problem with LM5018

Andrew Frazer

Expert 2305 points

I have this circuit built, but i'm getting a result that is not expected.

No Load:

Vin = 48V

Vcc = 7.4V

Vout = 6.8V

UVLO = 4.72V

With no load, i get 6.68V on the output, but when i put any load on it, the voltage drops considerably.
with a 47R resistor across the output, I am only getting a 1.8V. The circuit is based on a Webbench design.

Under no load I am measuring 7.4V on the VCC pin

When i probe the SW and FB pins i'm not seeing any ripple at all, its almsot like the thing is'nt switching..

If i stick a 47R/5W resistor across the output Vpout dropps to 1.7V or so...

I've swapped the IC in case i had a faulty one.

Kind Regards

Andrew.

over 11 years ago

0 Juan Paulo Fung over 11 years ago

TI__Expert 3840 points

Hello Andrew,

I've forwarded your request to the Applications Team in charge of this device. Expect a reply shortly.

Best Regards,

-Juan

0 Andrew Frazer over 11 years ago in reply to Juan Paulo Fung

Expert 2305 points

Thanks i really look forward to their assistance. iS there anyone i can contact directly? I have a bit of a urgnet time frame looming on this.

0 Vijay86929 over 11 years ago in reply to Andrew Frazer

TI__Genius 11490 points

What is the VCC with load? Is your power supply current limit set to allow loading?

0 Andrew Frazer over 11 years ago in reply to Vijay86929

Expert 2305 points

Vcc drops to 2.24V when i have a 47R resitor across the output, Vout = 1.47 in this situaiton.

When i remove the load, Vcc = 7.29V, Vout = 6.8V

The power supply is not limiting the the output

Again i've checked, there is no sign of any switching going on;

0 Andrew Frazer over 11 years ago in reply to Juan Paulo Fung

Expert 2305 points

This is rapidlly becoming urgent. Is anyone able to assist?

Kind Regards

Andrew Frazer.

0 Vijay86929 over 11 years ago in reply to Andrew Frazer

TI__Genius 11490 points

Hi Andrew,

I will need to look at the layout of the board. Do you have any input bypass capacitor close to VIN-RTN pins? If not is it possible to add one close to the IC to see if solves the problem.

Thanks and regards,

Vijay

0 Vijay86929 over 11 years ago in reply to Vijay86929

TI__Genius 11490 points

oh I see it. The BST and SW connections are reversed in your design. Please compare it to the typical application schematic in the datasheet or user guide and you will see the difference.

0 Andrew Frazer over 11 years ago in reply to Vijay86929

Expert 2305 points

Thanks Vijay. that was in deed the problem. Forutnatly on my prototype PCB, I was able to make a coupel of easy 'hacks' Its running happily and to specificaiton now.

I kind of feel stupid, that i did'nt see this myself, or diagnose it.

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problem with LM5018