Integrated FET-based hot swap designs, subsequently called eFuses, are commonly used in input power path protections for high-current enterprise server power supply designs because of improved power density and cost competitiveness. TI released the TPS25984, TPS25985 (both without PMBus® interface), and TPS25990 (with PMBus® interface) eFuses in the aforementioned market space.

In addition to providing several input power path protection features, these three eFuses allow the systems to monitor the output load current accurately by providing an analog current output (IMON) on the IMON pin. IMON is proportional to the current through the FET inside the eFuse. This IMON output is fed to the ISYS pin to implement PSYS™ in the VR14 controller, as shown in Figure 3-4.

Figure 3-4 PSYS Monitor Functionality Implemented Using TPS25984, TPS25985, or TPS25990 eFuses

The benefit of having a current output is that the signal can be routed across a board without adding significant errors due to voltage drop or noise coupling from adjacent traces. The current output also allows the IMON pins of multiple eFuse devices in a parallel configuration to be tied together to get the total current. As shown in Figure 3-5, the IMON signal is fed to the ISYS pin of the VR14 controller and is converted to a voltage by dropping it across a resistor (R_ISYS or R_IMON) close to the VR14 controller to implement the PSYS™. The current monitoring circuit inside these three eFuses is designed to provide high bandwidth (> 500 kHz) and high accuracy (<± 1.5%) across load and ambient temperature conditions, irrespective of board layout and other system operating conditions. This helps in achieving the required response time (10 μs) and error specifications (± 2%) for PSYS™ implementation. The analog current monitoring response of these eFuses is depicted in Figure 3-6 to demonstrate the large signal bandwidth of IMON. There is no need to add any external components to implement the PSYS monitor in the design.

Figure 3-5 Parallel Connection: TPS25984, TPS25985, and TPS25990 eFuses

Figure 3-6 Large-Signal Current Monitoring Bandwidth of TPS25984, TPS25985, or TPS25990 eFuses