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LM3648: Maximum Vout in pass mode

Zoey Wu
Expert 5855 points
Part Number: LM3648

Hi team,

One of my customer is using LM3468 and the schematic is as attached. 

The LED works between 2.5-3V and they want LM3468 to work in pass mode . But in actual test , some LEDs can work normally and some can't. I guess that some LED's voltage may be too high and the LM3468 will work from pass mode to boost mode. And it may cause large current ripple in Vin without inductor between SW and VIN. Do u think it is the possible reason?

And one more question is that what is  the maximum voltage of LED  that can work normally with about 400mA driving current in LED in pass mode? Do we have any ways through the I2C configuration to increase the voltage of VLED in pass mode?

  • 0
    TI__Genius 15776 points

    Hi Dear E2E users. Since April 3rd to 5th is Chinese "Qingming" Holiday, our expert will reply your question on April 6th.
    Thanks for your understand.

  • +1
    TI__Genius 13001 points

    Hi Zoey,

    LM3548 will switch to boost model when Vout-VLED < Vhr. Since the input voltage is low, I think for some LED, in pass mode, they can not work.

    If you want switch to boost mode to light the unworked LED, you have to reset bit2 of 0x07 register.

    In fact, LM3648 doesn't control the output voltage. The output voltage in pass mode is depends on Vin-Vpmos.

    Robin

  • 0 in reply to Yuting Shao
    TI__Expert 5855 points

    Hi Robin,

    Does this mean that in pass mode, the maximum Vf equals to Vin-Vpmos-Vhr?

    Another question at first the customer doesn't configure the pass mode and some LEDs unworked. But when they configure the 0x07=0x0D, the unworked LEDs turn to normal state. Could u explain the possible reason?

  • 0 in reply to Zoey Wu
    TI__Genius 13001 points

    Hi Zoey,

    Yes, your understanding is correct.

    I find there is no inductor in the schematic. So even customer configures the 0x07=0x0d, the boost function can't work.

    Robin

  • 0 in reply to Yuting Shao
    TI__Expert 5855 points

    Hi  Robin,

    Thanks for your reply! But  I still have some other questions.

    1. When we configure LM3468 as IR mode, does this mean VHR in IR mode can be estimated as VHR in flash mode?

    2. The VHR data shown as the following table is tested in 1.5A . So what the VHR will be in 400mA? It seems that smaller the current is, smaller VHR will be.

    3.VHR shown in the table is the typical value, not the maximum value. So what the margin of VHR should be reserved in your opinion?

    4. When I configure LM3468 as  IR mode,  what will happen when VLED >Vin-ILED*Rpmos-Vhr? Does the ILED will decrease to adjust or will the LED just can't light?

  • 0 in reply to Zoey Wu
    TI__Genius 13001 points

    Hi Zoey,

    Please allow me to reply you later.

    Robin

  • 0 in reply to Yuting Shao
    TI__Expert 5855 points

    Hi Robin,

    Hope for your reply!

  • 0 in reply to Zoey Wu
    TI__Genius 13001 points

    Hi Zoey,

    Sorry for forgetting to reply you!

    1.Yes.

    2.The VHR is 290mV(typ) when current is 1.5A in flash mode and this is the highest VHR.

    3.I'm sorry I can't guarantee the maximum VHR at present.

    4.Device will enter boost mode.

    Robin