In high-frequency switching power supply applications where high-current gate drivers such as the UCC27284 are used, parasitic inductances, parasitic capacitances and high-current loops can cause noise and ringing on the gate of power MOSFETs. Often external gate resistors are used to damp this ringing and noise. In some applications the gate charge, which is load on gate driver device, is significantly larger than gate driver peak output current capability. In such applications external gate resistors can limit the peak output current of the gate driver. it is recommended that there should be provision of external gate resistor whenever the layout or application permits.

Use Equation 9 to calculate the driver high-side pull-up current.

Equation 9.

where

• I_OHH is the high-side, peak pull-up current
• V_DH is the bootstrap diode forward voltage drop
• R_HOH is the gate driver internal high-side pullup resistor. Value either directly provided in datasheet or can be calculated from test conditions (R_HOH = V_HOH/I_HO)
• R_GATE is the external gate resistance connected between driver output and power MOSFET gate
• R_GFET(int) is the MOSFET internal gate resistance provided by MOSFET data sheet.

Use Equation 10 to calculate the driver high-side sink current.

Equation 10.

where

• R_HOL is the gate driver internal high-side pulldown resistance

Use Equation 11 to calculate the driver low-side source current.

Equation 11.

where

• R_LOH is the gate driver internal low-side pullpullup resistance

Use Equation 12 to calculate the driver low-side sink current.

Equation 12.

where

• R_LOL is the gate driver internal low-side pulldown resistance

Both high and low-side channels of the gate driver have a peak current rating of ±3 A. These equations help reduce the peak current if needed. To establish different rise time value compared to fall time value, external gate resistor can be anti-paralleled with diode-resistor combination as shown in Section 8.2. Generally selecting an optimal value or configuration of external gate resistor is an iterative process. For additional information on selecting external gate resistor please refer to External Gate Resistor Design Guide for Gate Drivers.