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THS6212: Using Linedriver as LED driver

Stefan Thorlacius
Intellectual 570 points
Part Number: THS6212
Other Parts Discussed in Thread: THS6012

Dear Sir or Madame

 

We want to use THS6212 to current control a string of LEDS, we understand that the THS6212 is aimed for a total different application area but we hope it can be used for our application.


The reason the THS6212 is chosen is it is cheap, can drive at least 1 ampere with a slew rate of 3800V/usat 150MHz bandwidth

Our idea is to let the THS6212 drive a string of LED’s to ground via a 0.1 ohm shunt resistor to ground and then feedback the voltage over the shunt to the - input.

In this way we can vary the current through the LED's as we wish by applying a chosen voltage to the + input.

The basic requirements for our application is to control the LEDS in two ways

  1. Continuously feed with up 100mA
  2. 1 Ampere with a 40kHz PWM.

 

Is it possible to drive the THS6212 on 24V (the datasheet says +-12 Volt)?

Is there any other suggestion to archive this LED control (the op amp gives such elegant and simple solution)?

Is there a dedicated LED driver chip that can archive this (which we obvious is not aware of)?

A second solution we been thinking of is a much simpler op feeding a Darlington that drives the LED’s, perhaps it is better?

Regards Stefan

Regards Stefan

  • 0
    Guru 48395 points

    Lot of what you notice on the THS6212 is correct, what is often not stated is these later wireline drivers are internally coupled differential pairs that have to be used in push pull differential driver config. 

    1. Almost sounds like you are trying to do a Howland current pump. But, a lot of these high output high speed parts are CFA, need to add the right Rf in your feedback circuit and account for the diode dynamic impedance in the compensation

    2. If your diode string builds up to require higher supply voltages, look at the some of the very early true dual op amp drivers like the THS6012

    3. Or, you might consider a composite with a VFA front end and high output buffer - 

  • 0 in reply to Michael Steffes
    Intellectual 570 points

    Well, the Howland current pump require very high accurate resistor values or being able to trim it (which is highly undesirable for production)

    Our basic configure is to be able to manoeuvre the current from, 10mA to 1A and being able to pulse it up to 40kHz.

     

    Is it possible to drive the THS6212 with 0 - 24V or does it require – to + 12 ?

    We intended to drive two independent LED strings with THS6212 (because THS6212 is cheap and have two amplifiers inside)

    THS6212 is more expensive and can only drive 500mA.

  • 0 in reply to Stefan Thorlacius
    TI__Genius 15555 points

    Hello Stefan,

    The THS6212 can be used in a single supply configuration of 24V.

    Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    Guru 48395 points

    Stefan, I am seeing two threads for this question - duplicates?

    Again THS6212 is not a single ended output device, only push pull differential - probably not useable and you should close this discussion and start a new one - 

    I thought I had replied yesterday, but not there - Anyway, using an op amp directly to drive 1A is maybe not the best idea, 

    Often folks will add an external power transistor inside the loop and construct a current source to either sink or source current. here is a starting circuit that appears stable and uses a Rohm power transistor - here, you would put your LED string up to a positive supply - or, if you absolutely need them to ground, reverse all this to a PNP drive. This one puts all the power in the external Q and the op amp needs are modest. 

    I have this set for a 100mA to 1.1A square wave, need to be careful to not shut off the Q and open the loop, once you set your diode string, put a shunt R across those that some small current will bypass but not develop enough IR to turn on the LED path. This is a pretty high power Q, but the Rohm models seem pretty good. Imported this one to TINA for you - I cannot go further without a diode model, 

    VFA plus NPN current source.TSC