Data Sheet

LT8418

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Rev 0

13 of 19

pull-down resistance, a strong 4A current sourcing, and 8A current sinking capability, which is suited to use to drive

a variety of GaN FETs with different QG or RDS(ON) values. All driver outputs, TGP and TGN, have default 500kΩ pull-

down resistance to SW node; BGP and BGN have default 500kΩ pull-down resistance to ground to prevent top and

bottom GaN FETs from false turn-on.

APPLICATIONS INFORMATION

The front page shows a typical LT8418 application circuit. This section serves as a guideline for selecting external

components for typical applications.

1.1. Selecting the VCCand BST Capacitor

The LT8418 can support a wide range of switching frequencies. Thus, the VCC and BST capacitors must be selected

properly based on the system switching frequency, QG, of GaN FETs.

The bypass capacitor CVCCprovides the gate charge for the top-side and bottom-side GaN FETs. The charge to turn on

the external GaN FET is referred as gate charge, QG, and is typically specified in the external GaN FET data sheet. Gate

charge depends on the gate drive level and external GaN FET, ranging from 1nC to tens of nC. The required bypass

capacitance can be approximated as:

CVCC >

Q

GT + QGB

∆V

(1)

where:

Q

GT and QGB are the gate charge of the top-side and bottom-side GaN FETs, respectively.

∆V

is the maximum allowable voltage drop across CVCC.

An external boost capacitor, CBST, connected between BST and SW, supplies the gate driver voltage for its respective

GaN FET driver. To turn on the external GaN FET, the driver places the CBST voltage across the gate and source of the

GaN FET. The CBST capacitance must have at least ten times the gate capacitance to fully turn on the external GaN

FET. For most applications, a capacitor value of 0.1uF for CBST is sufficient. However, if multiple GaN FETs are

paralleled and driven by the LT8418, the CBST capacitance must be increased correspondingly and should be

maintained in the following relationship:

CBST >

10Q

GT,TOTAL

1V

(2)

A BST switch is integrated into the LT8418 to keep charging the CBST, eliminating the requirement of the external VCC-

to-BST Schottky diode. An additional diode from VCC to BST may cause the overcharge of the BST rail and gate

damage of high side GaN FET. When the BG/TG is low, the total current from VCC is typically 250μA; when the BG/TG

is switching at 400kHz with a 1nF capacitance load at driver outputs, the total current from VCC is typically 2.4mA.

1.2. Selecting the Gate Resistance

The LT8418 provides split gate drivers for top and bottom GaN power switches. A typical gate resistor up to 5.6Ω can

be added to TGP/TGN and BGP/BGN pins to adjust the turn-on/turn-off rate of two GaN FETs for electromagnetic

interference (EMI) and efficiency optimization.

1.3. Power Dissipation

The major power dissipation on the LT8418 is the sum of the power loss on the gate driver and the BST switch. The

gate driver loss is caused by charging and discharging the gate capacitance of the GaN FETs. Because the gate loss