The LM6x4xx-Q1 is protected from overcurrent conditions by cycle-by-cycle current limiting on both the high-side and the low-side MOSFETs.

High-side MOSFET overcurrent protection is implemented by the nature of the peak current mode control. The HS switch current is sensed when the HS is turned on after a short blanking time. The HS switch current is compared to the minimum of a fixed current setpoint, or the output of the voltage regulation loop minus slope compensation, every switching cycle. Since the voltage loop has a maximum value and slope compensation increases with duty cycle, the HS current limit decreases with increased duty cycle if duty cycle is above 35%. See Figure 8-15.

Figure 8-15 Maximum Current Allowed Through the HS FET - Function of Duty Cycle for LM62460-Q1

When the LS switch is turned on, the current going through it is also sensed and monitored. Like the high-side device, the low-side device turn-off is commanded by the voltage control loop. For a low-side device, turn-off is prevented if current exceeds this value, even if the oscillator normally starts a new switching cycle. See Section 8.4.3.4. Also like the high-side device, there is a limit on how high the turn-off current is allowed to be. This is called the low-side current limit; see the Electrical Characteristics for values. If the LS current limit is exceeded, the LS MOSFET stays on and the HS switch is not turned on. The LS switch is turned off once the LS current falls below its limit. 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 8-16 Current Limit Waveforms

The net effect of the operation of high-side and low-side current limit is that the IC operates in hysteretic control. Since the current waveform assumes values between I_L-HS and I_L-LS, output current is close to the average of these two values unless duty cycle is very high. Once operating in current limit, hysteretic control is used and current does not increase as output voltage approaches zero.

If duty cycle is very high, current ripple must be very low to prevent instability; see Section 9.2.2.3. Since current ripple is low, the part is able to deliver full current. The current delivered is very close to I_L-LS.

Under most conditions, current is limited to the average of I_L-HS and I_L-LS, approximately 1.4 times the rated current. If input voltage is low, current can be limited to approximately I_L-LS. Current does not exceed the average of I_L-HS and I_L-LS as output drops to 0.4 times the output voltage setting. Below 0.4 times the output voltage setting, the peak current does not exceed the average of I_L-HS and I_L-LS and the hiccup mode activates, preventing excessive heating.

Figure 8-17 Output Voltage versus Output Current

Once the overload is removed, the device recovers as though in soft start; see Section 8.3.11. Note that hiccup can be triggered if output voltage drops below approximately 0.4 times the intended output voltage.