LM74700-Q1: When spec over source current to FET gate

Hi ACGUY,

The controller will limit Igate to 11mA (typ) which means it takes longer time to charge FET having higher gate capacitance.

Igate = Qgs/ dt.  => turn-ON time dt = Qgs/11mA.

For fast turn-on, please consider LM7480Q1 device.

Best Regards, Rakesh

Hello Rakesh,

Thank you for reply.
When I using low_Ron FET with LM74700, the LM74700 repeats source and sink on Gate-pin, the output voltage oscillates.
I inserted resister(18k-ohm ~ 47k-ohm) between to Gate-pin and FET Gate, the output voltage oscillation was suppressed.
I think it is because the peak current to the FET Gate is suppressed.

Please let me know your opinion.

Best regards,
ACGUY

Hi ACGUY,

What is the load during your test ?

Hello Rakesh,

Vin = 15V

Iout(max) = 30A

Using FET : BSC030N08NS5 (Infineon)

When Iout less than 1A, the output voltage oscillates.

Best regards,

ACGUY

Hi ACGUY,

Thanks for the details.

We should not see oscillations at 1A. What is the Cout ?

The gate resistor in the order of 18k-ohm ~ 47k-ohm is not recommended as it affects the turn-off response.

Best Regards, Rakesh

Hi Rakesh,

Cout is 1000uF (470uF * 2). And the circuit board has 10-cm cable on output. Do you have any concern for this composition?

Regards,

ACGUY

Hi ACGUY,

Is the load switching? The ripple might be due to a switching load through the cable ?

Can you share block diagram and schematics.

Can you share test waveform of load current, Vout, GATE and input voltages.

Hello Rakesh,

I correct the information. Its resistance is between FET_Gate and FET_Sourse.

Load current is constant. I attached waveform and schematic. 

The output voltage oscillates when the load current decreases.

Regards,

ACGUY

Hi ACGUY,

Is 0.1A the load current here ? Do you see similar ripple on Vin ?

Can you please share test waveform in a single scope-shot.

1. Load current, Vout, GATE and input voltages. include Math function (Vin-Vout)