and one-half the supply voltage appears across the output transistor sourcing the current the power dissipation is 17 V x 0.7 A, or about 12 Watts. Therefore, if the POWER DISSIPATION AND HEAT SINKING instructions on datasheet Pg. 8 are followed there shouldn't be any issues.

    Do note that the Output Voltage Swing in the Electrical Characteristics table lists for RL = 8Ω a typical output swing of ±21 V, and tested of ±18 (min) V. The tested limit equates to about 2.25 Amps, and the swing limit is 7 V from the specified +/-25 V supplies. The 2.25 Amps is greater than 3x the 0.7 Amps your circuit will source. I doubt if there will be an issue swinging within 4 V of the +34 V supply you intend to use, but do be aware there is the possibility that some devices may not be able to swing as high as +30 V. If you want to add a couple of volts of supply margin that would help assure that the output gets to the +30 V level when loaded.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    TI__Genius 11135 points
    Mikhil

    We haven't heard back from you so we assume this answered your questions. If you need additional help just post another reply below.

    Thanks
    Dennis