The converter protects from overcurrent conditions with cycle-by-cycle current limiting on both the high-side and the low-side MOSFETs. High-side MOSFET overcurrent protection is implemented by nature of peak-current mode control. The HS switch current is sensed when the HS switch is turned on after a short blanking time. Every switching cycle, this switch current is compared to the minimum of a fixed current setpoint or the output of the voltage regulation loop minus slope compensation. Because the voltage loop output has a maximum value and slope compensation increases with duty cycle, the HS current limit decreases with increased duty cycle when the duty cycle is above 35%. See Figure 7-8.

Figure 7-8 HS Switch Maximum Current as a Function of Duty Cycle for the LM63460-Q1

When the LS switch is turned on, the switch current is also sensed and monitored. Like the HS device, the LS switch turns off as commanded by the voltage control loop and low-side current limit. If the LS switch current is higher than I_L-LS at the end of a switching cycle, the switching cycle is extended until the LS current reduces below the limit. The LS switch is turned off after the LS current falls below the limit, and the HS switch is turned on again as long as at least one clock period has passed since the last time the HS device has turned on.

Figure 7-9 Current Limit Waveforms

Because the current waveform assumes values between I_L-HS and I_L-LS, the maximum output current is very close to the average of these two values. Hysteretic control is used and current does not increase as output voltage approaches zero.

The converter employs hiccup overcurrent protection if there is an extreme overload, and the following conditions are met for 128 consecutive switching cycles:

• The output voltage is below approximately 0.4 times the output voltage setpoint.
• Greater than t_SS2 has passed since soft start has started; see Soft Start and Recovery from Dropout.
• The converter is not operating in dropout, which is defined as having minimum off time controlled duty cycle.

In hiccup mode, the device shuts down and attempts to soft start after t_W. Hiccup mode helps reduce the device power dissipation under severe overcurrent conditions and short circuits. See Figure 7-10. After the overload is removed, the device recovers as though in soft start; see Figure 7-11.

Figure 7-10 Inductor Current Bursts During Hiccup

Figure 7-11 Short-Circuit Recovery