Hot Swap and Digital Power Monitor Introduction

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Overview:
1 What is hot swap?
During hot swap, the board or module can be plugged in or unplugged from the powered power bus without affecting the overall system operation. This idea is likely to come from a telephone line card whose printed circuit board sockets need to be suitable for connectors of different lengths. When plugging a telephone line card into the backplane, always connect the ground first, then connect the power bus and I/O pins. This is sufficient for simple and robust electronic devices in public switched telephone networks. But today's routers, servers, and embedded systems often require a hot-swap controller, sense resistor, and FET trim, and need to provide a more robust, intelligent solution.

2 Which type of module requires a hot swap controller?
A hot-swap controller is required for a module with a large capacitance bypass capacitor connected to both ends of the input power line or a failure mode capable of reducing the performance of the main power bus. The closed-loop current limiting capability limits the initial charging current of the bypass capacitor, preventing the mains bus voltage from dropping when a new load is added (see Figure 1). It also prevents the power supply contact of the connector from being overloaded due to excessive instantaneous current.



Figure 1: A typical power-up waveform is presented here, using a linear current control loop that limits the surge (shock) current at startup when plugging a hot-swappable module into a powered backplane.
Once the input capacitor is charged, the current control loop can be used as a protection mechanism to limit any subsequent large currents that exceed the module design specifications and can be used to indicate a short or other fault. Therefore, when a large current exceeding the required current is detected, this method can relatively simply use the controller to turn off the module circuit, or to shut down the circuit until a manual reset or an automatic reset.
(Please read the PDF for details)