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LMG1020EVM-006: LMG1020EVM-006

Juan santana
Prodigy 20 points
Part Number: LMG1020EVM-006
Other Parts Discussed in Thread: LMG1020

Dear,

I am using this module to drive an infrared LED (OSRAM SPL PL90).

Before attaching the LED to the module I tested the module without removing the resistors, just as it came out of the box. I measured IN+ and Vg and I got a proper signal, at 100KHz and 100ns pulse width as recommended in the manual. Then I measured the voltage at Vbus (TP5) and I can see a signal following the Vg, this in principle should tell me that the FET is switching properly, but I can see that this Vbus voltage has an offset of -1.92V. I am a bit puzzeld about it and it is very important for me to fully understand the issue. I am afraid that if I connect the LED I will damage it becuase of exceeding absolute ratings when reverse biasing the LED because of the offset.

I will really appreciate your help and guidance.

Regards,

Juan

  • 0
    TI__Genius 12990 points

    Hi Juan,

    thanks for reaching out and welcome to e2e!

    To confirm that you are applying the correct voltage to VBUS, What bus voltage are you applying to J2? are you measuring TP5 (+VBUS) with respect to TP7 (-VBUS)?

    The voltage at TP5 (and TP7) should be constant with some ripple and should not follow Vg. To confirm if the FET is switching and not damaged you can probe TP6 to make sure the FET is bringing ground to K during the Vg pulse. If TP5 is following VBUS then its possible the probe points are not correct or the FET is damaged.

    The schematic for this circuit is on page 6 of the EVM User's guide below.

    thanks,

  • 0 in reply to Jeffrey Mueller
    Prodigy 20 points

    HI Jeffrey,

    Thanks for the anwer.

    I am applying 5.5V to J2 and meassuring as you mentioned with respect to TP7. The reason for this is that I cannot go higher that 5.5V otherwise the LED I am driving with the board will be damaged.

    Indeed the voltage at TP5  is constant and I can see some ripple. I was mistaken when I said it follows Vg.

    When I solder the LED and I measure the voltage at the KA node of the board with respect to groung (TP3) I expected to see some switching (as it happened when the resistors R5, R6, R7 and R8 were still in place), but I cannot see any switching. If I connect again a resistance between AN and KA nodes I see again switching, but when placing the LED I see absolutely nothing. I am using new LEDs so to be sure they are not damaged.

    Regards,

    JUan

  • 0 in reply to Juan santana
    TI__Genius 12990 points

    Hi Juan,

    thanks for the update,

    To measure KA you can use the pigtail probe connection TP6. When you see the switching, are you still seeing Vg or TP4 switch with the input signal?

    are you applying 5V to LMG1020 via TP1, or the 5.5V to TP2 or J2? (LMG1020 needs 5V nominal to operate)

    When you connect the LD are you connecting to the middle (KA pad) and one of the top or bottom (AN pad)? (check out Figure3 from the users guide to see the 3 LD pad connections)

    thanks,