The TPS92642-Q1 monitors the CSN voltage to detect output short circuit faults. A short failure is indicated by open drain FLT output when the CSN voltage drops below 1.5 V (typical). The device continues to regulate current and operate without interruption in case of short circuit. A short-circuit fault does not impact the device behavior. The device continues to operate and regulate current without interruption.

An LED open-circuit fault ultimately causes the output voltage to increase and settle close to the input voltage. When this event occurs, the TPS92642-Q1 switching operation is then controlled by the fixed on-time and minimum off-time resulting in a duty cycle close to 100%. The COMP pin voltage exceeds the COMP overvoltage threshold, V_COMP(OV), and the fault in indicated by FLT output. However, during open circuit, the dynamic behavior of the device and buck converter is influenced by the input voltage, V_IN, and the output capacitor, C_OUT, value. The device response to open circuit can be categorized into the following two distinct cases.

Case 1: For a Buck converter design with a small output capacitor, the switching operation in open load condition excites the tank resonance forcing the output voltage to oscillate. The frequency and amplitude of the oscillation are based on the resonant frequency and Q-factor of the second order tank network.

Figure 6-9 Open-Circuit Condition with Output Voltage Oscillation

Case 2: For a buck converter design with large output capacitor the inductor Q-factor and resonant frequency are much lower than the switching frequency. In this case, output voltage rises to input voltage and the converter continues to switch with minimum off-time.

Figure 6-10 Open-Circuit Condition with Minimum Off-Time Operation

The voltage transient imposed on CSP and CSN inputs during short circuit and open circuit is dependent on the output capacitance and is influenced by the cable harness impedance. The inductance associated with a long cable harness resonates with the charge stored on the output capacitor and forces CSP and CSN voltage to ring above VIN and below ground. The magnitude of the voltage overshoot above VIN and below ground are dependent on the parasitic cable harness inductance and resistance.

Figure 6-11 Cable Harness Parasitic Inductance

When using a long cable harness, TI recommends diodes to clamp the voltage across CSP and CSN input, as shown in Figure 6-12. TI recommends a low forward voltage Schottky diode or a fast recovery silicon diode with reverse blocking voltage rating greater than the maximum output voltage. The diode is required to be placed close to the output capacitor.

Figure 6-12 Transient Protection Using an External Diode