LM73605: abnormal SW waveform

Jiawei Hu1

Expert 5625 points

Part Number: LM73605

Hi team,

The SW waveform of LM73605EVM_5V_400K is abnormal when I made the following changes.

R2 changed from 100k to 52k and the output became 3.07V
add 4.7R in series to boot cap C7

As the input voltage increases to 20V, the SW waveform is abnormal as follows.

Could you tell me why this happened? Thanks in advance.

Best Regards

Jiawei

over 6 years ago

0 David Baba over 6 years ago

TI__Mastermind 40060 points

Hello Jiawei,

Please can you follow the recommendations in table 3 of the datasheet for your configuration. I would try the following additional modifications. L = 4.7uH, Cout = 200uF.

Also check that your load current is not at a level that causes the current limit of the device to be violated?

Hope this helps?

0 Jiawei Hu1 over 6 years ago in reply to David Baba

TI__Expert 5625 points

Hi David,

Thanks for your feedback. I have another question.

Is the internal bootstrap circuit of the LM73605 as shown below?

Best Regards

Jiawei

0 David Baba over 6 years ago in reply to Jiawei Hu1

TI__Mastermind 40060 points

Hello Jiawei,

Referring to your left hand diagram, you are correct. During the conduction period of the LSF, the Boot cap charge is refreshed.

Referring to the right hand drawing current flows from the boot cap to Ho out as you have indicated. However, current does not flow directly through the gate drive circuit to the Vsw [Hs].

Hope this helps?

0 Dongbao Zhou over 6 years ago in reply to David Baba

TI__Expert 4170 points

Hi David,

May I know the reason why Rboot affect the operating of LM73605?

All the test results are under no-load condition, usually, the recommended range for Rboot is 0ohm-10ohm. However, abnormal waveforms are captured when Rboot is more than 4.7ohm in LM73605 EVM. What's the recommended range of Rboot?

Similiar question http://e2e.ti.com/support/power-management/f/196/p/823504/3050772?tisearch=e2e-quicksearch&keymatch=lm73606%20boot%20resistor#3050772, but it doesn't talk too much how Cboot-uvlo affect the switching.

Thanks

Dongbao

test results.docx