The external gate-driver resistors, R_G(ON) and R_G(OFF) are used to:

1. Limit ringing caused by parasitic inductances and capacitances
2. Limit ringing caused by high voltage or high current switching dv/dt, di/dt, and body-diode reverse recovery
3. Fine-tune gate drive strength, specifically peak sink and source current to optimize the switching loss
4. Reduce electromagnetic interference (EMI)

The output stage has a pull up structure consisting of a P-channel MOSFET and an N-channel MOSFET in parallel. The combined peak source current is 4.5 A Use Equation 1 to estimate the peak source current as an example.

Equation 1.

where

• R_GON is the external turnon resistance.
• R_{GFET_Int} is the power transistor internal gate resistance, found in the power transistor data sheet. We will assume 0Ω for our example
• I_OH is the peak source current which is the minimum value between 4.5A, the gate-driver peak source current, and the calculated value based on the gate-drive loop resistance.
• V_GDF is the forward voltage drop for each of the diodes in series with R_GON and R_GOFF. The diode drop for this example is 0.7 V.

In this example, the peak source current is approximately 1.7A as calculated in Equation 2.

Equation 2.

Similarly, use Equation 3 to calculate the peak sink current.

Equation 3.

where

• R_GOFF is the external turnoff resistance.
• I_OL is the peak sink current which is the minimum value between 5.3A, the gate-driver peak sink current, and the calculated value based on the gate-drive loop resistance.

In this example, the peak sink current is the minimum of Equation 4 and 5.3A.

Equation 4.

The diodes shown in series with each, R_GON and R_GOFF, in Figure 9-1, Figure 9-2, and Figure 9-4 ensure the gate drive current flows through the intended path, respectively, during turn-on and turn-off. Note that the diode forward drop will reduce the voltage level at the gate of the power switch. To achieve rail-to-rail gate voltage levels, add a resistor from the V_OUT pin to the power switch gate, with a resistance value approximately 20 times higher than R_GON and R_GOFF. For the examples described in this section, a good choice is 100 Ω to 200 Ω.

The UCC23514S provides split output pins, OUTH and OUTL, which provide separate paths for turn-on and turn-off current. The series diodes are not necessary when this device option is used, as shown in Figure 9-3. For this case, substitute V_GDF = 0 V in the equations above. The UCC23514S provides rail-to-rail gate voltage levels without need for additional parallel resistors.