3 Negative Hot-Swap Controller – TPS2352x

The TPS2352x is an advanced featured integrated hot-swap and OR-ing controller to deal with negative input voltage systems with stringent transient requirements. The TPS2352x provides intelligent control of the power supply connections during the insertion and removal of the systems from a live back-plane. TPS2352x provides in-rush current control to limit system voltage droop and transients, a programmable current limit, input insertion, and fault detection timer, and input over-voltage and under-voltage lockout levels and hysteresis. Additional features described in the following are implemented in the TPS2352x series to tackle common hot-swap design challenges.

• Dual Current Limit:
  The device has two current limit levels depending on the drain-to-source voltage (VDS) of the series pass FET. The current limit levels are programmable with the selection of a sense resistor and the VDS threshold for the current-limit switch-over is also programmable with an external resistor.

  Inevitably, the hot-swap design runs into a challenge with FET selection as the pass FET can exceed the safe operating area (SOA) region during startup when the VDS is high. The design requires a larger FET than the need of the normal operating condition. Two different current limit levels can address this conundrum by utilizing the SOA region of pass FET in the high VDS range. With a high VDS sensed from the pins during the startup, TPS2352x applies the lower current limit level, thereby effectively limiting the power across the pass FET. Once the output voltage level is stabilized and VDS becomes low, TPS2352x sets the higher current limit level to provide the power needed during normal operation but still protect the pass FET within the SOA region.

• Soft Start Disconnect:
  The device has a disconnection switch implemented between the soft-start capacitor and FET gate pin to make sure an agile operation if transients or short circuits are encountered.

  The use of a soft-start capacitor in hot-swap is to limit the inrush current into the output capacitor. The capacitor is tied to the gate of pass FET to limit the slew rate of gate voltage. However, the capacitor can interfere with the normal operation during transients, deteriorating the fast charge and discharge of gate voltage.

• Secondary Gate Drive:
  The device features a second hot-swap gate drive, which can save the boll of material (BOM) cost and size when multiple pass FETs are designed. The device drives the FET only during normal operating conditions where the main FET is enhanced, reducing the SOA requirement of the secondary FET.
• OR-ing:
  The device features integrated OR-ing that controls the external MOSFET in a way to emulates an ideal diode. TPS2352x regulates the forward drop across the OR-ing FET to 25mV by controlling the gate-to-source (VGS) voltage of the FET. The device blocks the reverse current if any system disorder happens and provides advantages in BOM cost and size as the design can save external OR-ing control devices.

For more detail and specific design examples, please refer Protecting Radio, Baseband and Active Antenna Systems with TPS2352x Hot Swaps.