• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 422 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM3409: PGATE no output in the low PWM duty cycle

Betty Guo
Expert 7300 points
Part Number: LM3409

Hi team,

customer have PGATE no output issue of LM3409.

The application is : VIN=12V, output is 3s LEDs, the VF of LED is about 2.75~3.25V. they use PWM to dimming ,the frequency is 100Hz

they found that when the PWM duty cycle is set to 0.043%, Power up, the PGATE is normal have output signals. After some minutes, PGATE is high, no output.

we have below questions:

1. why the PGATE have this issue? customer use PMOS is BUK6Y24-40P, the Qc is 23nC,35nC(max), datasheet require the Qc should be less than 30nC. is that the reasone why PGATE no output?

2. customer increase the capacitor Cf between VIN and VCC from 1uF to 10uF, can solve this problem. The datasheet show if the Qc of PMOS >30nC, the Cf shoud not connect VIN.could you help give more introdcution about this. how to modifiy this circuit in customer application?

I attached customer schematic and waveform, and test result (chinese).please help to check, thanks.

TI 相关芯片使用问题咨询_20201014-LED.docx

  • 0
    TI__Mastermind 28655 points

    Hello Betty,

    The MOSFET gate charge is not the problem.  What I think is happening, due to very low duty cycle and pulling EN below 0.5V, is that Vcc capacitor is discharging over time causing it to go into low power shutdown.  The EN high time is not enough to recharge Vcc.  To avoid this EN should not be pulled below 0.5V for PWM.  You can see this in the last oscilloscope picture, the gate pulses are low voltage and climbing as the EN is high.  Another problem is at such low duty cycle the output voltage is dropping so the off-time gets long.  There is a 10 uF output capacitor that will prevent the LED from illuminating until it is charged.  If the output is discharging and the recharge PWM is at very low duty cycle there will be no light output.  Run the same tests as shown without the output capacitor and the results will look different though it probably still won't be as wanted.

    During operation watch the VCC voltage when it stops working.  Look over many cycles to see if it is recharging or not.  You can also trying using UVLO for PWM dimming to prevent low power shutdown.  Just use a diode to pull UVLO low to allow the UVLO function to still operate.

    "8.4.1 Low-Power Shutdown
    The LM3409/09HV can be placed into a low-power shutdown (typically 110 μA) by grounding the EN terminal
    (any voltage below 0.5 V) until VCC drops below the VCC UVLO threshold (typically 3.73 V). During normal
    operation this terminal should be tied to a voltage above 1.74 V and below absolute maximum input voltage
    rating."

    Best Regards,