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TL072: TL072 LED Driver stability issues

Jordan Nedev
Prodigy 10 points
Part Number: TL072

Hi team,

I’m currently designing op amp based 16bit LED driver for controlling arrays of 14 high CRI LEDs.

I build the schematic shown below on pcb, but I have some issues with stability. I try a lot of different values for the capacitor and resistor in the feedback circuit. The last that I try was C1 = 47n, R7 = 100R, R8 = 10k. With those values the ouput current is more or less stable. The circuit is very slow and don’t react fast on changes of set current. Always need a time to settle on the set point. With small cap in he control loop the circuit is very unstable and start to oscillating. The simulation results are very unclear. The control source is 16bit DAC. From the input side for sure there are no problems.

Linear Driver Tina TI file

If you have suggestions how to improve it please share.

Thanks in advance.

Best regards.

Yordan Nedev

  • 0
    Guru 140200 points

    Hi Jordan,

    OP2 is introducing too much time delay and by this too much phase lag which cannot be fully compensated. By the way, do not only look at the phase of OP1 but also OP2. You can easily improve the phase margin of OP1, but not of OP2.

    Kai

  • 0
    TI__Guru**** 150916 points

    Hello Jordan,

    Not only is there a stability concern with OP2, there is a  DC accuracy concern. The full temperature input offset voltage can be as high as 13mV. The 'noise' gain of OP2 circuit is 3.9k/0.1k+1 = 40. That is a error up to 40*13mV = +/-520mV. Then add in another +/-13mV for OPA1

    I suggest using a more accurate op amp for OP1. Remove OP2. Add a resistive divider to lower IN+ voltage as needed. Also consider increasing sense resistor resistance (improves accuracy, but decreases headroom and increases power dissipated in resistor, decreases power for T1)