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LED driver with PNP BJTs and Zener diode

Andy Yan
Prodigy 65 points
Other Parts Discussed in Thread: TINA-TI

Hi Everybody,

There is an automotive lamp project, and I find some stuff about the LED driver design. The following picture is the schematic for the application. 

There are 5 pcs 0.5W white LEDs for the reverse lamp in the 12V cars application.  

In my opinion, if taking away components marked with black boxes, this circuit can make constant current function.

The voltage VM1 & VM2 can be caculated by following:

E node voltage of PNP(T1) VE = VIN(input) - 3.3V(VZ1) + VR5 + VEB    (1)

Resistors (R1/R2/R3/R4) voltage drop VR = VIN - VE                                 (2)

According equation (1) and (2), we can get VR = 3.3V - VR5 - VEB.

LED current I = VR / R = (3.3V -VR5 - VEB) / R.

From TINA-TI simulation resaults, input voltage chages form 10V ~ 16V, VR5 is almost 200mV and VEB is 650mV. And LED current I = 2.45 / R.

My question is what is the function of the circuit labled in the black boxes and these added components(1 PNP and 3 diodes) are used for what?

Do me a favor. Thank you.

Best Regards~

Andy

  • 0
    TI__Mastermind 25500 points

    Hello Andy,

    I am not sure what the intent of that circuit is exactly, perhaps some sort of soft-start? In either case as far as I can tell (without knowing the initial intent) it won’t do anything until the cap charges beyond Vbe, then it pulls up on the regulating node to about ½ of the Zener voltage but RC decays as the caps charge, then it’s out of the circuit. I don't know the exact timing of everything but it seems like any effect it would have would come a bit late if it is for soft-start. So I could be wrong but I don't believe they are required to regulate current.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Prodigy 65 points
    Hi Clinton,

    Thanks for your kind help. This reply is useful for me and I will do some test according your answer.
    Anyway, thank you and Leonard Ellis. TI E2E community is very nice.

    Best Regards

    Andy
  • 0 in reply to Clinton Jensen
    Prodigy 65 points

    Hi Clint,

    It's been a long time since you answered this question for me. Thanks for your help.

    And now I am sorry for the mistake in the shematic. The C3 CAP should be a PTC thermistor so that the circuite labled in the black boxes implemented temperature protection.

  • 0 in reply to Andy Yan
    TI__Guru 71526 points

    Hi Andy,

    Glad you were able to find out about the PTC.  I'm just wondering, where did this circuit come from, and is it in production now?  What is the function in the vehicle for this circuit?

    Regards,

    ~Leonard~

     

  • 0 in reply to LeonardEllis
    Prodigy 65 points
    Hi Leonard,

    Thank you. This circuit came from a car's tail lamp production. I got the lamp and then reviewed the driver solution.
    You know the total power of the tail lamp is usually not big (< 5W), in this case we use linear circuit to driver LEDs and then EMC test will be easy and the production cost is cheap. I find out many tail lamps (such as STOP lamp, Turn lamp, Backup lamp) use the discrete components to implement const current function.


    Best Regards,

    Andy