LM7480-Q1: start up issue on fast turn ON/OFF

Ann_Lien

Part Number: LM7480-Q1

Hello expert,

Customer has a issue on fast turn ON/OFF. There is a chance that system turn on fail. Please see below waveform for different conditions and I will send schematic with you via mail. May I know if LM74800 has turn on limitation that we need to meet? Turn on delay or other requirement.

Issue : Below is the original design from LM74800, there is a chance that turn on will fail.(in red rectangle). We thought it might related to driving power so we change VCAP and remove soft start.

Step 1: remove soft start circuit and test. Result --> 19Vin would not see this turn on issue but there still chance to see this issue on 39.6V(higher Vin) condition.(we also make sure OV is not triggered)

Step 2:remove SS and change VCAP from 1uF to 2.2uF. Result --> issue is still there.

Step 3:remove SS and change VCAP from 2.2uF to 2.2uF*2.2uF. Result --> issue is still there.

Best regards,

Ann Lien

over 4 years ago

0 Praveen GD over 4 years ago

TI__Guru 57737 points

Hi Ann,

As you mentioned, please share the schematic on email. We can take it forward from there.

0 Ann_Lien over 4 years ago in reply to Praveen GD

TI__Genius 14430 points

Hello Praveen,

Thanks for your help and I've already sent schematic to you. Thanks a lot!

Best regards,

Ann Lien

0 Praveen GD over 4 years ago in reply to Ann_Lien

TI__Guru 57737 points

Thanks. Let me look into the schematic and get back to you.

0 Ann_Lien over 4 years ago in reply to Praveen GD

TI__Genius 14430 points

Hello Praveen,

May I know if you have any update for this issue? Thanks a lot!

Best regards,

Ann Lien

0 Praveen GD over 4 years ago in reply to Ann_Lien

TI__Guru 57737 points

Hi Ann,

Before the HGATE driver is enabled, following conditions must be achieved:

The EN/UVLO pin voltage must be greater than the specified input high voltage.
The CAP to VS voltage must be greater than the undervoltage lockout voltage.
Voltage at Vs pin must be greater than Vs POR Rising threshold.

Can you send us zoomed in waveforms at the instant when Gate voltage doesn’t ramp up.

Please measure the above mentioned 3 signals – EN/UVLO, VCAP-VS and VS and check if they are above theor respective thresholds.
Use a differential probe to measure the VCAP-VS voltage

Power management

Power management forum

LM7480-Q1: start up issue on fast turn ON/OFF