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Original question:

LM3409HV: Iadj voltage change when LM3409HV is enabled

LM3409HV: Suggestion for specifications of the LM3409.

Roger Wolff
Intellectual 550 points
Part Number: LM3409HV
Other Parts Discussed in Thread: LM3409

(if the system wokrs, there must be a link somewhere to a related question.... ) 

I ran into the same issue: The LM3409 draws current from the IADJ pin when EN is low. This current is much larger than the 5 uA that it sources from the pin when operating normally. In my case there are multiple LM3409 chips on one PCB They share a common, adjustable IADJ voltage. 

Referring to the datasheet, you expect that the voltage source that drives this will need to sink about 5 uA per LM3409. However, when the PWM dimming hits "0" on one or more channels, the disabled chip will draw much more than 5 uA from the voltage source. 

I think that mentioning: "when disabled, the LM3409 will draw typically xxx uA from the iadj pin" in the specifications section of the datasheet would have helped me and others not to fall for this pitfall. 

  • 0
    TI__Mastermind 18890 points

    Hello Roger,

    Thanks for your comments.  The 5uA is for the IC when enabled.

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Intellectual 550 points

    Right! And it is in the datasheet. 

    What is unexpected to me and at least one other person is that the input does not become high impedance  when the device is disabled for longer than a few ms. 

    The pin is clearly described as being able to be used as an input. (just take care that the chip will source about 5uA into whatever drives it). 

    It is a valid practice to connect multiple inputs to one output. I have 16 LM3409 chips that require ONE dynamically configurable current limit. So the "please apply a voltage to this node to set the current limit" is an appropriate method to set the current among this set of LM3409 drivers. I use PWM on the EN pin to set the intensity of the individual led lights.

    But when the intensity of ONE of the 16 drivers reaches "0", the chip that is disabled suddenly no longer sources 5uA into the iadj pin (I have no issue with that), but switches to drawing about 100uA from that pin! This is totally unexpected to many engineers: It is an input. When the chip is disabled, you do not expect the input to stop being high-impedance. 


    I'm not suggesting: "please fix the chip that's a bug! ". I'm suggesting that if you put it prominently in the datasheet you can point to the datasheet and say: "ahh, but you forgot to read the datasheet" when someone falls for this trap again in the future. Now people are pissed at TI because the datasheet simply didn't mention it. 

    Just to double check on my assertions I double checked with the datasheet just now. I now must come back on my assertion that the behavior of the chip is "within datasheet specifications, but just unexpected", 

    The datasheet says: 

    The IADJ, COFF, CSN and CSP pins are all high-impedance control inputs which c ...

    which is not true when EN is used for PWMing and happens to be "0". 

    Furthermore the datasheet says: 

    IADJ IADJ pin current min 3.8, typ 5, max 6.4 µA

    this is true as long as "en" is high (at least some percentage of the time. I'm currently using 1/4096), but not true when EN goes low for too long. (I measured -100uA wihch is outside the given 3.8 to 6.4 uA range. I didn't measure remotely accurately. That was not necessary for my purposes.). 

    So, the chip is defective, does not conform to datasheet specifications. Will you replace my chips until I die or refund me for the extra prototype turnaround ?

  • 0 in reply to Roger Wolff
    TI__Mastermind 18890 points

    Hello Roger,

    In section 7.1, the Absolute Maximum Rating we do specify that IADJ is listed at +/-5mA.  The Electrical Characteristic is for the part when it's enabled and operational unless otherwise specified.  Thanks Tuan