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LM3414: Inquiries regarding IADJ pins

JAEMIN LEE
Prodigy 10 points
Part Number: LM3414
Other Parts Discussed in Thread: TPS40210

Tool/software:

Dear [TI Support Team],

My name is Jaemin Lee, an engineer at PABLO AIR in KOREA.

We are using the LM3414 part.

I have a question regarding the IADJ pin.

* Question

1. The circuit is designed with I_led (1A) and R_iadj (3.24kJ),  what happens if the I_led current exceeds 1A?

2. R_iadj = 3125  / I_led 

  2-1. Does it mean 3125V?

  2-2. What are the selection criteria for 3125?

I would like to request your response.

  • 0
    TI__Mastermind 30401 points

    Dear Sir,

    When you select 3.24kohm R_iadj, the current in theory should be 3125/3240=0.9645A.

    JAEMIN LEE said:
    1. The circuit is designed with I_led (1A) and R_iadj (3.24kJ),  what happens if the I_led current exceeds 1A?

    It does not mean 3125V, it's a factor considering the internal circuit design and you don't to choose this factor by yourself, just follow the datasheet. As the average output current of the LM3414 depends on the current being drawn from the IADJ pin, thus the LED current can be increased or decreased by applying external bias current to the IADJ pin.

    JAEMIN LEE said:

      2-1. Does it mean 3125V?

      2-2. What are the selection criteria for 3125?

    Thank you!

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    Prodigy 10 points

    Are you saying that when designing R_iadj (3.24kΩ) on the IADJ pin, there is no problem even if the LED current flows at 1.5A?

    According to the datasheet, R_iadj(3.24kΩ) = 3125 / I_led(1A), so isn't the LED current limited to 1A?

  • 0 in reply to JAEMIN LEE
    TI__Mastermind 30401 points

    Hi Jaemin,

    Let me check your question and get back to you soon. Thank you!

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    TI__Mastermind 30401 points

    Hi Jaemin,

    I didn't mean that when R_iadj=3.24kohm, there is no problem even if the LED current flows with 1.5A. In theory, it should be 0.9645A.

    Thank you!

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    Prodigy 10 points

    Hi Lucia.

    I designed the circuit with R_iadj=5.681kohm. According to the datasheet, I_led=550mA, but the actual measured value is I_led=880mA.

    Why is the LED current not controlled? (Since there is no current probe, the output current is measured using a DC supply.)

  • 0 in reply to JAEMIN LEE
    TI__Mastermind 30401 points

    Hi Jaemin,

    Could you pls let me know how you measure the output current with DC supply? I would like to first check the measurement method;

    What is input voltage, LED string forward voltage and what is the FS resistance? (Not clear from the above figure, looks like 40.2k, kindly correct me if not) I would like first review the schematic based on the application condition.

    Thank you!

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    Prodigy 10 points

    Hi Lucia.

    # How to measure the output current with DC supply

    1. Apply 16.8V as DC supply to Boost IC (TPS40210) to generate 25V output.

    (DC supply 16.8V is supplied as a replacement for the drone battery voltage.)

    2. DC 25V is applied as LM3414 Vin and VCC of RGB LED.

    3. Check power and output current, voltage with DC supply.

    And, I will share the parameter values you requested.

    Vin = 25V, V_led_green = 22.7V, Rfs =40.2kΩ

    Thank you.

  • 0 in reply to JAEMIN LEE
    TI__Mastermind 30401 points

    Hi Jaemin,

    Thanks for your feedback. You mentioned that there is no current probe, so that the output current is measured with DC supply? May I know is this current measured with 16.8V DC power supply? Since there is power loss for the Boost device and LED driver, not sure how you take them into consideration? And not sure if you have multimeter that can directly read the current? 

    Thank you!

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    Prodigy 10 points

    Hi Lucia.

    1. DC suppy current is the current value when DC 16.8 V is applied.

    2. Since I don't have a multimeter that can measure current, I'm simply checking the power and current with a DC supply.

    3. Request for explanation of the method of measuring the power loss for the Boost device and LED driver.

      (When the LED is OFF, the DC supply current value is 50mA.)

    Thank you.

  • 0 in reply to JAEMIN LEE
    TI__Mastermind 30401 points

    Hi Jaemin,

    Directly read from DC supply is not a good and accurate way to get the LED current. For example, for a general DC/DC converter, let's say VIN=12V, IIN=1.35A, VOUT=5V, IOUT=3A. Efficiency=5*3/12/1.35=92.6%. We can't measure the output current directly from input power supply measured current.

    Thank you!

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    TI__Mastermind 30401 points

    Hi Jaemin,

    Kindly let us know any further support need from us. Thank you!

    BRs

    Lucia