LM76003: FPWM perform not as expected

Hedy Hung1

Part Number: LM76003

Hi TI team,

I pull MODE pin (#17) to 3.3V and power on the device with Vin=24V, Vout=4V, Iout= 2.5A.

The switching frequency should be fixed in 580kHz, but the waveform doesn't appear that. I attached the switching waveform for troubleshoot.

Since I cannot find the waveform in datasheet at the same condition. Need your help to comment.

Btw, if I reduce Vin to 12V, Vout=4V, Iout =2.5A, the switching waveform shows fixed.

Thanks.

Hedy

over 5 years ago

0 Daniel Li14 over 5 years ago

TI__Mastermind 27470 points

Hi Hedy:

some suggestion:

1. Whether the output capacitor is enough

2. Whether the inductor value is too small which make the sub harmonic oscillation

Thanks

0 Hedy Hung1 over 5 years ago in reply to Daniel Li14

Intellectual 325 points

Hi Daniel,

The output capacitor is near 100uF, and the select the inductor XGL4030-103ME/ 10uH.

I ran the webech before designed and followed the recommendation as we could.

Can you provide the switching frequency waveform at 24Vin, 5Vout with 2.5A by TI's EVM?

Thanks.

Hedy

0 Daniel Li14 over 5 years ago in reply to Hedy Hung1

TI__Mastermind 27470 points

pls see EVM board for 24Vin, 5V 2.5A waveform

0 Daniel Li14 over 5 years ago in reply to Hedy Hung1

TI__Mastermind 27470 points

pls see below waveform:

CH2=SW CH4=Io

0 Hedy Hung1 over 5 years ago in reply to Daniel Li14

Intellectual 325 points

Hi Daniel,

Thanks your quick reply.

The waveform seems great and reasonable. Can you indicate what's root cause on my waveform?

Thanks!

B.R.,

Hedy

0 Hedy Hung1 over 5 years ago in reply to Hedy Hung1

Intellectual 325 points

Hi Daniel,

What is your PGOOD and MODE configuration? Thanks.

Hedy

0 Daniel Li14 over 5 years ago in reply to Hedy Hung1

TI__Mastermind 27470 points

it is default setup. Mode=FPWM, PGOOD OPEN.

for the waveform you shows, even at 12V, the duty is not very stable, can you share the output part number, it is ceramic or AL?

it is better that you can share the layout files see anything improper.

Thanks

Daniel

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LM76003: FPWM perform not as expected