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TLV9102: Voltage-to-current (V-I) converter circuit with BJT

Milan Parekh
Prodigy 20 points
Part Number: TLV9102

Hello Sir,

Hope this message finds you well,

I have few questions mentioned below, it will be appreciated if you provide your inputs here.

I am using below Voltage-to-current (V-I) converter circuit with BJT,

https://www.ti.com/lit/an/sboa325/sboa325.pdf?ts=1703735825123&ref_url=https%253A%252F%252Fwww.google.com%252F

My application is  ViMin = 0V , ViMax = 10V ,  IoMin= 0A  ,  IoMax = 600mA 

We are getting the output as expected with below configuration and same BJT 2N5686.

Please find attached simulation file.Section_AB_combined. 1.TSC

Could you please help us to understand,

1)Why in the your application selected BJT is 2N5686(Which has Icmax = 50A) and output required is Icmax = 1A?

2)Is there any relation of VBE vs Ic? 

3)We are planning to use https://www.onsemi.com/pdf/datasheet/2n3019-d.pdf this 1A BJT because of size concern. is it workable solution? 

4)How to select R3, C1 and R4 of your application? is there any equations or you can share any application notes?

Kindly guide us, your earlies response will be appreciated.

  • 0
    TI__Mastermind 21750 points

    Hi Milan,

    Hope you are having a good day. 

    Milan Parekh said:
    1)Why in the your application selected BJT is 2N5686(Which has Icmax = 50A) and output required is Icmax = 1A?

    I didn't work on this application but I have a few ideas why they chose the 2N5686.

    1. They had this model in the library.
    2. This part can easily handle 1A. Having a part that can handle the 
    3. They wanted a part that would be easy for someone to be able to easily increase the amount of current needed to be on the output. 

    Milan Parekh said:
    2)Is there any relation of VBE vs Ic? 

    I'm not a transistor expert, but if I'm remembering correctly, Ic = Is*(e^(Vbe/Vt)) in the forward bias region. Is there a reason you are wanting to know the relationship? 

    Milan Parekh said:
    3)We are planning to use https://www.onsemi.com/pdf/datasheet/2n3019-d.pdf this 1A BJT because of size concern. is it workable solution? 

    I'd be a little worried about having the transistor operating at it's maximum current. You want to have some tolerance in the design. Passive component values can be 20% of their rated values, input signal might be a little larger than what was expected, etc. 

    Milan Parekh said:
    4)How to select R3, C1 and R4 of your application? is there any equations or you can share any application notes?

    We have material that covers this in detail! You will want to look into the Riso + Dual Feedback architecture. I'd take a look at the TI Precision Lab Video: Isolation Resistor for more details on how to calculate these values. 

    Let me know if you have any additional questions. 

    Best Regards, 

    Robert Clifton