LM5119: Synchronous Buck "Freewheel" FETs must have Qgd/Qgs <1?

Andrew Massey

Part Number: LM5119
Other Parts Discussed in Thread: PMP10260, PMP20818, CSD18534Q5A

Hi,

It is said that Synchronous Buck "Freewheel" FETs must have Qgd/Qgs <1?

As such, please advise why ti.com has two reference designs of synch bucks, which violate the "Qgd/Qgs < 1 rule"?

PMP10260 uses BUK7Y6R0-60EX for bottom FET
www.ti.com/.../PMP10260

PMP20818 uses CSD18534Q5A as bottom FET
www.ti.com/.../PMP20818

..Both of these use bottom FETs with Qgd/Qgs > 1

Qgd/Qgs <1:

0 Chanakya Mandela 3 months ago

Hi Andrew,

Our team will get back to you.

Best Regards,
Chanakya.

0 Mahmoud Harmouch 2 months ago in reply to Chanakya Mandela

Hello,

the gate to source capacitance is higher than the gate top drain capacitance. The charge Q depends on both capacitance and voltage.

Where this rule "It is said that Synchronous Buck "Freewheel" FETs must have Qgd/Qgs <1?" coming from?

Regards

Power management forum