WEBENCH® Tools/LM25118: buck no boost

jeff scott

Part Number: LM25118
Other Parts Discussed in Thread: LM5118

Tool/software: WEBENCH® Design Tools

i have an LM25118 on a board with 3-7v input and 5v output and it only runs in buck mode as soon as it goes below 5 v the output starts to sag with basically no load we can start it on 6v and get 5v out just fine but if the input goes under 5v the output sags at 4.5v in the output is 3.3v

I was based on a webench design that isnt in my folder now but I submitted it to TI support along with a SCH and they sent me here.

over 5 years ago

0 Garrett Roecker over 5 years ago

TI__Mastermind 21720 points

Hi Jeff,

Thanks for reaching out with your questions and for considering the LM5118 in your design.

What is the maximum output current for your design?

Can you please post the schematic? That will help me understand what is happening.

Thanks,

Garrett

0 jeff scott over 5 years ago in reply to Garrett Roecker

Prodigy 50 points

2 amps is plenty 3 would be nice its a proto, atm its just an LED as a load so a few mA.

0 Garrett Roecker over 5 years ago in reply to jeff scott

TI__Mastermind 21720 points

Hi Jeff

It sounds like the LM25118 might be running into VCC UV. Can you please probe the VCC and VCCX pin voltages?

Thanks,

Garrett

0 jeff scott over 5 years ago in reply to Garrett Roecker

Prodigy 50 points

that was the first thing I thought of but it was well over 2V at the time. as you can see in the sch. its a 50/50 voltage divider and we checked for resistor miss loads as well.

0 Garrett Roecker over 5 years ago in reply to jeff scott

TI__Mastermind 21720 points

Jeff,

Just to confirm, the VCC voltage was 2V? Or are you referring to the voltage on the UVLO pin? I would like to know the voltage of the VCC pin.

Thanks,

Garrett

0 jeff scott over 5 years ago in reply to Garrett Roecker

Prodigy 50 points

input voltage is on a variable supply its fine at 5V and above.

at 4.7V in the output drops to 3.3V with the UVLO at ~2.3V

0 Garrett Roecker over 5 years ago in reply to jeff scott

TI__Mastermind 21720 points

Jeff,

Please be aware the the LM25118 requires 5V on the VIN pin to start-up. After start-up the VIN pin can fall to 3V as long as the VCC voltage is supplied through VCCX or externally biased through a diode. When the VIN voltage is coming close to 5V can you please measure the voltage on the VCC pin? I want to see if the VCC voltage is falling below the VCC undervoltage lockout voltage specified in the datasheet (3.7V typical). If the VCC voltage falls below this the output voltage might not regulate. Please let me know what the VCC voltage is when the input voltage is around 5V.

Let me know if you have any questions.

Thanks,

Garrett

0 jeff scott over 5 years ago in reply to Garrett Roecker

Prodigy 50 points

So this seems to have been a combination for a miss load on the Vcc Bypass Cap and a copper pour issue that corrupted the current feedback negative side.

it now goes down to 4.3V on the input before having issues.

0 Garrett Roecker over 5 years ago in reply to jeff scott

TI__Mastermind 21720 points

Jeff,

When the input is going down to 4.3V what is the voltage on the VCC pin? I have a feeling that it is falling below the UV threshold of the VCC pin. The UV threshold is 3.7V.

Thanks,

Garrett

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WEBENCH® Tools/LM25118: buck no boost