LM74502: GATE to SRC Zener protect

James Panko

Part Number: LM74502

Tool/software:

Hi TI Team,

The LM74502, LM74502H SNOSDE5A datasheet revision A Section 9.3.3 suggests "An additional zener diode from GATE to SRC can be needed to protect the external FET in case output is expected to drop to the level where it can exceed external FET VGS Rating."

Assuming the External Fets have a VGS Max of +/-20V:

Could you explain what condition or case would cause the LM74502 Gate to Source voltage to exceed the external FET VGS rating?

What specification should the zener diode meet?

Regards,

Jim

over 1 year ago

0 Shiven Dhir over 1 year ago

TI__Mastermind 48475 points

Hi Jim,

You can select a 17-18V Zener for a 20V FET.

Regards,

Shiven Dhir

0 James Panko over 1 year ago in reply to Shiven Dhir

Prodigy 160 points

Hi Shiven,

Under what circumstances would the VGS be exceeded as mentioned in "Section 9.3.3 suggests "An additional zener diode from GATE to SRC can be needed to protect the external FET in case output is expected to drop to the level where it can exceed external FET VGS Rating."?

Look forward to your input.

Jim

0 Shiven Dhir over 1 year ago in reply to James Panko

TI__Mastermind 48475 points

Hi Jim,

In cases where VOUT drops suddenly, HGATE to VOUT might go high by the time HGATE responds to dropped VOUT. Here Zener can help by keeping it in safe VGS rating of the FET.

Regards,

Shiven Dhir

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LM74502: GATE to SRC Zener protect