The TPS4022 device has overcurrent fault and warning thresholds for each channel which can be independently set, when operating in dual-output mode. When operating in two-phase mode, both channels share the same overcurrent fault and warning thresholds. The overcurrent thresholds are set via the PMBus interface using the IOUT_OC_FAULT_LIMIT and IOUT_OC_WARN_LIMIT commands. (See the Section 8.5.1 section for more details.)

The device generates an internal voltage corresponding to the desired overcurrent threshold, using the IOUT_OC_FAULT_LIMIT threshold and the IOUT_CAL_GAIN setting, and adjusting for temperature using the measured external temperature value. The current sense amplifier amplifies the sensed current signal with a fixed gain of 15 and then compares that value to this internal voltage threshold. The device uses a similar structure to activate an overcurrent warning based on the IOUT_OC_WARN_LIMIT threshold.

Figure 8-4 Overcurrent Protection

The programmable range of the overcurrent fault and warning voltage thresholds places a functional limit on the input voltage of the current sense amplifier. The minimum overcurrent fault and warning thresholds correspond to a voltage from CSxP to CSxN of 6 mV and 4.7 mV, respectively. If the voltage across these pins does not exceed the minimum thresholds, then overcurrent fault and warning cannot be tripped, regardless of the setting of IOUT_OC_FAULT_LIMIT and IOUT_OC_WARN_LIMIT. There is also maximum overcurrent fault and warning thresholds corresponding to a voltage from CSxP to CSxN of 60 mV and 59 mV, respectively. If the voltage across these pins exceeds this maximum threshold, the overcurrent fault or warning will be tripped, regardless of the setting of IOUT_OC_FAULT_LIMIT and IOUT_OC_WARN_LIMIT. The result is that for higher values of inductor DCR, a resistor across the current sensing capacitor may be required to create a voltage divider into the current sensing inputs.

The TPS4022 device implements cycle-by-cycle current limit when the peak sensed current exceeds the set threshold. In a time constant matched current sensor network, the signal across the CSxP and CSxN pins has both dc and ac inductor current information, so an overcurrent fault trips when the dc current plus half of the ripple current exceeds the set threshold. When the time constant is not well-matched, the dc current which trips the overcurrent changes accordingly.

When the controller counts three consecutive clock cycles of an overcurrent condition, the high-side and low-side MOSFETs are turned off and the controller enters hiccup mode or latches the output off, depending on the IOUT_OC_FAULT_RESPONSE register. In continuous restart hiccup mode, after seven soft-start cycles, normal switching is attempted. If the overcurrent has cleared, normal operation resumes; otherwise, the sequence repeats.