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LP8545 not powering up correctly?

Christopher Allen
Prodigy 40 points
Other Parts Discussed in Thread: LP8545

I am having issues with the LP8545 backlight display driver. The I2C interface is attached and VDDIO is connected to 3.3V. However, I am using it in PWM mode. The I2C interface is not used but the signals are pulled high. On power up the enable and PWM signals are held low. Enable is driven high, we wait 10mS then drive PWM high, wait another 20ms, and then start driving a 20kHz PWM signal to the chip. We have tried other variations on this boot up but seem to have the same results. ISET has been populated with a 23.7k resistor and FSET is populated with a 90.9k resistor. No registers are written or read.

We built 5 boards. 3 of the 5 boards work great. 2 of the 5 show the following behavior. When powered up all 6 lanes turn on after about 1-2 seconds lane one goes out completely, then lane two will drop. Some random time after that all six lanes will dropout for a period of time. By drop out I mean that they stop conducting and the LEDS on the backlight go out. Eventually they will come back on. Sometimes all of them will come back on and other times only a few. Also adjusting the PWM does not effect this at all. It does not even change the brightness of the LEDs that are on.

 

Any help here would be greatly appreciated. I have been working at this for a week and a half. I have done the following.

 

  1. Replaced the device on one of the two boards.
  2. Cut the etches on VDDIO, SDA and SCL. Then tied them directly to ground
  3. Cut and rerouted one of the ground ties to the filter circuit for noise immunity on pin 20 before I realized that it is not being used.
  4. Rerouted the Feedback etch for pin 21 again for immunity to noise.
  5. Adjusted the resistors on ISET and FSET. May be not to the right values but I did change them.

 

It seems like the internal clocks are not starting up properly but I have no way of knowing. I did manually read the fault registers and there is no complaint of over temp, fault LED strings or anything.

 

Thanks

Chris

  • 0
    TI__Genius 11020 points

    Hi Chris,

     

    Default configuration of LP8545 uses VSYNC for reference clock to generate PWM. Since you are not using VSYNC, you should connect VSYNC pin and FILTER pin to GND to avoid internal PLL to locking some external noise etc. LP8545 will then default to internal clock generation (once VSYNC and FILTER are grounded) and startup should work fine.

     

    Thanks.

    Best Regards,

    Tomi Koskela

  • 0 in reply to Tomi Koskela
    Prodigy 40 points

    Thanks Tomi,

    I do have the VSYNC pin tied to ground. The filter pin is tied to the filter circuit. Is it important that the filter pin be tied to ground as well? The datasheet says to let it float?

    Chris