LM3464AEVAL: The 4 FETs are not behaving identically.

Hello Sana,

If these are the EVMs they should work. It must be connected correct for it to work. How are you connecting this to the power source? How is the dynamic headroom feedback connection made, along with it's return? Was the power source modified for operation with the LM3464AEVAL? What does the input voltage to this board look like?

I'm not sure I understand what 4.7 Kohm you installed? Are these the RG1-4 resistors? If so they are not gate drive, they are pull-downs. They are not needed. Did you read through the EVM instructions for power up?

What LED stack voltage(s) are you using?

Best Regards,

Hi Irwin,

Just to clarify, I have purchased two evaluation boards. I made the modification on the first one but not the second one. On the second one, I have the EVM connected to a DC power supply with no connection to feedback vfb signal. Only FETs 1 and 4 could light up the LEDs.
The data sheet recommends adding a 4.7k between the gate and ground and this is why I’ve added a resistor at the gate of the first board. I had a very intermittent operation (led light up but it is not consistent). I don’t know if that relates to gate charging speed or to the layout of the board (distance between the gate and the LM3464A part).

Please see below additional answers to your specific questions.

If these are the EVMs they should work. It must be connected correct for it to work. How are you connecting this to the power source?

---DC power supply 60V

How is the dynamic headroom feedback connection made, along with it's return?

---No connection

Was the power source modified for operation with the LM3464AEVAL?

---Not the second board. Only the first one where I added pull down 4.7k resistor to GND, per the data sheet. I made no modification on the board.

What does the input voltage to this board looks like?
--Very stable 60VDC

I'm not sure I understand what 4.7 Kohm you installed? Are these the RG1-4 resistors?

---See data sheet please, if the leds are connected with more than 1m length cables then 4.7k resistors should be added.

If so they are not gate drive, they are pull-downs.

--I know that.

They are not needed.

---The data sheet says otherwise.

Did you read through the EVM instructions for power up? Yes.

--I’ve also added 1uf on The first board for power startup cap but still not working and I don’t ink that it is an issue on the second board since the FETs 1 and 4 are operational.

--VDHC is higher than 0.9V on the drains of the non-operational FETs

What LED stack voltage(s) are you using?

--I’m using retrofit leds, which turn on at 48V for sure and they do with FETs 1 and 4 but not with 2 and 3.

Thank you for your help with this matter.
Sana

Hi Irwin,

Have you powered the LED strings up independently to make sure that is all working correct? Have you tried lowering the input source voltage?
--I have tried both and same behavior.
It is not being used as the EVM document intends it to be used.
--What did you mean by that?

Have you looked at the fault signal, gate signals, etc?
--The fault signal on these second EVM is at 6V and the gate signal is not toggling. I have checked the same signals on the LM5021+3464 board. The gate signal is a PWM (200mV which duty-cycle with PWM input signal duty-cycle). I'm quite aware of how the part should operate.
The LM3464AEVM board however is simply not consistent at the FETs outputs. The datasheet states that if the cables are longer than 1m and are quite inductive, one should add 4.7KOhms pull-down resistor at the gate, so we certainly can use that part that way.

I don't know if you have the capability to do it at your facility but if you try the board the way that I'm describing it, I would expect you to see the same issues.
Thanks.

Hello Sana,

Have you tried connecting the EVM to your other setup which works?

"It is not being used as the EVM document intends it to be used.
--What did you mean by that?" What I mean is that the headroom control is not being used. It is not powering up in a sequence where the voltage slowly rises and the LM3464 can determine when all channels are regulating and feed back the headroom control. I don't know how you are powering this up. If you could add oscilloscope pictures during power up while watching the functions of the IC such as fault, gate, drain, etc it may be helpful.

Best Regards,

Got it and it makes sense; I also believed that that may be part of the issue, especially when all the 4 channels did not light up on the first board. To remediate to that, I've added a 1uF for the startup cap CDHC as well as to CFault but no stability on the functionality of the part, to slow down the startup of the LM3464. Please not that all board modifications were made only on the first board. The second one is intact and it is the one that made me rethink the startup issue, as only two FETs are operational. I'm quite taken by another project right now, however, I will take the time to get you the scope snapshots and send them to you. I will aim to do that by next week. thank you again for your help ad have a nice WE.

Have you tried connecting the EVM to your other setup which works?

--The other setup is the LM5021/LM3464 EVM board so the feedback line is already coming from its own LM3464. I don't think that I can connect the standalone LM3464EVM board to it. Does that make sense?

--I strongly believe that the layout maybe contributing to this issue.

Hello Sana,

It would require disabling one and adding the other, it may not be that easy to do. It was just an idea to make sure the board was working correct and may be more hassle than it's worth.

As for layout, I'm not sure which layout you are referring to, the long wires?

Best Regards,