8.2.2.5 Selecting Gate Resistor R_ON/R_OFF

Resistor R_ON and R_OFF are sized to achieve the following:

• Limit ringing caused by parasitic inductances and capacitances.
• Limit ringing caused by high voltage/current switching dV/dt, dI/dt, and body diode reverse recovery.
• Fine-tune gate drive strength to optimize switching loss.
• Reduce electromagnetic interference (EMI).

As mentioned in Output Stage, the UCC27712 has a pull up structure with a P-channel MOSFET providing a peak source current of 1.8A.

For this example 3.3-Ω resistors for R_ON and 2.2-Ω resistors for R_OFF were selected to provide damping for ringing and ample gate drive current.

Equation 6.

Therefore the peak source current can be predicted with:

Equation 7.

Equation 8.

where

• R_ON: External turn-on resistance
• R_{GFET_Int}: Power transistor internal gate resistance, found in the power transistor datasheet.
• I_O+ = Peak source current. The maximum values between 1.8 A, the UCC27712 peak source current, and the calculated value based on the gate drive loop resistance.

In this example:

Equation 9.

Equation 10.

Therefore, the high-side and low side peak source current is 1.6 A. Similarly, the peak sink current can be calculated with:

Equation 11.

Equation 12.

where

• R_OFF: External turn-off resistance
• V_DGATE: The diode forward voltage drop which is in series with R_OFF. The diode in this example is an MBRM130L.
• I_O- = Peak sink current. The maximum values between 2.8 A, the UCC27712 peak sink current, and the calculated value based on the gate drive loop resistance.

In this example:

Equation 13.

Equation 14.