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LM3421: Unstable / Flickering output

Part Number: LM3421

Figure1: LM3421 Boost application

I am redesigning the LED driver circuit on the PCB which provides a constant current of 0.9 to 1A to the LED module. On my PCB I have 4 repeated lamp driver circuits. Every circuit uses the same components has the same clearances but are located in different positions and are orientated differently on the PCB.

I would like to mention that this design has been created and tested before, and the performance of the circuit is perfectly fine. The only differences between this new design and the old design are the resistor and capacitor size (0603 old design to 0805 new design), the different N-channel MOSFET, and little bit of the layout and the clearance.

Figure2: LM3421 PCB layout

The issue with the new design appears when I apply the load (LED module) to each circuit. All LED driver circuits are performing differently even though they have implement the same enable input (3.0V – 3.3V), and PWM signal (100% duty cycle & 4 kHz frequency).

Top left circuit & Bottom left circuit

I have captured some of the waveforms seen at various pins on the circuit. These are shown in the figures below.

From Figure 3 to Figure 5, the top left LED driver circuit and bottom left LED driver circuit represents the output waveforms seen at the DDRV, GATE pins and voltage output (Vout). Both circuit have the same component, clearance and orientation.

The top left LED Driver is receiving a 100% PWM duty cycle from our microcontroller and the output current (Iout) is approximately to the LED module is reading 0.8-0.9A which is what is expected. However, the bottom left circuit is receiving the same 100% duty cycle signal, and the output current was approximately 0.6A.

Figure3: DDRV output (CH1 Top left circuit and CH2 Bottom left circuit).

Figure4: Gate output (CH1 Top left circuit and CH2 Bottom left circuit).

Figure5: Voltage output (CH1 Top left circuit and CH2 Bottom left circuit).

I have also captured some waveforms on the other circuits for comparison as well. Figure 6 shows the output voltage seen at the LED Module, as you can see the on and off time are slightly different as well. I am unsure why this is happening.

Bottom left circuit & Top right circuit


Figure6: Voltage output (CH1 Bottom left circuit and CH2 Top right circuit).

Top left circuit & Top right circuit

The LED Driver circuits on the right hand-side record different output voltages comparted to the bottom left LED Driver circuit. The result, shows that there seems to be more power loss and less duty cycle compare to the bottom left circuit (as shown in figure6). The duty cycle from the outputs of the top right circuit was around 30% and only pushed 0.15A through the LED module.

Figure7: Voltage output (CH1 Top left circuit and CH2 Top right circuit).

Figure8: Gate output (CH1 Top left circuit and CH2 Top right circuit).

Figure9: DDRV output (CH1 Top left circuit and CH2 Top right circuit).

From the schematic drawing, to our prototype I changed Co capacitance from 3 x 4.7uF to 4x 10uF to improve the performance of the LED driver. I have also changed the RT from 47k to 39k ohms to increase the switching frequency. However, these three circuits are still operating as what they were before.

Can you please help me troubleshoot this problem, and what is actually causing system instability in this design?

  • Hello Araya,

    The design works before and this is just a relay out with caps and capacitor changing from 0603 to 0805 which would not be an issue.  The most likely scenarios is your layout as you have mentioned.  Have you tried operating one channel at a time to see if it operates fine?  If it operates fine one channel at a time then it's cross talk between the channel and you can look at the layout to see if there is a path that this can happen.  Groundings and ground planes on the board can also be an issue.  Other than that you will have to trouble shoot to figure out what is wrong with the layout.

    Thanks Tuan

  • Hi Tuan,

    I have tried what you suggested, and run one channel at a time. However, the output is still the same as before. I also checked my layout and I can't see any obvious non-optimal route.     

    This is 4-layer PCB where 1st layer with signal tracks, and polygon GND plane, 2nd layer with 24V power plane, 3rd layer with GND plane and 4th layer with only signal tracks. I also checked my soldering and tried to implement the circuit on the other PCB. However, the second prototype board is still having the same issue.

    Thanks Araya.

  • Hi Araya,

    All we can do here is to point you to our layout guideline in the data sheet.  You will need to debug and figure out where the discrepancy is within you own layout.

    Thanks Tuan