PMP4334: LED's blink from time to time

Hi Jose,

    Glad to see you here. I will looping David here to resolve your problem.

Thanks!

Regards,

Richard

Hi, Jose,

Could you please share your schematic? That will help us to check whether there is any founding.

How do you dimming your LED? PWM or Analog. If PWM, what is the duty cycle.

Once blinking, do you have check the UCC28810 single stage flyback output voltage whether it is stable?

Hi David,

thank you for reaching out!

I have been using PWM and the duty cycle range between 20 to 100%.

Regarding the UCC28810, you mean checking the EAOUT (2) pin correct? I have not, I will have a look at it and let you know the typical values recorded. 

Hi, Jose,

Yes, we can check the EAOUT voltage to see whether it is stable. Also, we can check the VOUT DC voltage and ripple voltage when LED blinking.

Thanks.

hi David,

Yes, I can confirm that the voltage on pin EAOUT is stable around 2.57V. Nevertheless I noticed some extra blinking just by making the measurement (it looks in a way that it got more interference). It is hard anyway to make a measurement while the blink happens once its asynchronous. The VOUT voltage on the LEDs varies from 45 to 48V. 

Hi, Jose,

Could you please test the current of blinking LED string also drive signal of this string PWM on FET?

Thanks.

Hi David,

I am sorry for the delayed answer. The current of the LED string is 300mA at normal operation. Note that all strings blink simultaneously but its not predictable when they'll blink. Regarding your second question you're asking about the gate voltage of the MOSFETs after the LEDs correct? The gate voltage is 4.3V at normal operation 

Hi, Jose,

Thanks for measuring this parameters. However, could you please test the waveform instead of testing its value? The waveform will help us to analysis, thanks.

Hi David,

thanks for your previous message and sorry for the delay answering you.

I have noticed that right before the LEDs blink, the voltage value on Rdhc (coming from the Outp pin) goes from around 3.5V to 0V. Due to this drop, the voltage on the external mosfets gates (in series with the LED lines)  go to 0V while the LEDs are blinking.

Could this sudden drop on Outp be the reason for the blink? And if you, you have any idea why it happens?

If you want I can also send you the screen shots took for further analysis. 

Thanks a lot!

Best regards,

Jose 

Hi, Jose,

Please also send me the waveform, that will be helpful.

1. In addition, it is recommended to test flyback by removing D1 firstly from no load to full load using E-load. Checking flyback stability.

2. Check the C54 value to ensure it is over 4.7uF. The higher the capacitance of the CDHC, the lower the frequency response of the DHC loop, and vice versa. I am not sure how fast your flyback is, make sure CDHC cap bigger is more safe.

Hi David,

thanks for your answer.

Please have a loot at the description of the following two waveforms;

waveform 1: Purple line: Rdhc, the resistance right after the OutP pin (it starts going down exactly at the same time as the MOSFET's gate (green line) goes to zero

  Blue line: Rfb2 on the 2.5V comparator on the  Power Supply side (just wanted to check if there are any weird fluctuations)

  Green line: MOSFET gate, goes to 0 and the LED series goes down (note that this behavior happens with all the LED lines)

waveform 2 - I added this waveform once I noticed that the DRAIN of each external MOSFET (in series with the LED lines) has a higher than expected voltage value. In this case you can see that the line has around 7V (accordingly to the LM3463 datasheet it should be somewhere around 1V). The same happens to the other lines but with a slightly lower value (around 5V). Also note that when the flicker happens on the LED lines, Vdrain has a spike.

Blue line: MOSFET's gate going low when the LED line flickers

Yellow line: MOSFET's drain with a value too high

Purple line: Rdhc going down as in waveform1

In your last post you also mentioned the Cdhc. I had a look, currently I have 2.2uF. Should I increase it the 4.7uF or higher?

Please let me know what you thing about the Rdhc variations (visible in waveform 1) and the high drain voltage of the MOSFET's (visible in waveform 2). 

Thank once again for your time.

Kind regards,

José

EDIT: as an extra from my last post I also wanted to add that Vdrain gets lower as the current increases. Accordingly the value you can see in the second image (yellow line Vdrain) gets lower.

At maximum current Vdrain drops to around 1V. But it does not make the lines to stop flickering from time to time.  

Looking forward for your answers/thoughts David!

Thanks in advance!

Best regards,

José

Hi, Jose,

Appreciate your waveform. From the waveform, I have some confusion on it.

1. as you mentioned before, it is PWM dimming. If so, the gate voltage will be PWM according to your PWM dimming signal. Why it is keep around 4.02V in the first waveform so long? It is nearly the same as analog dimming. Could we please confirm which dimming control is used? If PWM dimming, what is the dimming frequency? From the waveform, the frequency is very lower because we just see a pulse. It is normal we can see the flicker of LED.

2. in the waveform 2. when voltage on Outp drops, it shows that there is current from the shunt regulator 2.5V node. the primary controller will increase the output voltage. LM3463 will detect the voltage increasing and react it. We will see a voltage increasing on the drain FET. This is a normal reaction. However, we need to find why voltage on Outp drops.

3. In addition, the logic is not right from the waveform 2. When gate voltage is 4.02V, the Rdhc voltage is very 3.76V. This means the diode inserted between Outp and FB node is blocked. The right logic the flyback output voltage will be higher when LED on and vice versa.

4. if possible, could you please test the startup, dimming function waveform according to test report? This will help us to analysis it quickly.

If we can make sure the Cdhc capacitance is 2.2uF, there is no need to try 4.7uF.

Hi David,

once again thank you very much for your feedback! I really appreciate your nice support! Regarding your questions please have a look at my answers bellow:

I can confirm with you it is PWM dimming. I have MODE (18) to GND enabling direct PWM. But in any case I also have a potentiometer for Analog Diming (I use it to test different values for the output current to the LED's - Poti is coupled to Vref IOUTADJ and REFRTN just like in the reference design PMP4334 - R27).

Please have a look at the waveform bellow to see the dimming startup. 

Regarding what you mentioned about Outp. The relation I have visualized between Outp and Vdrain is that  Outp voltage gets higher if Vdrain gets also higher (for example: Vdrain: 8V Outp: 3.8V and if Vdrain: 1.55V Outp: 2.7V). I get these values simply varying the Analog Dimming potentiometer, and by consequence changing the output current. 

Let me also an extra behavior I noticed between Vdrain and Vgate of the external MOSFET. I could see that synchronously in time there is a disturbance in both lines. Vdrain rises for some miliseconds and Vgate drops exactly at the same time for some miliseconds. This happens in fixed intervals of time. I am not sure but maybe from time to time this disturbance might be a bit higher making the gate go low for a larger amount of time and the LEDs to go off. Have a look at what I am talking about in the following waveform: 

Looking forward to hear back from you!

Thank you once again.

Kind regards,

José

EDIT: just as a quick extra it looks like the LED blink is more often when the current flowing through the LED lines it's higher. 

Once again thanks for your time.

Best regards,

José

Hi, Jose,

Could you please send me the PWM control logic? It seems that you control every LED channel respectively? If you check our test report, 6 paralleled LED is controlled by one signal. Anyway, LM3463 can support 4 individual PWM control.

In addition, when LED blink, is it just one channel or every channel?

Please also send me the schematic if possible, thanks.

Hi David,

Yes that is true, we use individual PWM currently. It basically goes from 0% to 100% and then we perform also some tests at 50% and 80%. 

When it blinks all the channels blink simultaneously. I also noticed (placing an oscilloscope probe) that when there is a blink the EN pin (on the LM3463) goes low and then after the blink it goes back up!

Currently I do not have access to the schematics as soon as I have I can send it to you. 

Just as a little extra input I have noticed that the evaluation board from the LM3463 uses a 0 ohm resistance on DRVLIM. I am currently using a voltage divider with 10k ohm.

Another thing that might be a trouble (not sure) is that we are using a FCAP cap of 2.2nF and the valuation board from the LM3463 uses 22nF (this capacitor is used to calculate the debouncer time for the fault feedback). 

Could the FCAP or DRVLIM be part of the problem or you still think its something else? 

Thanks once again for your time!

Best regards,

José

Hi, Jose,

DRVLIM will MOSFET power limit, could you please check the MOSFET temperature using thermal imager to check whether there is over temperature protection?

EN is a input signal determined by you setting signal. We need to check EN input source.

FCAP capacitance will influence the fault denounce time. It is just a fault dictator not the cause.

In addition, I have checked the stock, there is the PMP4334 boards. Please contact your local FAE or sales for applying this board. That will help you debug it quickly.