TPS92682-Q1: Flickering of the LED on PWM ramp up

HI Andreas,

The PWM duty cycle you are talking about is it the GATE signal or the PWM input signal to the TPS92682.  You are using CV mode to drive and PWM current to the LED's or is there a CC mode controller after the CV mode to drive the current.  

Thanks Tuan

Hi Tuan,

Thanks for your answer. We are using an externel PWM Driver on PWM Input on the TPS92682. There is in CV and we drive LEDs with series resistors. This is one of our use cases.

The mode depends on the load device.

BR

Andreas

Hi Andreas,

Without knowing all the register settings and your schematic along with operating condition it's hard for me to comment.  If my understanding is correct, when first start up you increase the PWM Dimming input from 0% to 100% duty cycle slowly to the TPS92682 you would get a flicker and then the second ramping of PWM input from 0% to 100% and after you do not get any flickering.  Is that correct?  If it is then my first guess is for you to increase the Soft-Start time in register 06 to max as a test.

If not then please have the register setting send to me along with schematic and operating conditions.  Please also include scope captures and more description on what you are seeing.

Thanks Tuan

Hi Tuan,

yes your understanding is correct. The Soft-Start time was also our first thought, but there was no improvement. 

Below you will find our schematic from the TPS92682 as in SEPIC topic. 

At the initialization we will set the register:

    // BASIC Init
    TPS92682_Write(spiChannel, RESET, 0xC3);
    TPS92682_Write(spiChannel, ENABLE, 0xB0);
    TPS92682_Read(spiChannel, FLT1);
    TPS92682_Read(spiChannel, FLT2);
    TPS92682_Write(spiChannel, CFG1, 0x01);
    TPS92682_Write(spiChannel, OV, 0b01010101); // ChxOV: 1,125 (threashold 12.5%)
    TPS92682_Write(spiChannel, SOFTSTART, 0b11011101); // SSDIV factor: 64
    TPS92682_Write(spiChannel, FM, 0b00110100); // FMMAG: 15% / FMFREQ: 1 / 2048
    TPS92682_Write(spiChannel, FEN1, 0b00111100); // UV: disabled / RFEN: disabled
    TPS92682_Write(spiChannel, FLATEN, 0x00); // all disabled
    TPS92682_Write(spiChannel, ISLOPE, 0b00010001); // 50mV Slope
    TPS92682_Write(spiChannel, SWDIV, 0x00); // DIV: 2
    TPS92682_Write(spiChannel, ILIM, 0b10100000); // 16 Err till trig /  CHxILIM: 75mV

For Testing with an LED and series resistor, we change to following:

CV-Mode, CH1ADJ-Register to 0x3E (10V),

We increase the PWM via a ramp or external slider from 0% to 100% Duty Cycle. 

The LED ramp up a little bit and then goes out. When we start again, it will works. 

Thanks Andreas

Hi Andreas,

Please give me a few days to review this and I will get back to you.

Thanks Tuan