Today, the seemingly simple “hot-swap” operation isn’t as simple as it once seemed. It involves more than making the ground connection ahead of the hot connection. Linear Technology’s new LTC4222 dual Hot Swap controller, developed for boards with multiple load supply voltages ranging from 2.9 to 29 V and inrush currents large enough to create a glitch on the load supply, it’s intended to control external N-channel MOSFETs to limit those inrush currents. But there’s more to it than that. The device measures card voltages and currents and records past and present fault conditions for both channels. The system can query each LTC4222 over its I2C bus periodically and read status and measurement information.
In basic protection mode, several conditions must be present before the external MOSFET for a given channel turns on. First the external supplies, VDDn, must exceed their 2.44V undervoltage lockout levels. Next an internally generated supply, must cross its 2.64V undervoltage threshold. This generates a power-on-reset pulse. During reset, fault registers are cleared and control registers are set or cleared.
During start-up, inrush currents are safely controlled using current foldback, Soft Start and dI/dt-limiting techniques. A 5% accurate circuit breaker threshold minimizes both power loss and overdesign of power supply requirements. Load currents are then monitored using the voltages sensed across current sense resistors and controlled by adjusting the MOSFET Gate-to-Source voltages accordingly without the need of external gate components. The LTC4222 is offered in commercial and industrial versions, supporting operating temperature ranges from 0°C to 70°C and -40°C to 85°C, respectively. The devices are priced starting at $5.85 each in 1,000 piece quantities and are available today in production quantities.