LM5148: EN hysteresis function 2

user5054474

Intellectual 450 points

Part Number: LM5148

Tool/software:

Hi,

This is a continuation of the thread below.

LM5148: EN hysteresis function - Power management forum - Power management - TI E2E support forums

I tried changing the resistance value of the EN resistive ladder using the LM5149-EVM, and got the attached results.

From these results, I surmise that setting the EN voltage low may cause unintended fluctuations.

You said that unintended fluctuations do not occur with the EVM you own, but have you ever changed the resistance value of the resistor ladder?

Please give it a try.

lm5148_en_hysteresis_function2.pdf

Please note that this measurement data was obtained at 48Vin.

If you check at a different voltage, the resistance value at which the symptoms are most likely to occur may be different.

Best regards,

7 months ago

0 David Baba 7 months ago

TI__Mastermind 40410 points

Hello,

The link you provided does not include the schematic, please provide me with the schematic. Thanks.

David.

0 user5054474 7 months ago in reply to David Baba

Intellectual 450 points

Hi,

We are currently in discussion with Taru Kapoor about this matter, and the circuit diagrams and board data have been sent via Private Message.

Best regards,

0 Taru Kapoor 7 months ago in reply to user5054474

TI__Genius 11995 points

Hi User,

I checked on the EVM with higher resistor values for Renb and Rent. (499kOhm and 105kOhm). I didn't observe any significant switching noise on the EN pin.

Green trace is output voltage (the output voltage is set to 11.5V) and yellow is the Enable Signal.

No load condition with 48V input.

Zoomed in Enable:

Startup:

I am guessing that either this is an issue with the measurement technique of the EN signal or the layout of your board.

Thanks,

Best Regards,

Taru

0 user5054474 7 months ago in reply to Taru Kapoor

Intellectual 450 points

Hi,

Have you looked at the pdf file (lm5148_en_hysteresis_function2.pdf)?

At 499kohm and 105kohm, no unintended fluctuations occur in my environment.

In my environment, the symptoms do not occur unless the EN voltage is low.

If Rent=499kohm, try Renb=15 to 16kohm.

Could you please try various other combinations of resistance values before replying?

Best regards,

0 user5054474 7 months ago in reply to user5054474

Intellectual 450 points

Hi,

How is the progress? Were you able to reproduce the symptoms?

Best regards,

0 Taru Kapoor 6 months ago in reply to user5054474

TI__Genius 11995 points

Hi,

I checked with 499kOhm and 22.1KOhm (This is the closest value to 15kOhm I have in my lab currently) - I do not see any switching noise being reflected on the EN pin of the EVM (whether that is at startup or during steady state).

This was captured at No Load and at ~50V input voltage.

user5054474 said:
In my environment, the symptoms do not occur unless the EN voltage is low.

I am not sure what you mean by this - If the EN voltage is low - the device will not turn ON.

If you mean the 'magnitude' of Enable voltage is low - then also i dont see any switching artifacts showing up on the EN pin. Please check your measurement setup - that seems to be the most likely root cause since I wasn't able to recreate the issue that you see on the EVM.

Thanks,

Best Regards,

Taru

0 user5054474 6 months ago in reply to Taru Kapoor

Intellectual 450 points

Hi,

The waveform you measured appears to be pulsating at about 400kHz.

I would like you to measure it in the PFM setting, but are you measuring it in FCCM operation?

Best regards,

0 user5054474 6 months ago in reply to user5054474

Intellectual 450 points

Hi,

- If the EN voltage is low -

This "Low" does not mean 0V.

lm5148_en_hysteresis_function3.pdf
As shown in the attached document, on my board, the symptoms occur when the EN terminal voltage is small, but do not occur when the EN terminal voltage exceeds 3V.