LM5066I: Start up problem

Hi Jonathan,

Can you please share the design calculator and schematic as attachments. What is the load at the output (Cout + any other load and the type of load)  ?

Best Regards, Rakesh

Hi Rakesh,

Below is the calculative sheet. I'm incorporating the LM5066I into a battery adapter and the load is mostly resistive. My wild guess is if the fault timing at 0.52ms is way too short for the mosfet?

LM5066I_CSD19536KTT.XLSX

Hi Jonathan,

The design sheet looks fine. 

Are you able to startup with just Cout load ?

Can you share the above test waveform with the input current information 

Best Regards, Rakesh

Hi Rakesh,

Thank you very much for your help. I have been digging deep into the problem and found the root of the startup hiccup is from a bad power supply. Swap out the power supply and the hiccups are gone.

That being said, the GATE Voltage is still very low, as long as I'm within the UVLO (11V) and OLVO (17V), the GATE pin will be at 7.12V and VOUT is stuck at 3.84V.

Below are the screenshots of the scope and the schematic. COUT total is 2720uF with no load attached. TP3 is measured at 14V, same as VIN.

Am I missing something?

Hi Jonathan,

Can you share the above test waveform with the input current information and timer votlage

Best Regards, Rakesh

Hi Rakesh,

At the moment I do not have the proper equipment to draw the current trace precisely with my scope. That being said, I did measure the current on two of my multimeters and both report MAX current @ 33.75mA, nominal at 15.44mA (both with no load at output).

Below is the Timer/Gate/VOut traces.

Hi Jonathan,

Sorry. We would need actual current information to understand the issue. Appreciate if you get that.

Best Regards, Rakesh

I understand and appreciate the help. Just ordered some tools, will update later.

Thanks Jonathan.

Hi Rakesh,

So I got a current clamp. Clamp was set at 1mV/10mA, I measured and these are the results:

Interestingly the gate voltage is saturated when the current is peak. Soldered a new PCB with just the LM747 & LM5066I like the schematic above, no COUT or load connected and the problem is still there. Really don't know where I took the wrong turn. Any ideas?

Thanks Jonathan.

What makes the current to rise abruptly when GATE=~7V ? What is the load connected ? type of load and value ?

What is the part number of Q9 in your schematic.?

Is the device able to power-up at no-load without any Cout ?

Best Regards, Rakesh

Hi Rakesh,

1. I have tried to resemble a new PCB with just the LM747 & the LM5066I (exactly like the schematic above) with no COUT or load connected and the problem is still there. Triple check on all nets and none are shorted so I really don't know where I took the wrong turn. 

2. Q9 is an ON Semi BJT MMBT5401LT3G.

3. Yes, the device is able to power up at no load & no COUT. GATE is still stuck at 7V and VOUT is at 4.1V.

Not sure what would happen if I remove the entire dv/dt start up (Q9, R30, D4, C19). I can give this a try unless you have found something else.

I have been really scratching my head around this problem for awhile now and I really appreciate your help Rakesh.

JR

Hi Rakesh, 

I did try to remove the entire dv/dt setup and the problem went away. Really wondering what did I do wrong at that section.