Why is there a residual voltage present on the gate outputs of the LTC3869 when the controller is powered off?

Here are some common reasons why residual voltage might be observed on the gate outputs of the LTC3869 when the controller is off: 1. Gate Capacitance Holding Charge MOSFET gates have parasitic capacitance. When the controller turns off, this charge doesn't immediately dissipate, especially if there's no discharge path. This can leave a small voltage lingering on the gate. 2. Leakage Currents Even when the LTC3869 is off, small leakage currents from nearby components or through the MOSFETs themselves can cause a slight voltage to appear on the gate pins. 3. External Pull-Up or Bootstrap Components If the circuit includes pull-up resistors or bootstrap capacitors connected to the gate drive circuitry, they might maintain a small voltage on the gate even when the controller is off. If not addressed, residual voltage can falsely trigger the MOSFETs, causing them to turn on prematurely. To prevent this, add gate-to-source resistors (e.g., 10kΩ) to help discharge the gate when the driver is off. Adding a gate-to-source resistor on a MOSFET provides several important benefits, especially in power electronics and switching applications. Here's a breakdown of the key advantages of adding a gate-to-source resistor on the MOSFET: 1. Prevents Floating Gate When the gate is left floating (not actively driven), it can pick up noise or retain charge, unintentionally turning the MOSFET on. A resistor from gate to source ensures the gate is pulled to a known low voltage when the driver is off. 2. Discharges Gate Capacitance MOSFET gates have capacitance. When the gate driver turns off, the resistor helps discharge this capacitance, ensuring the MOSFET turns off completely and quickly. 3. Improves Noise Immunity In noisy environments, especially in switching power supplies, the gate can pick up transients. A gate-to-source resistor helps suppress these by providing a discharge path, reducing the risk of false triggering. 4. Enhances Safety During Startup or Faults If the gate driver is unpowered or fails, the resistor ensures the MOSFET remains off, preventing accidental conduction that could damage the circuit. 5. Supports Predictable Turn-Off Behavior The resistor defines a known RC time constant with the gate capacitance, allowing designers to control how quickly the MOSFET turns off, which can help manage EMI and switching losses.

Can the LTC3869 enter a cross-conduction state that causes shoot-through in the external MOSFETs?

The LTC3869 incorporates a fixed dead time of 30 nanoseconds between the switching of the top gate (TG) and bottom gate (BG) MOSFET drivers. This dead time is a critical design feature that ensures the high-side and low-side MOSFETs are never on simultaneously. By enforcing this non-overlapping switching interval, the controller effectively prevents cross-conduction, a condition where both MOSFETs conduct at the same time, which would otherwise result in a shoot-through current directly from the input supply to ground. This protection mechanism enhances the efficiency and reliability of the power stage by minimizing power loss and thermal stress on the MOSFETs.