LM5069: LM5069: LM5069MM-2/NOPB In-rush current limiting issue

Balaji K R

Part Number: LM5069

Dear Sir,

We have used LM5069 IC in our power supply module for below output configuration.

Output Voltage: 40V
Load Current: 15A
At start, 10.5mF capacitive load applied to LM5069 output.
Current limit set: 18.33A (55mV/3mohm)
Timing Capacitor: 3.3uF (Fault time: 155ms)
Power limit resistor: 47K (Power limit: 125W)
There is no Soft start (dv/dt) circuit used.

When module powered ON with 10.5mF capacitor load at start, expected 40V output voltage is reached @120ms (<155ms(Fault timer)) and observed waveforms is attached here. Figure 1- Green-LM5069 output Voltage & Blue - Load current.
Figure 2- Green - Gate voltage & Blue- Load current.

From the figure 2, we could see load current is reaching 0A during inrush current limiting period & Gate voltage abruptly drops & raises repeatedly.

Please clarify the reason why load current is reaching 0A instead LM5069 allowing (125W/Vds drop) current during inrush current limiting period.

Waiting for your reply. Thank you..

over 4 years ago

0 Rakesh over 4 years ago

TI__Guru** 104466 points

Hi Balaji,

Can you please capture startup waveform with Vin, GATE, Timer voltage and input current in a single scope shot.

How is the startup if you enable the soft-start circuit ?

Also please share me the schematic and filled up LM5069 design calculator.

Best Regards, Rakesh

0 Balaji K R over 4 years ago in reply to Rakesh

Prodigy 195 points

Hi Sir,

Please find the required details below,

Can you please capture startup waveform with Vin, GATE, Timer voltage and input current in a single scope shot.

Figure 3: Red- LM5069 input voltage; Green- LM5069 output voltage; Blue- Load current (LM5069 input/output current)

Figure 4: Blue – Timer Voltage; Green- LM5069 Output voltage

For gate voltage waveform, please refer already sent Figure 2.

How is the startup if you enable the soft-start circuit ?

Provision for Soft-start circuit is not given in our design.

Also please share me the schematic and filled up LM5069 design calculator.

Attachment sent to your TI mail id.

0 Rakesh over 4 years ago in reply to Balaji K R

TI__Guru** 104466 points

Hi Balaji,

Is there any other load apart from 10.5mF capacitor at the output during startup ? If yes - what is the load value and the type of the load. ?

Best Regards, Rakesh

0 Rakesh over 4 years ago in reply to Rakesh

TI__Guru** 104466 points

Hi Balaji,

Can you please share the schematic also. It is not displaying in the excel file you sent.

Best Regards, Rakesh

0 Balaji K R over 4 years ago in reply to Rakesh

Prodigy 195 points

Hi sir,

Only 10.5mF capacitive load available at the output during startup.

0 Balaji K R over 4 years ago in reply to Rakesh

Prodigy 195 points

Hi Sir,

Attachment sent to your TI mail ID.

0 Rakesh over 4 years ago in reply to Balaji K R

TI__Guru** 104466 points

Hi Balaji,

The schematic is missing in the attachment. Even if you found the reason for improper startup, the design will lack SOA margin. Hence, I recommend to use soft-start circuit as mentioned in the front page of LM5069 datasheet.

Can you add that soft-start circuit externally on your board and test again.

Best Regards, Rakesh

0 Rakesh over 4 years ago in reply to Rakesh

TI__Guru** 104466 points

Hi Balaji,

Did you get chance to test the design with soft-start circuit ?

Best Regards, Rakesh

0 Balaji K R over 4 years ago in reply to Rakesh

Prodigy 195 points

Sir,

Sorry for the delayed response.

I haven't tried adding Soft-start circuit externally yet.

Selected MOSFET has SOA margin:

For 155ms - Ids-4.1A@Vds- 40V - which is higher than load current allowed by LM5069 during MOSFET power limiting condition(125W/40V=3.15A). So selected MOSFET meets SOA right?

During LM5069 limiting power dissipation in MOSFET at startup, my load Capacitor 10.5mf gets charged to 40V & load current settles before fault timeout period elapsed, in this case what is the requirement of soft start Circuit (reducing dv/dt).

Kindly clarify my above doubts.