For a buck converter, during the on-time of the high-side FET, the switch current increases at a linear rate determined by input voltage, output voltage, the on-time, and the output inductor value. During the on-time of the low-side FET, this current decreases linearly. The average value of the switch current equals to the load current.

The output overcurrent limit (OCL) in the TPS548B28 device is implemented using a cycle-by-cycle valley current detect control circuit. The inductor current is monitored during the on-time of the low-side FET by measuring the low-side FET drain-to-source current. If the measured drain-to-source current of the low-side FET is above the current limit threshold, the low-side FET stays ON until the current level becomes lower than the current limit threshold. This type of behavior reduces the average output current sourced by the device. During an overcurrent condition, the current to the load exceeds the current to the output capacitors, thus the output voltage tends to decrease. Eventually, when the output voltage falls below the undervoltage-protection threshold (80%), the UVP comparator detects it and shuts down the device after a wait time of 68 µs. The device then enters a hiccup sleep period for approximately 14 ms. After this waiting period, the device attempts to start up again. Figure 7-4 shows the cycle-by-cycle valley current limit behavior as well as the wait time before the device shuts down.

If an OCL condition happens during start-up, the device still has cycle-by-cycle current limit based on low-side valley current. After soft start is finished, the UV event which is caused by the OC event shuts down the device and enters hiccup mode mode with a wait time of 68 µs.

The resistor, R_TRIP connected from the TRIP pin to AGND sets current limit threshold. A ±1% tolerance resistor is highly recommended because a worse tolerance resistor provides less accurate OCL threshold. Equation 4 calculates the R_TRIP for a given overcurrent limit threshold on the device. To simplify the calculation, use a constant, K_OCL, to replace the value of 12x10⁴. Equation 4 calculates the overcurrent limit threshold for a given R_TRIP value. The tolerance of K_OCL is listed in Section 6.5 to help you analyze the tolerance of the overcurrent limit threshold.

To protect the device from unexpected connection on TRIP pin, an internal fixed OCL clamp is implemented. This internal OCL clamp limits the maximum valley current on LS FET when TRIP pin has too small resistance to AGND, or is accidentally shorted to ground.

where

• I_OCLIM is overcurrent limit threshold for load current in A
• R_TRIP is TRIP resistor value in Ω
• K_OCL is a constant for the calculation
• V_IN is input voltage value in V
• V_O is output voltage value in V
• L is output inductor value in µH
• f_SW is switching frequency in MHz

Equation 5.

where

• I_OCLIM is overcurrent limit threshold for load current in A
• R_TRIP is TRIP resistor value in Ω
• K_OCL is a constant for the calculation
• V_IN is input voltage value in V
• V_O is output voltage value in V
• L is output inductor value in µH
• f_SW is switching frequency in MHz

Figure 7-4 Overcurrent Protection