LM5050-1: Wondering if Vgs voltage is too low

Ann_Lien

Part Number: LM5050-1

Hi expert,

My customer uses LM5050-1 in their USB-C / Adapter Oring application. And they tested on their mother board and result is shown below. We can know Vgs(Vgate-Vin) would be 3V maximum. We are wondering if it is too low causes Rdson too high. Or this test result makes sense, could you please kindly let us know why it would be?

They tested with below schematic without the red circle resistor. Another question is if we can place pull down resistor at GATE pin? Thanks a lot!!

	S0	Full Load	S5
Watt	15w	63w	>15W
Vin	19.86V	19.9V	19.86V
Vg	20.4V	23.08V	21.77V
VSD	0.44V	0.023V	0.022V

Best regards,

Ann Lien

over 5 years ago

0 Ann_Lien over 5 years ago

TI__Genius 10640 points

Also have one question, is gate floating when Vin is floating? Thansk a lot!!

0 Karikalan Selvaraj over 5 years ago in reply to Ann_Lien

TI__Genius 16305 points

Hi Ann Lien,

Having a resistor from Gate of MOSFET to GND will take away the gate charge current.
Gate charge current is 30uA for LM5050 device and now if you have a 150k resistor, with 20V VIN it can leak 133uA.
Gate may not be even turned on if you connect the 150k resistor.

Measurements taken after removing the 150k resistor are all correct for the MOSFET PE5E6BA N-MOSFET.

When Vin is left floating, Gate is has an internal clamping diode from VIN to Gate, with Anode connected to Vin and Cathode connected to GATE.
This will not allow Gate's voltage beyond VIN.

Regards,
Kari.

0 Ann_Lien over 5 years ago in reply to Karikalan Selvaraj

TI__Genius 10640 points

Hi Karikalan,

Let me confirm again. We tested without pull down resistor with GATE and result is shown as previous thread. Gate-Source voltage is around 1V~3V and it is normal . Am I right? Thanks a lot!!

Best regards,
Ann Lien

0 Ann_Lien over 5 years ago in reply to Ann_Lien

TI__Genius 10640 points

Hi Karikalan,

May i have your update? Thanks a lot!!

Best regards,
Ann Lien

0 Karikalan Selvaraj over 5 years ago in reply to Ann_Lien

TI__Genius 16305 points

Hi Ann Lien,

Yes, at 63W full load, load current is 3.15A. At this current with MOSFET PE5E6BA, we expect 4V of Vgs.
At 15W load, we expecet 3.75V of Vgs (load current 0.75).

MOSFET seems to oversized for this application and that is why we see low gate voltage even at max load current.

Regards,
Kari.

0 Karikalan Selvaraj over 5 years ago in reply to Karikalan Selvaraj

TI__Genius 16305 points

Hi Ann Lien,

I would like to close the thread now as we are understand why the gate voltage is low.
Please restart the thread if you have more questions.

Regards,
Kari.

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LM5050-1: Wondering if Vgs voltage is too low