Upon enabling our device into a capacitive load, TPS272C45 defaults the current limit to 2 times ICL for a period of time programmed set by ILIMD. In Figure 10-4, TPS272C45 charges a 1-mF capacitor using the inrush current handling feature. During the first 4 mS after enabling the device, IOUT1 is 2 times the ICL programmed (4 A). After the 4-mS period, the current folds back to the programmed ICL (2 A).

Figure 10-4 TPS272C45 Capacitor Charging

If the device has a no-load case due to an open load or wire-break, the device registers the fault even in an off-state if the DIA_EN pin is high. Figure 10-5 shows the device behavior when an open load event is registered with EN low and DIAG_EN is raised. Systems can PWM DIA_EN to lower system power losses while still watching for open load events and the same timing applies.

Figure 10-5 Open-Load (t_OL) Detection Time

If the output of the TPS272C45 is short-circuited, the device protects the system from failure. Depending on R_ILIM, the current limit set-point varies. The waveforms below show examples of the current limit behavior when the device is enabled into a short circuit.

In Figure 10-6, the output is permanently shorted. Upon enabling the device, the current reaches the 2 times current limit is enabled for 6 mS set by ILIMD resistor. After the inrush current period, the current is reduced to the programmed current limit set by R_ILIM. In this case, because the power dissipation is low enough, the device is able to constantly act as a current source.

Figure 10-6 Enabling Into Short (R_ILIM = 10 k, V_S = 6 V, Delay = 6 mS)

In Figure 10-6, the output is also permanently shorted. Upon enabling the device, the current reaches the 2 times current limit, however because the power dissipation is greater due to the higher input voltage at V_S the device reaches its thermal shutdown threshold and disables itself before reaching the programmed delay time.

Figure 10-7 Enabling Into Short (R_ILIM = 10 k, V_S = 24 V, Delay = 6 mS)

Figure 10-8 shows that after the device has been enabled into a short and due to the high power dissipation, the device reaches its thermal shutdown threshold. The device then shuts down the FET for a period of tRETRY and re-enables the FET into the short. The capture shows the continuous retry cycle and protection because the short is permanently applied.

Figure 10-8 Permanent Short Behavior (R_ILIM = 10 k, V_S = 24 V, Delay = 6 mS)

In the event a short is applied to the output while a load is being driven, the device activates its fast trip comparator and shutdown the output to limit the inrush current. The device then immediately re-enables into the short and limit the current to the programmed current limit value. Figure 10-9 describes the behavior.

Figure 10-9 On-State Short Circuit (R_ILIM = 10 k, V_S = 24 V )