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LM3509 Excessive Heating

myo_motor
Prodigy 250 points
Other Parts Discussed in Thread: LM3509

We have an LM3509 driving two sets of two white LEDs.

With L = 4.7 uH and C = 2.2 uF, we had the output voltage at 21.3 V and the LM3509 got hot (over 50 C)

Switching to L = 4.7 uH and C = 22 uF, the output voltage went to 7 V so we should be dissipating less heat on the LM3509.  But it still heats up to 50 C, and is dissipating 500 mW.  How to reduce this?  The diode is Diodes Inc SD103AWS-7-F

  • 0
    TI__Intellectual 1040 points
    I think you might have enabled the device in OLED mode and LED mode, and without the resistive divider connected from OUT to SUB/FB to GND. This has caused the device to operate at its over voltage protection threshold (21.3V) and this has caused a large drop across the internal current sinks (21.3V - VLED(7V)) = 14.3V (14.3V x 40mA = 572mW). If this device does not need the constant output voltage and is only used to drive LED's, then you should program register 0x10 with bits [1:0] = 11 (both LED current sinks ON) and bit[5] = 0 (OLED mode off). In this mode the output will regulate to whatever voltage it needs to in order to bias the LED strings at the correct current (around 7V).

    Thanks
    Travis
  • 0 in reply to Travis Eichhorn
    Prodigy 250 points

    Travis,

    I agree with your statements; at first I was looking at the disconnected output (OVP to SUB, while MAIN had a string of LEDs attached).  Looking at the connected output the device is regulating properly.  Is Table 10 valid for the non-OLED mode?

    With both outputs enabled and connected to 2 LEDs each, the power dissipation is much better, but still seems like there's losses on the order of 200 mW or so (I see a 10 C rise above ambient).  This still seems like it's only 50% efficient (100 mW per LED, 3.2 V * 30 mA).

    Also, the datasheet implies that the high order bits of the GP Register should always be 1: 0b1100000 on reset.  I am always seeing them 0 (verified on the scope, also my I2C code seems to work everywhere else for this device).  Is this a typo?

  • 0 in reply to myo_motor
    TI__Intellectual 1040 points

    Table 10 only applies for OLED mode.  For non-OLED mode you can use a 1uF ceramic.  Yes, bits 6 and 7 should read back 11.  They are not used.  For the power dissipation, you should have the following (can you confirm):

    2 Strings of 2 LEDs each at 3.2V per LED and 30mA per string =  (3.2V x 30mA x 2 x 2 = 384mW)

    0.4V of regulation voltage at the current sinks (minimum) = (0.4 x 30mA x 2) = 24mW

    The questions I have are:

    What is VIN, what is the part number of the inductor?  Maybe the inductor is dissipating too much power.

    Thanks

    Travis

  • 0 in reply to Travis Eichhorn
    Prodigy 250 points
    Travis,

    VIN is 3.3V. The inductor we're using is VLF3010AT-4R7MR70,

    www.digikey.com/.../1132858

    I'm not sure what this means but I never see the two high bits come back as 1. The GP Register reads 0x0 on POR.