LM25011 voltage drops after very little current draw

Fredrik

Other Parts Discussed in Thread: LM25011

We have built a relay board with a dc/dc regulator LM25011, a relay and a buzzer.

The problem I have is that the voltage drops already after putting a 50ma+ load onto it. And if I put a 30ohm resistor across the output (Equals 180mA current draw) the voltage drops to 0.8V and it can't manage to pull it self back up.

We have looked over the design and can't find any misplaced components or anything.

Specs the regulator is designed for:
Input: 10V-30V
Output: 5V@1.5A
Inductor man.fact.nr: VLP8040T-220M

Attached is schematic and a picture of one of the assembled boards. Any help would be greatly appreciated.

Thanks.

-Fredrik

BU_LM25011 (1).pdf

over 9 years ago

0 David Pace over 9 years ago

TI__Intellectual 1435 points

Frederik,

My first guess is that noise from the switching regulator, buzzer or PC board layout are interfering with the LM25011 regulator or current limit comparator. You could try adding a capacitor, maybe 10nF, directly across CS and CSG pins of the LM25011 (as close as possible). An R-C filter between 120mohm sense resistor and the CS-CSG pins would be even better.

Is the buzzer operating when you test the regulator. If yes, try disconnecting it to remove one noise source.

0 Vijay86929 over 9 years ago

TI__Genius 11490 points

Hi Fredrik,

Can you attach some waveforms? Usually it helps to see the SW & IL (inductor current).

Also it helps to double check all the components are mounted properly.

0 Fredrik over 9 years ago in reply to David Pace

Intellectual 350 points

Hi,

Thank you for your answers.

I am running the board with the regulator only to eliminate all sources of noise.

I will supply waveforms when I get back to the lab in some days.

One thing I have thought about is if the ripple voltage over the sense resistor is to low. Is this a possibility?

-Fredrik

0 Vijay86929 over 9 years ago in reply to Fredrik

TI__Genius 11490 points

It is possible. But output dropping at 50mA current seems like too extreme a case to arise just from noise. Is the VIN pin staying constant or is it dropping as well?

0 Fredrik over 9 years ago in reply to Vijay86929

Intellectual 350 points

Hi,

The VIN pin is staying constant.

How should I measure the SW and IL?

Thanks.

0 Fredrik over 9 years ago in reply to Fredrik

Intellectual 350 points

The problem was that the CSG pin was lacking ground. Once this had a proper ground connection the reguator worked perfect even from 5.5V-32V!

Thanks.

-Fredrik

Power management

Power management forum

LM25011 voltage drops after very little current draw