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  • TI Thinks Resolved

XTR106: External transistor

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Replies: 21

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Part Number: XTR106

The recommended transistor packages are all too big/too tall for my device.  Is there a substitute in surface mount package I can use?  The smaller the better as I have space constrain.  Please also educate me what specifications of external transistor I need to watch out for and why.

  • Hi Jason,

    keep in mind that this transistor has to dissipate lots of power. What is your loop power supply voltage?

    Also read section "External Transistor" of datasheet of XTR106.

    Kai

  • Hello Jason,

    The XTR106 datasheet advises that the external transistor have a VCEO = 45V min, β = 40 min and PD = 800mW. Power dissipation requirements may be lower if the loop power supply voltage is less than 36V. Since the external transistor is inside a feedback loop its characteristics are not critical. When using a surface mount package such as an SOT-223, make sure your board layout can support the heat of the power being dissipated. You can refer to this app note for more information. This reference design for the XTR116 uses the FCX690BTA in SOT-89 package which meets the requirements above. 

    Best,

    Katlynne Jones 

    Best,

    Katlynne Jones

  • My loop power is generally 24 Vdc.  I want to use SOT-89 package because of size constrain.  I'm investigating the suggested app note AN-1028 by Katlynne Jones.  I was going to put the transistor right next to the XTR106.  Is there any suggestion on how to keep power dissipated?  

  • In reply to Jason Chang4:

    Hi Jason,

    Are you looking for suggestions on how to dissipate the heat? The output stage with the external transistor for the XTR106 is going to look the same as the output stage of the reference deign for the XTR116 that I linked previously. Page 12 of the Design Guide shows the board layout. Q1 is the transistor we are talking about. They have the collector pins of Q1 connected to a small copper fill on the top and bottom layers of the board, which will act as a heat sink. 

    Q1 is used on these XTR devices to to avoid on-chip thermal-induced errors. As you can see in the layout, the XTR (U1) and Q1 are relatively close, but the ambient temperature changes from the heat produced by Q1 can still affect the XTR106, so you can place Q1 further away to minimize these effects. 

    Best,

    Katlynne Jones

    Best,

    Katlynne Jones

  • In reply to Katlynne Jones:

    This is very helpful, thank you so much.

    What is the smallest package I can use?  What's the gain bandwidth fT, maximum DC collector current I should choose for a smaller package?

  • In reply to Jason Chang4:

    Hi Jason,

    using a transistor providing the smallest package is no good idea. 24V x 20mA = 0.48W in the short circuit case is quite a lot...

    Kai

  • In reply to kai klaas69:

    Hi Kai,

    Can you elaborate more about your concern on 24V x 20mA = 0.48W in SOT-89 package.  What package would you recommend?  I have a very limited space.

  • In reply to Jason Chang4:

    Hi Jason,

    Kai is correct that you don't want to use the smallest package possible. You would probably want to go for the largest package that your design allows. The external transistor is important to the accuracy of the XTR106. In smaller packages without the proper heat sink, 0.48W is going to cause the transistor to get hot which will introduce the thermal-induced errors to the XTR106 that I mentioned before. For this reason, power dissipation is going to be a main concern. When you select a transistor, check the datasheet to see if it has any recommendations for the layout in order to meet its specified power rating. 

    To answer your earlier questions, the current output of the system will be 4-20mA. You need the external transistor to be able to provide that + some margin. This plot on page 5 of the XTR106 datasheet shows the bandwidth:

    Depending on the gain, the BW will be in the kHZ range. Most transistors have a higher fT than this in the MHz range, and will not limit the frequency response of the system. Most transistors can provide more than 20mA of current as well. I believe that is why these specific requirements weren't mentioned for the selection of the external transistor.  As long as you choose a transistor that meets the voltage, current, and power requirements, in addition to properly heat sinking the transistor, then you should be alright. I don't have any data that would support giving you an answer to what the smallest package you can use is. However, you can take confidence in the part selection and heat sinking of the reference design I shared with the SOT-89 package. 

    Best,

    Katlynne Jones

    Best,

    Katlynne Jones

  • In reply to Katlynne Jones:

    Hi Katlynne,

    Can you point me which pin on the transistor needs to provide more than 20mA?  Is it emitter or DC collector current spec I need to watch out for?  Does that means I must have .01uF cap at the V+ and Io?  

    I used mouser to filter out all of the spec I understand

    www.mouser.com/.../N-ax1sh

  • In reply to Jason Chang4:

    Hi Jason.

    In datasheets and Mouser, this value would be the maximum collector current. It looks like the lowest Mouser even has as an option to filter is 50mA which is sufficient.

    The 0.01uF capacitor is a recommended supply bypass capacitor. 

    Best,

    Katlynne Jones

    Best,

    Katlynne Jones

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