The LMR3650x is protected from overcurrent conditions by cycle-by-cycle current limiting on both high-side and low-side MOSFETs.

High-side MOSFET overcurrent protection is implemented by the typical peak-current mode control scheme. The HS switch current is sensed when the HS is turned on after a short blanking time. The HS switch current is compared to either the minimum of a fixed current set point or the output of the internal error amplifier loop minus the slope compensation every switching cycle. Because the output of the internal error amplifier loop has a maximum value and slope compensation increases with duty cycle, HS current limit decreases with increased duty factor if duty factor is typically above 35%.

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 has a turn-off commanded by the internal error amplifier loop. In the case of the low-side device, turn-off is prevented if the current exceeds this value, even if the oscillator normally starts a new switching cycle. Also like the high-side device, there is a limit on how high the turn-off current is allowed to be. This limit is called the low-side current limit, I_VALMAX in Figure 8-8. If the LS current limit is exceeded, the LS MOSFET stays on and the HS switch is not to be turned on. The LS switch is turned off after the LS current falls below this 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 8-8 Current Limit Waveforms

Because the current waveform assumes values between I_PEAKMAX and I_VALMAX, the maximum output current is very close to the average of these two values unless duty factor is very high. After operating in current limit, hysteretic control is used and current does not increase as output voltage approaches zero.

If the duty factor is very high, current ripple must be very low to prevent instability. Because current ripple is low, the part is able to deliver full current. The current delivered is very close to I_VALMAX.

Figure 8-9 Output Voltage versus Output Current

Under most conditions, current is limited to the average of I_PEAKMAX and I_VALMAX, which is approximately 1.3 times the maximum rated current. If input voltage is low, current can be limited to approximately I_VALMAX. Also note that the maximum output current does not exceed the average of I_PEAKMAX and I_VALMAX. After the overload is removed, the part recovers as though in soft start.