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LM3549 Eval Board - Cannot Affect LED Output Behavior Via I2C Configuration

Prodigy 20 points

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Hi All,

I'm having what I'm sure is a brain fart here, but I've been unable to figure out what I'm doing wrong and would like some help. Here are the steps I'm going through, and what I see at each step:

1. Plug in LM3549 eval board on USB and read all I2C registers. Initial values.png is the result of this initial read:

2. Configure MFE bit in register 0x14 to be asserted and reduce MASTER FADER to 0, commit these values to LM3549. 'I2C Commands.png' indicates the values that are written to the device:
3. Enable Red LED via R_EN button on config tool.
4. Observe Red LED intensity is unchanged relative to MASTER FADER being set to 0xFF. The same is true for all 3 LEDs. While LEDs are on, changing MASTER FADER has no effect. The same is true when MASTER FADER is configured after clearing all _EN lines for several seconds in an effort to return the LM3549 to Standby state. I have also tried writing 0 to ALL I2C registers during both standby and active LED operation, this also had no effect. Performing a read all returns the values that have been written to the device until Enable is cleared and re-asserted, at which point the values read back are the EEPROM defaults.
Any advice in getting this chip to respond to input would be appreciated here. I must be missing something painfully simple.
Thanks!
-Erik

1 Reply

  • Hi Erik,

    LED brightness can't be changed while LED is on. If you change the master brightness setting while LED is on it wont have effect until you toggle the LED enable button off and back on. Try the following, start the automatic sequence by checking the Enable sequence check box next to B_EN button. This will generate a sequence which turns on and off all the LEDs one by one. Then try changing the Master fader setting. You should now be able to see the change in the LED currents.

    Please note that minimum LED current is limited to 100mA which is already very bright. If you set the master fader to zero LED current will be set to 100mA, not 0mA. It is better to check the LED current with oscilloscope to see the change in LED current.

    Regards,
    Anssi

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