LM5121: LM5121 Switching not happens

Manikumar s

Prodigy 30 points

Part Number: LM5121

Hi,

We are using LM5121- Synchronous Boost converter in one of our application.

Details On converter design are given below.

Input Voltage : 16V to 36V (Capable to source Maximum of 30A)

Output voltage : 40V / 9A

Switching Frequency : 600KHz.

LO FET was driven by gate driver IC UCC21520ADW. Since it's Vgs should not go above 7V.

Below are the testing observation:

3V UVLO voltage, 7.6V VCC pin voltage and DG to DS voltage was 15V.

We get a output voltage which follows the dropout of input voltage. Power switching not happened.

Please go through the below schematic, and provide feedback if any issue is there?

Thanks and Regards,

Manikumar

over 4 years ago

0 Yinsong Cai1 over 4 years ago

TI__Expert 7450 points

Hi Manikumar,

Thank you for using LM5121.

Could you please confirm the operating condition that it runs into this issue? Does it happen at certain input or the full range 16 to 36V? Does it happen at 9A or light load as well?

Thanks,

Yinsong

0 Manikumar s over 4 years ago in reply to Yinsong Cai1

Prodigy 30 points

Switching and softstart not happens and HO FET operating in diode mode. So we have not loaded.This issue happens at No load and at full input range (16-36V).

0 Manikumar s over 4 years ago in reply to Manikumar s

Prodigy 30 points

Hi Yinsong,

Above mentioned issue not happens when we probing DG vs DS pin by scope probe(PR500B).

You have told that if Disconnection switch not used , its DG and DS can left open refer below link.

LINK: https://e2e.ti.com/support/power-management/f/196/p/904093/3342298?tisearch=e2e-sitesearch&keymatch=LM5121#3342298

But while probing DG vs DS pin only my boost converter (LM5121) provides 40V output when its input voltage was 28V and If i not probed DG vs DS pin then my output voltage was 28V.

Do I need to connect any passive element between DG to DS pin if I not used Disconnection switch?

Please provide suitable solution to solve this issue.

0 Yinsong Cai1 over 4 years ago in reply to Manikumar s

TI__Expert 7450 points

Hi Manikumar,

You mentioned that DG to DS is 12V, which is above the VGS detection threshold so it should be able to pull up the SS. Could you please check

1. SS voltage

2. FB voltage

Thanks,

Yinsong

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LM5121: LM5121 Switching not happens