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WHAT'S YOUR ENGINEERING QUOTIENT?

Problem 6—Johnny built a 12 V solenoid low-side driver using a low-threshold MOSFET driven by an output pin on a microprocessor. Later, testing determined that the driver was generating unacceptable noise levels. Johnny decided to increase the switching time of the MOSFET to roughly 0.5 V/ýS by adding a resistor in series with the MOSFET gate.

Typical MOSFET specs are Ciss = 1500 pf, Crss = 90 pf, Rdson <= 0.1 ohm, Gate Threshold Voltage (Vgsth) = 1.9 V.

The microcontrollerýs output pin high voltage (Voh) is 4.5 V

How did Johnny compute the gate resistor value?

Gate resistor Rg = (Voh-Vgsth) / Ig = (Voh-Vgsth) / (Crss × dVds/dt) =

(4.5 V ý 1.9 V) / (90×10^ý12 ý 0.5×10⁶) = 58k ohm

Keep in mind that Crss can vary considerably between devices, and lead capacitance or external capacitance between the drain and gate would have to be paralleled with Crss in this computation for a more exact value of Rg. An external capacitance between the drain and gate could be used to swamp Crss for a more precise turn-on time.