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Original question:

LM3409: LM3409 PWM Dimming Question

LM3409: LM3409 PWM Dimming Question in low frequency

user4370495
Expert 1155 points
Part Number: LM3409

Dear E2E:

I Use lm3409 IC to driver two leds :  VIN = 8V;  VO = 5.5V  fSW = 30Hz  Duty=56%   ILED = 1.A  
But i found  Led Power on  delay time is longer than datasheet show 3.5us.  
Compared  with waveform, The fsw is different , the fsw=200Khz in datasheet.
The reason why it affect the power on delay time ,and how to improved it .thanks 
schematic and waveform as fellow:  yellow= En_pin,  cyan=Out_led

  • 0
    TI__Mastermind 28655 points
    Hello,

    The LM3409 is not a constant frequency device. It is constant off-time which is really constant if Vout stays the same such as an LED load. If the input voltage goes up the frequency will go up. If the Input voltage goes down the switching frequency will go down. I calculate about 860 nS off-time and about 360 Khz. They duty cycle should be about 69% or slightly higher because of circuit losses.

    You are running into the maximum off-time. This means there isn't enough energy in the first on pulse to get the output voltage high enough to start running so it goes through one maximum off-time (see the datasheet, spec is 300 uS). The turn-on will not be 3.5 uS because of V=L*(di/dt). V is 8V - 5.5V to charge the inductor, L is 33 uH and di is 1A. This makes dt 13.2 uS. You can add a resistor and diode to the Coff capacitor where the resistance is much higher than your Roff resistor, try 51K to start with to speed up the turn-on.

    Look at the output voltage when it tries to start, it has to get above 1.243V for the off-timer to work otherwise it has to go through a maximum off-time. Is there a capacitor on the output? The LEDs should be mostly open at low voltage so it should start with the first pulse. If there is reduce this capacitor value (you'll be able to see the LED voltage with your oscilloscope when PWM dimming). Is the load just LEDs or is there something else connected?

    Regards,
  • 0 in reply to Irwin Nederbragt
    Expert 1155 points

    Dear  Irwin Nederbragt ：

    thank a lot for you help. 
    The load is just two LEDs (Power: 3.4W) ,and there is no capacitor on the output.
    i have do so me test ,add a capacitor (such as 1uf/4.7uf) the performance worse than no capacitor.
    i need you double check the resistor and diode position,  thanks 


  • 0 in reply to user4370495
    TI__Mastermind 28655 points

    Hello,

    The anode of the diode needs to go to VCC the resistor needs to go from the cathode of the diode to the connection at pin 4 of U1.  It may need to be a lower value depending if it works or doesn't pull high enough.  What is the voltage of the LED string?  I still don't understand why the output doesn't rise with the first pulse, an LED load should not be drawing current at very low output votlage.

    Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 40 points

    Hi  Irwin Nederbragt ：

        is the diode and resistor position as below ?? 

  • 0 in reply to Irwin Nederbragt
    Prodigy 40 points

    Dear  Irwin Nederbragt ：

    is the circuit need to change as below??

  • 0 in reply to user654629
    TI__Mastermind 28655 points
    Hello,

    Not quite. The modification is correct for the added resistor but you deleted the Original R4. You need to add the original R4 Roff resistor back in. It should be Roff of 5.1K (which was R4 originally) and the added diode and 51K resistor as drawn above.

    Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 40 points

    I modify the circuit as up, the delay is improved after power on,  but after work a few minutes,

    It became 300us again.

  • 0 in reply to user654629
    TI__Mastermind 28655 points
    Hello,

    If the output voltage is 5.5 volts something else is wrong. Can you measure the output voltage when it is a long delay? Is it only after you power down adn back on? Try changing the 51 Kohm to 20 Kohm to see if it operates correct. Is the load only LEDs? Is there some other resistive load on the output? The LM3409 does not like to start with a resistor load due to the voltage not getting high enough from the first energy pulse.

    Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 40 points

    Is it only after you power down adn back on?

    -> No, it appear when the PWM duty less than 57%.

    Try changing the 51 Kohm to 20 Kohm to see if it operates correct.

    -> Yes , change the 51 Kohm to 20 Kohm the wafer is normal,  great!!

    Is the load only LEDs? Is there some other resistive load on the output? The LM3409 does not like to start with a resistor load due to the voltage not getting high enough from the first energy pulse.

    -> Yes , Only 2 pcs LEDS , Osram SFH4725S

  • 0 in reply to user654629
    TI__Mastermind 28655 points

    Hello,

    The load must be discharging during the off-time.  The 20 Kohm will ensure that the Off-time voltage can rise above the 1.24V threshold, the 51 Kohm isn't enough for that since the output is somehow going low.  Make sure your switching frequency is still close to where you need it to be.  If it is too high now you may need to raise the 5.1 Kohm value a little.

    Regards,