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LM3409HV: LM3409HV

Ambar Pakhira
Prodigy 80 points
Part Number: LM3409HV

Hi,

We want to use IADJ pin of LM3409HV, to control the LED current as a function of externally applied control voltage. We are planning to drive this IADJ pin from a voltage source of 0V - 4V. Does the IADJ pin reference of 1.24V, has zener diode or precision linear regulator? If only zener is there, what is the biasing current range?

We couldn't find the voltage rating of this pin from datasheet. What is the maximum allowed voltage of ADJ pin?

If the external voltage applied to IADJ pin is below 1.24V, will the internal reference be overridden by external voltage reference?

Warm Regards,

Ambar

  • 0
    TI__Mastermind 28655 points
    Hello Ambar,

    The Iadj pin is clamped at 1.24V (this means you cannot apply higher voltage without limiting the current). The ABS MAX current of the pin is +/- 5 mA. If you used a 1K resistor in series with your 0-4V it would be fine since the maximum current would be ((4V - 1.24V)/1K = 2.76 mA which is less than 5 mA). Better would be to use a resistor divider that sets your 4V to 1.24V at the pin, this give you more resolution. The 1.24 volts internal to the part is a reference voltage to be accurate. There is an internal 5 uA current source to Iadj that has to be taken into account when designing the interface to Iadj.

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 80 points
    Hi Irwin,

    We suspect that internally there is some additional linear circuit attached to the IADJ pin parallel with the Zener Diode. Can you confirm that? That will help us design our controlling voltage circuit design.

    Warm Regards,
    Ambar
  • 0 in reply to Ambar Pakhira
    TI__Mastermind 28655 points
    Hello Ambar,

    The Iadj pin is clamped at 1.24V, it will be damaged if it is pulled higher with more than 5 mA of current. There is a 5 uA current source supplying Iadj. This should be enough information to design the interface to this pin. The 'zener diode' is a representation of the internal IC function, not what is actually in there. Clamped from going above 1.24V (one of the internal references) and can source or sink up to 5 mA with a 5 uA current source, this should be all that is necessary.

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 80 points
    Hi Irwin,

    Thanks for the clarification.

    Warm Regards,
    Ambar