Figure 3-4 shows a state machine that best describes the TIDA-020065. Initially, the MCU boots up and enters precharge mode to charge the load capacitors for 15ms. After this 15ms period has expired, the system transitions over to active mode by pulling nLPM high.

Figure 3-4 TIDA-020065 State Machine

In active state, the main FET is being driven for loads up to 30A. S3 can be pressed to transition the system from active to low power mode. S2 can also be used to switch between the programmed time-current fuse characteristics.

The ADC peripheral of the MSPM01306 collects samples every 100μs to constantly monitor the output current in active mode. When an overload current is detected, the system promptly transitions from active to shutdown mode. Depending on how long the current pulse is active, and how high the pulse is, the software uses the programmed time-current characteristics to protect the wire harnesses and load for the current ranges shown in Figure 3-5.

Figure 3-5 TIDA-020065 Time-Current Characteristics

The software I²t algorithm replicates the behavior of an actual melting fuse; the output is shutdown before peak load transients or other overcurrents cross the wire current capability and damage the vehicle wiring. The I²t algorithm is best explained by Equation 10.

where

• I²t is the melting fuse constant
• I_Nom is the nominal current rating of the fuse channel in use
• t_Shutdown is the expected shutdown time of the fuse when there is an overcurrent detected

To better simulate a real fuse, is subtracted from the measurement every time an ADC sample is taken. This addition allows the smart fuse to operate at the Inom threshold and to avoid falsely shutting down due to normal load transients.

To bypass limitations with only being able to run the I²t algorithm up to the maximum monitorable current, which is 66A in this design, a fixed-delay shutdown threshold was added to shutdown the output at a fixed time up until the SC threshold. Although 4ms was chosen for the fixed-delay shutdown time for this design, set this time according to what current pulses are allowed in the system.

When either fuse shutdown or fixed-delay shutdown occurs, the system transitions over to cooldown mode, where INP is pulled low and the system is allowed to recover from the overload event. By default, the MSPM0L1306-Q1 automatically pulls INP high again in 4s to recover the output. Nonetheless, the software can be configured by pulling TP14 low for latch-off behavior in cooldown mode. The output now stays off indefinitely until user input is received by pressing either S2 or S3. Comparing this behavior to a melting fuse shows that melting fuses have to be replaced whereas this design allows for resettable overcurrent protection. In a more realistic application, the software considers the thermals of the vehicle wiring to make sure that enough time has elapsed to cool the wire harnesses down.

For immediate shutdown, the short-circuit protection feature of the TPS1213-Q1 is used. nFLT is pulled low to signal to the MSPM0L1306-Q1 that a hardware fault was detected. In this state, pressing S2 or S3 toggles INP which clears the SC protection latch of the TPS1213-Q1.