• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 47 subscribers
  • Views 5845 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Constant current LED driver with Opamp

Faisal Rehman
Prodigy 180 points

Other Parts Discussed in Thread: TLV4110, OPA547, TINA-TI, OPA350

Hello,

I want to design an opamp based constant current driver to drive an infrared led. The maximum current of led is 200mA and forward voltage is 1.8V.

Can someone guide me which opamp should I select with atleast 200mA output constant current, low input/output offest voltage, low noise, and single supply operation?

Thanks,

Faisal

  • 0
    TI__Guru* 93105 points

    Hi Faisal,

    You didn't mention the supply voltage you would be using, but if it is something on the order of 2.7 to 5.5 V take a look at the TLV4110. It is a CMOS, single-supply operational amplifier that can sink/source up to 300 mA. The data sheet can be viewed at:

    http://www.ti.com/product/tlv4110

    If your supply voltage is higher, 8 to 60 V, then you may want to consider the OPA547:

    http://www.ti.com/product/opa547

    You can configure either amplifier as an Improved Howland Current source with the current set current for 200 mA.

    Regards, Thomas

    PA - Linear Applications Engineering

     

     

     

     

  • 0 in reply to Thomas Kuehl
    Prodigy 180 points

    Hi Thomas,

    Thanks for your suggestions.

    The input voltage is in the range of 1 to 5.5 V.

    I was looking at the Laser driver circuit provided in the application report SBEA001-Sept 2001 Optoelectronics circuit collection by Mr. Neil Albaugh using Howland current pump.

    I have simulated the circuit given in Figure5 of above report with TINA-TI software with slight changes; replacing the R4 with a potentiometer of 10K to change the current    

    through the resistor (0-200mA) as well as the Shunt resistor of 500m to acheive a maximum output current of 200mA. I am a bit confused with the circuit, I checked the datasheet of OPA350 and there the max output current is 40mA.

    How can we acheive a 200mA or may be more from this opamp which has a  max output current limit of 40mA? Is the rest of current provided by the transistor. Can you please help me understand how a Howland current pump works and how we acheive a constant current?

    I checked for transistor FTZ869 but could not find its spice model. So i just replaced with a simple NPN transistor 2n2222 with max output current of 800mA. Is there any  other transistor for this application.

    Meanwhile I simulated the same circuit with TLV4110 but it does not work for some reason. 

    8865.OPA350 LED driver.TSC

     

  • 0 in reply to Faisal Rehman
    TI__Guru* 93105 points

    Hi Faisal,

    A complete discussion of the Improved Howland Current Pump is covered in TI Application Report, SNOA474 (AN-1515), A Comprehensive Study of the Howland Current Pump. That should answer most any question you may have about the fundamentals behind its operation. I have attached that application report for your convenience.

    The OPA350 is only called upon to provide the base current for the FTZ869 transistor in Neil’s circuit. Therefore, it is only sourcing a few milliamperes at most. The FTZ869 emitter provides the LED current. TINA does not have a model for the FTZ869. That series of transistors was produced by Zetex but appears to be out of production. The 2N2222 may work, but that is towards the high end of its current capability and power dissipation. The 2N4401 has higher current capacity and power dissipation, and may be a good substitute for the Zetex device.

    The simulation circuit you sent doesn't converge in its present arrangement. The input is being driven by a step function that is initially zero. It appears that TINA is having difficulty resolving the operating point in that condition. If you try a Transient Analysis and change the selection to Zero Initial Values, the circuit will converge but the setup doesn't result in 200 mA current.

    If you replace the ac generator with a dc source having a voltage level set at a mid-supply voltage of 1.35 V and change R2 and R4 (the potentiometer) to 6.04 K, the current pump will provide a dc current of approximately 200 mA. A potentiometer can be placed at R4 as discussed in the applications report.

    The TLV4110 that I suggested will provide 200 mA output current alleviating the need for the driver transistor.

    Regards, Thomas

    PA – Linear Applications Engineering

    Howland Current Source.pdf