LM3409HVEVAL: Start Up delay at 20kHz PWM

hi Martin

Do you mean you apply PWM signal on EN pin or PWM2? the waveform of EN looks weird since the result should be square wave if you apply PWM signal on EN. what is your input and output voltage condition?

Hi Mason,

we apply the PWM2 with the jumper between EN and R4. I changed R4 to 110 ohms. C9 has not been changed.  C9 pulls R4 and EN low every time PWM2 goes high. Why is that? Does the LM3409 need a reset? 

We don't understand why this circuit is made this way and if this is the fastest possible PWM driver.

How can we reach start up times <<100ns between 1kHz and 40kHz external PWM?

Martin

hi Martin

you can try to decrease inductor to 10uH and see any improvement.

Hi Mason,

the inductance is 33 uH, the value of the EVAL board.

I think the EN pin looks right because the high pass filter of R4 and C9 pulls it down a bit.

Can you please provide us the scope screenshots of the EN pin what you understand it should look like?

Switching frequency:

Figure 7 in the user guide shows 2us = 500kHz @ 20kHz PWM.

Figure 8 & 9 in the user guide shows 1us = 1MHz @ 100kHz PWM. 

Why does the switching frequency changes with the pwm frequency? 

Martin

hi Martin

For the switching frequency, I do not think the dimming frequency will have an impact on switching frequency. After comparing Figure 7 and Figure 8&9, the current rating is different. This will lead to a different toff and thus different switching frequency. 

For EN coupling, I do not think this is really necessary and the waveform is right. with the PWM signal, the voltage of red circle also varies between 0V and 5V which will further change EN voltage. 

Hi Mason,

without EN coupling it does not work well. The LED flickers. 

We still do not understand the function of R4 and C9 and what is the best approach for fast start up times. 

The User guide does not help here well.

Regards

Martin

hi Martin

Pls allow me some time to further check this. At the same time, do you mind trying the action with smaller inductor with EN coupling? 

Hi Mason,

I'll try a smaller inductor next week. 

Yes, please gather more information about the internal processes of the LM3409HV. This would help a lot. 

hello Martin

sure, I will get back to you once I have some progress.

Hello Mason,

we need some progress in this timing issue.

What exactly happens inside the LM3409HV?

Who has this detailed information?

Any ideas?

Regards
Martin

Hi Martin – our experts for this issue are currently celebrating the Chinese New Year.  Please expect a delay in response. Thank you for your understanding.