LM25118 output problem

Varaprasad Malisetty

Other Parts Discussed in Thread: LM25118, LM5118

Hi,

I have started working with LM25118 recently. I have designed it in buck-boost mode for 24V/3A. But, the output i was getting is only 6V. Please find the attached circuit and let me know if there is anything wrong.

Design

Frequency : 300KHZ

Thanks,

Varaprasad.

over 10 years ago

0 EricLee over 10 years ago

TI__Mastermind 49771 points

Due to low resolution issue, it is hard to read.

0 Varaprasad Malisetty over 10 years ago in reply to EricLee

Intellectual 460 points

Please find the schematics in the below link.

6114.LM25118_Schmatics.pdf

0 EricLee over 10 years ago in reply to Varaprasad Malisetty

TI__Mastermind 49771 points

Please connect VCCX to AGND.

Please connect low side feedback resistor.

Please connect negative connection of sense resistor to ground.

Please reserve foot prints for RC subber.

Please connect negative connetion of output capacitors to ground.

Please double check coupled inductor winding direction or just use single inductor.

Regards,

0 Varaprasad Malisetty over 10 years ago in reply to EricLee

Intellectual 460 points

Hi Eric,

Thanks for all the inputs. Now the regulator is working fine under no load. When the load of 3A is connected the output drops to around 3V. When 1.5A load is connected it drops to 20V. I am bit confused here with the inductor. I am sure there is a problem with the inductor. Please suggest me the kind of inductor i need to use.

Vmin=18V

Vmax=36V

Vout = 24V

Iout = 3A

SwFreq = 300khz

Thanks,

Varaprasad.

0 EricLee over 10 years ago in reply to Varaprasad Malisetty

TI__Mastermind 49771 points

Please go to http://www.ti.com/product/lm5118 and try WEBENCH.

WEBENCH will suggest you an inductor which you can try.

Regards,

0 Varaprasad Malisetty over 9 years ago in reply to EricLee

Intellectual 460 points

Hi Eric,

I tried out the design according to the TI WEBENCH. But there seems to be a problem still, as i am unable to achieve total 3A current. Only upto 2.2A it was stable. Below attached is the TI WEBENCH circuit.

Changes did in the above circuit :

MOSFET(M1,M2) used : FQB50N06TM

DIODE(D1,D2,D3,D4) used : STPS1045

Cramp used : 940PF

RSENSE used : 0.015E

Please let me know if i am wrong anywhere.

Regards,

Varaprasad.

0 EricLee over 9 years ago in reply to Varaprasad Malisetty

TI__Mastermind 49771 points

Is UVLO pin voltage oscillating when you see the output voltage drop ?

Since LM5118 has a built-in hiccup operation if a cycle-by-cycle current limit tripped, you can see a hiccup operation at UVLO pin.

Regards

0 Varaprasad Malisetty over 9 years ago in reply to EricLee

Intellectual 460 points

The voltage is not oscillating @ UVLO pin. A constant 8V can be seen when the output is dropping at full(3A) load. The voltage dropping to 19.5V when full load is applied.

Regards,

Varaprasad.

0 EricLee over 9 years ago in reply to Varaprasad Malisetty

TI__Mastermind 49771 points

Please check COMP pin voltage. During the output voltage drop, COMP pin voltage should increase and be clamped to internal VDD rail. If not, please double check your wire connections on the board.

Does it happen over the input voltage range ? or happens only at low input voltage ?

Regards,

0 Varaprasad Malisetty over 9 years ago in reply to EricLee

Intellectual 460 points

Hi Eric,

I found the problem. The trace width at gate of the mosfet is small hence not able turn on the mosfet completely. Now its been solved.

Thank you,

Varaprasad.

0 Brian Cavanagh over 8 years ago

Intellectual 285 points

Hi Mr. Malisetty, I have been working with the LM25118, and used the evaluation board in my design but unfortunately it's not functioning correctly.
Can you offer some advice on this issue? My posting is at e2e.ti.com/.../1807819. I apologize if I'm not following the correct protocol, but I'm having a difficult time getting feedback about my request. Best Regards, Brian

0 Varaprasad Malisetty over 8 years ago in reply to Brian Cavanagh

Intellectual 460 points

Hi Brian,
Sorry for the late reply. I see, people suggested some points to you in the forum. So can you tell me the current status of your design.
Regards,
Varaprasad.

0 Garrett Roecker over 8 years ago

TI__Mastermind 21720 points

Hi Varaprasad,

First, can you post a schematic that has clearly stated component values? The one you supplied is hard to read and I want to make sure I'm getting the correct values. Also any waveforms of the IC operating will be helpful in debugging the issue.

One thing I did notice, for many higher output voltage designs there needs to be a resistor between VCC and RAMP. This is used to increase the RAMP voltage to work properly at 24V. Once I have the schematic values I'll be able to suggest a resistor value.

Thanks,

Garrett

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LM25118 output problem