LM5007MM and LM5010MH design problem - unstable output

Krystian Woliński

Other Parts Discussed in Thread: LM5007, LM5010

Hello!

I am prototyping with these two devices. As a reference I used QuickStartCalculator sheet, Evaluation Board documentation and datasheet - all available on the product pages at ti.com. My calculations, schematics, PCB layout and output waveform are all attached to this post.

Problem with LM5007MM: If I use SS16 diode (slow) everything is OK whatever load is applied. The problem arise if I use MURA110T3G diode (ultrafast) - the same diode is used in "AN-1298 LM5007 Evaluation Board User's Guide" and is suggested in datascheet "10V, 400mA Demo Board". With LED (5 mA) connected on the output the voltage is stable 5V. When resistive load of 134 Ohm is connected the output voltage gets unstable - its waveform attached below. The design makes audible noise whatever load is connected.

Problem witch LM5010MM: The same as with LM5007MM.

Why the design of LM5007MM made "according to the book" does not work for me? Why the problem appears if I use recommended diode type? Is the problem caused by incorrect properties of other component selected?

I appreciate any help or clue.

4834.LM5007QuickStartCalculator.xls

1200.LM5010QuickStartCalculator.xls

over 11 years ago

0 JohnTucker over 11 years ago

TI__Guru**** 191445 points

I think the first place I would look is your layout. It is hard to tell from the photo, do you have a ground plane on the bottom side? I would recommend you closely follow the kayout of the EVM, or modify an EVM board with your components for testing.

0 Krystian Woliński over 11 years ago in reply to JohnTucker

Prodigy 30 points

It is one sided PCB, no ground plane on the bottom side. Unfortunately I cannot afford for an Evaluation Board to check your idea. What can I do to make my existing design operate properly (except to change the diode to SS16-like)?

0 Vijay86929 over 11 years ago

TI__Genius 11490 points

Hi Krystian,

It is difficult to identify the root cause based completly based on the information provided. As John mentioned, the layout may have big impact on the operation. The layout does not have the reference designator so I can't tell if the components and bypass caps are placed properly.

But, to you point, a schottky diode is the recommended type to use with LM5007 and LM5010. If it fixes the problem I would recommend going with that. Ultrafast diodes have fast switching transition that can exacerbate any layout issue.

Thanks and regards,

Vijay

0 Krystian Woliński over 11 years ago in reply to Vijay86929

Prodigy 30 points

Hi,

Today I assembled my new design with LM5010MH. I have used the fast schottky diode ES2B (2A, 100V, trr=20ns). I compared operation of my previous design with the new one using the same ES2B diode mounted in both designs.

Old design & layout + fast schottky diode: unstable output, unpredictable output noise, could not provide required current at maximum load, random high pitch sound.
*** Test results (Vin=30V)
no load: Vout=5.06V
Rl=5.1Ohm: Il=0.32A, Vout=1.8V (max allowed load)
Rl=1.8Ohm: Il=0.49A, Vout=1.1V (overload)
Vin=24V and Rl=5.1Ohm: output noise 1.94Vrms, high pitch sound.
The design operates OK if a normal speed diode (STPS2150A) is installed.

New design & layout + fast schottky diode: stable output regardless of input voltage and output load, proper operation even at suddenly changing load. The design operates properly with a fast and normal speed type of diode.
*** Test results (Vin=30V)
no load: Vout=5.06V
Rl=5.1Ohm: Il=0.94A, Vout=5.06V (max allowed load)
Rl=1.8Ohm: Il=1.29A, Vout=2.66V (overload)
Vin=8V..32V and Rl=5.1Ohm: always stable and meets requirements.

The new design includes following changes: slightly different voltage divider resistors values, input bypass capacitor value, SMT devices packages, PCB includes bottom ground plane with vias to components at top PCB side.

Old design & layout: