LM74700-Q1: How many pcs FET can be driven?

user4979989

Intellectual 2800 points

Part Number: LM74700-Q1

Hi team,

I would like to know how many pcs MOSFETs can be driven by LM74700-Q1.

The consideration should be base on inrush current flowing through FET body diode during startup.

Could you let me know there is any calculations need to use?

Here is what I am thinking, but this is also a little bit weird to me, based on my calculation as below, the more pcs FET get less inrush, this seems wrong.

Please help!

1. Calculate T(DRV_EN) in datasheet equation #1

2. Calculate turn on time = (Qg*FET#)/Igate

3. T(DRV_EN) + turn on time = Ttotal

4. Cout*Vout/Ttotal = inrush current?

5. Check if inrush current is lower than the spec shown in FET datasheet?

Thank you.

over 6 years ago

0 Karikalan Selvaraj over 6 years ago

TI__Genius 16305 points

LM74700-Q1 drives the MOSFET with its source pin as its input and drain as its output.

Please note this. Due to this we will not have inrush current, because body diode of the MOSFET will always conduct even when the MOSFET is off.

Regards,

Kari.

0 user4979989 over 6 years ago in reply to Karikalan Selvaraj

TI__Intellectual 2800 points

Hi Karikalan,

We connect enable to input, once input exists, the current flows through body diode to output before FETs are fully conducted, is it correct?

I would like to know if we need to consider the current and FET rating (body diode current spec), and also does this relate to gate driver sourcing strength and MOSFET parallel quantity?

Can LM74700 drive 4pcs MOSFETs? (FET PN: SQJ886EP)

How to calculate the current before FETs are conducted?

Thanks.

0 Karikalan Selvaraj over 6 years ago in reply to user4979989

TI__Genius 16305 points

Yes, before enable signal is active, if power is connected at the input, MOSFETs body diode would conduct until gate is turned on.

Gate drive maximum sourcing current is 11mA.

With 2.9nF of gate capacitance of SDJ886EP , it will take (2.9nF * 4 * 2.5V) / 11mA = 2.6us for 4 MOSFET connected in paralllel to turn ON.

Regards,

Kari.

0 user4979989 over 6 years ago in reply to Karikalan Selvaraj

TI__Intellectual 2800 points

Hi Karikalan,

Can we say in Oring device, for gate driver we don't need to consider parallel MOSFET quantity?

Thank you.

0 user4979989 over 6 years ago in reply to user4979989

TI__Intellectual 2800 points

Hi Karikalan,

Could you check my question?

Thanks.

0 Lokesh Ghulyani over 6 years ago in reply to user4979989

TI__Expert 8630 points

Hello,

The quantity MOSFETs add more capacitance at gate drive of LM74700 and turn-on time of MOSFETs will increase as gate current is 11mA. The current through body diodes of FETs cannot be controlled by LM74700 before the FETs are ON. It will be a function of source impedance, drop across the diode, input voltage and load impedance.

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LM74700-Q1: How many pcs FET can be driven?