9.1.1 Constant-Current and Impact on Output Voltage

During normal operation the constant-current device (TPS2553-Q1) has a load current that is less than the current-limit threshold and the device is not limiting current. During normal operation the N-channel MOSFET is fully enhanced, and V_OUT = V_IN – (I_OUT × r_DS(on)). The voltage drop across the MOSFET is relatively small compared to V_IN, and V_OUT ≈ V_IN.

During the initial onset of an overcurrent event, the constant-current device (TPS2553-Q1) limits current to the programmed current-limit threshold set by R_ILIM by operating the N-channel MOSFET in the linear mode. During current-limit operation, the N-channel MOSFET is no longer fully-enhanced and the resistance of the device increases. This allows the device to effectively regulate the current to the current-limit threshold. The effect of increasing the resistance of the MOSFET is that the voltage drop across the device is no longer negligible (V_IN ≠ V_OUT), and V_OUT decreases. The amount that V_OUT decreases is proportional to the magnitude of the overload condition. The expected V_OUT can be calculated by I_OS × R_LOAD, where I_OS is the current-limit threshold and R_LOAD is the magnitude of the overload condition. For example, if I_OS is programmed to 1-A and a 1-Ω overload condition is applied, the resulting V_OUT is 1 V.

The constant-current device (TPS2553-Q1) operates during the initial onset of an overcurrent event, if the overcurrent event lasts longer than the internal delay deglitch circuit (7.5-ms typical). The constant-current device (TPS2553-Q1) asserts the FAULT flag after the deglitch period and continues to regulate the current to the current-limit threshold indefinitely. In practical circuits, the power dissipation in the package will increase the die temperature above the overtemperature shutdown threshold (135°C minimum), and the device will turn off until the die temperature decreases by the hysteresis of the thermal shutdown circuit (10°C typical). The device will turn on and continue to thermal cycle until the overload condition is removed. The constant-current devices resume normal operation once the overload condition is removed.