The OCP circuit monitors the LMG341xR150 drain current and compares that current signal with an internally-set limit. Upon detection of an overcurrent condition, the family of GaN FETs has two optional protection actions.
LMG3410R150 device provides the latched OCP option, by which the FET is shut off and held off until the fault is reset by either holding the IN pin low for more than 350 microseconds (tRESET) or removing power from VDD. The timing sequence is shown in Figure 17.
LMG3411R150 device provides the cycle-by-cycle OCP option. In this mode, the GaN FET is shut off and held off when overcurrent happens but the output fault signal will clear after the input PWM goes low. In the next cycle, the FET can turn on as normal. The cycle-by-cycle function can be used in cases where steady state operation is below the OCP level but transient response can still reach high current, while the circuit operation cannot be paused. It function also prevents the GaN device from overheating by having overcurrent induced conduction loss.
During cycle-by-cycle operation, after the current reaches the upper limit but the PWM input is still high, the load current can flow through the third quadrant of the other FET of a half-bridge with no synchronous rectification. The extra high negative voltage drop (–6 V to –8 V) from drain to source could lead to high third quadrant loss, similar to dead time loss but with much longer time. An operation scheme of cycle-by-cycle current limitation is shown as Figure 18.Therefore, it is critical to design the control scheme to make sure the number of switching cycles in cycle-by-cycle mode is limited, or to change PWM input based on the fault signal to shorten the time in third quadrant conduction mode of the GaN device.
OCP circuit has a 55-ns (typical) blanking at slew rate of 100 V/ns to prevent false triggering during switch node transitions. The blanking time increases with respect to lower slew rates accordingly since lower slew rates results in longer switching transition time. This fast response OCP circuit protects the GaN device even during a hard short-circuit condition.