The device is protected from over current conditions by cycle-by-cycle current limiting on both high-side and low-side MOSFETs. High-side MOSFET over current 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 is called the low-side current limit, I_VALMAX in Figure 8-10. 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 8-10 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.

The LMQ664x0 employs hiccup over current protection if there is an extreme overload, and the following conditions are met:

• Output voltage is below approximately 0.4 times the output voltage set point.
• Greater than t_SS has passed since soft start has started.
• The part is not operating in dropout, which is defined as having a minimum off time controlled duty cycle.

In hiccup mode, the device shuts itself down and attempts to soft start after t_HICCUP. Hiccup mode helps reduce the device power dissipation under severe over current conditions and short circuits. See Figure 8-11.

After the overload is removed, the device recovers as though in soft start; see Figure 8-12.

Figure 8-11 Hiccup Entry

Figure 8-12 Hiccup Exit