The LT8418 Integrated Circuit (IC), a recent launch by ADI at the APEC in California, is a unique component that features striking similarities to the output stage used in the newly launched GaN driving DC-DC controller ICs, namely, LTC7890 and LTC7891.
Unique Features of LT8418
The LT8418 IC stands out for its high-side and low-side outputs which have distinct pull-up and pull-down connections located on the IC package channels. This structure permits the use of additional resistors for fine-tuning the GaN transistor’s turn-on and turn-off times independently. This split gate drivers design is intended to allow customers to customize the on-off behaviour to effectively address control and flexibility issues.
Output Current Sourcing and Sinking
For both high-side and low-side drive, output current sourcing is achieved via a 0.6Ω pull-up (4A peak), while sinking is facilitated through a 0.2Ω (8A peak). This design ensures an optimized and balanced output current flow.
Over-voltage Protection
Given the sensitivity of GaN gates to over-voltage, which could cause damage slightly above the advised drive voltage, the high-side bootstrap is not via a traditional diode, but instead, a ‘smart-switch’. This smart bootstrap switch comprises power switches that can fully manage bootstrap charging or blocking, thus efficiently preventing overcharging and potential damage.
Bootstrap Switch and Under-voltage Lock-out Protection
The bootstrap switch activates to start charging the bootstrap capacitor when the bottom switch is turned on and the switching node voltage nears the ground level. The switch has a typical on-resistance of 6Ω, dropping 0.6V when 100mA is flowing into the bootstrap capacitor. Additionally, bootstrap under-voltage lock-out protection is included to prevent the GaN transistors from turning on with insufficient drive voltage, tripping at 3.1V and clearing at 3.35V (250mV hysteresis).
Package and Pin Configuration
The LT8418 is housed in a 1.7 x 1.7mm WLCSP BGA package, with two internally connected pads for the high-side output. One of these pads is designed for providing a low-inductance path to the bootstrap capacitor. The Vcc pin is also doubled, one for the power rail and one for the IC’s decoupling capacitor. Two input pins are provided, one for the high-side GaN PWM waveform and another for the low-side.
Considerations and Applications
Regarding the LT8418 applications, ADI has stated that it can be configured into synchronous half-bridge, full-bridge topologies, or buck, boost, and buck-boost topologies. However, it is important to note that no anti-over-lap protection is provided. Therefore, if both inputs are set to logic ‘1’, both GaN output transistors will be turned on simultaneously causing high current flow. Thus, a thoughtful approach must be applied to the input interface to avoid a possible shoot-through condition.
In summary, the LT8418 is a promising addition to the world of ICs, specifically designed for GaN driving DC-DC controller ICs. With its unique features and functionalities, it is poised to offer great flexibility and control to users. It is indeed a step forward in the realm of technology and innovation.
