7.3.1 Synchronization Input

The PWM switching frequency can be synchronized to an external frequency source. If this feature is not used, connect this input either directly to ground, or connect to ground through a resistor of 1.5-kΩ or less. The allowed synchronization frequency range is 650 kHz to 950 kHz. The typical input threshold is 1.4-V transition level. Ideally the input clock must overdrive the threshold by a factor of 2, so direct drive from 3.3-V logic through a 1.5-kΩ Thevenin source resistance is recommended. Applying a sustained logic 1 corresponds to zero Hz PWM frequency and will cause the module to stop switching.

7.3.2 Output Overvoltage Protection

If the voltage at FB is greater than a 0.86-V internal reference, the output of the error amplifier is pulled toward ground, causing V_O to fall.

7.3.3 Current Limit

The LMZ23603 is protected by both low-side (LS) and high-side (HS) current limit circuitry. The LS current limit detection is carried out during the OFF-time by monitoring the current through the LS synchronous MOSFET. Referring to the Functional Block Diagram, when the top MOSFET is turned off, the inductor current flows through the load, the PGND pin and the internal synchronous MOSFET. If this current exceeds the low-side current limit level the current limit comparator disables the start of the next switching period. Switching cycles are prohibited until current drops below the limit.

The HS current limit monitors the current of top side MOSFET. Once HS current limit is detected, the HS MOSFET is shutoff immediately, until the next cycle. Exceeding HS current limit causes V_O to fall. Typical behavior of exceeding LS current limit is that f_SW drops to 1/2 of the operating frequency.

7.3.4 Thermal Protection

The junction temperature of the LMZ23603 must not be allowed to exceed its maximum ratings. Thermal protection is implemented by an internal Thermal Shutdown circuit which activates at 165°C (typical) causing the device to enter a low power standby state. In this state the main MOSFET remains off causing V_O to fall, and additionally the C_SS capacitor is discharged to ground. Thermal protection helps prevent catastrophic failures for accidental device overheating. When the junction temperature falls back below 150°C (typical hysteresis = 15°C) the SS pin is released, V_O rises smoothly, and normal operation resumes.

Applications requiring maximum output current especially those at high input voltage may require additional derating at elevated temperatures.

7.3.5 Prebiased Start-Up

The LMZ23603 will properly start up into a prebiased output. This start-up situation is common in multiple rail logic applications where current paths may exist between different power rails during the start-up sequence. Figure 46 shows proper behavior in this mode. Trace one is Enable going high. Trace two is 1.5-V prebias rising to 3.3 V. Rise-time determined by C_SS, trace three.

Figure 46. Prebiased Start-Up

7.4.1 Discontinuous Conduction and Continuous Conduction Modes

At light load the regulator will operate in discontinuous conduction mode (DCM). With load currents above the critical conduction point, it will operate in continuous conduction mode (CCM). In CCM, current flows through the inductor through the entire switching cycle and never falls to zero during the OFF-time. When operating in DCM, inductor current is maintained to an average value equaling I_OUT. Inductor current exhibits normal behavior for the emulated current mode control method used. Output voltage ripple typically increases during this mode of operation.

Figure 47 is a comparison pair of waveforms of the showing both CCM (upper) and DCM operating modes.

V_IN = 12V, V_O = 3.3V, I_O = 3 A / 0.3 A 2 μs/div

Figure 47. CCM and DCM Operating Modes