slew rate of a mosfet

Question

Is there way to calculate slew rate of following circuit?

^{simulate this circuit – Schematic created using CircuitLab}

Datasheet for Transistor

Datasheet for Diode

I chose the PMOS and Diode arbitrarily from what I could choose from Circuit Lab.

If VTRIG goes from 5V to 0V, how can I calculate the slew rate (V/us)?

Answer 1

• A rough approximate is to use the RdsOn for -5V.
• It is guaranteed to be 0.3Ω but that is for -10V @25'C with 7.2A Pulse, width = 300 μs; duty cycle = 2 %.
• your situation is different and vague;
  • Initial conditions :
  • Vcap (unknown )
  • Cap part number (unknown ) and
  • trace inductance (unknown )
  • cap ESR or Dissipation Factor (unknown )
  • Diode ESR 1N4148 0.1W ~ 10ohm ( assumed to be larger than 1nF
  • Cap ESR which is expected to have ESR*C value=T < 0.1us for ceramic
  • The RdsOn starts at a high value due to Vdes =5V and Vgs=-5V and not being a "logic level" gate controlled FET requires a certain amount of calculations from the datasheet below

With the slew rate being dV/dt=Ic/C for the cap and Ic = (V+-Vcap)/(RdsOn+ESR(diode)) it becomes highly nonlinear.

But with initial conditions of ESR diode=10, and ESR or RdsOn of FET=10Ω then dropping to 0.34 Ω as Vds drops below 3V.

• Ic=C dV/dt=1e-9 * (5V)/(10Ω+10Ω) and C= 1e-9F for Vds=3~5V

slew rate dV/dt= Ic/C= 5V/20Ω*1e9= = 0.25V/ns

Low confidence in results due to inadequate info.