7.3.1 Synchronization Input

The PWM switching frequency can be synchronized to an external frequency source. The PWM switching will be in phase with the 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 314 kHz to 600 kHz. The typical input threshold is 1.4 V. Ideally, the input clock should overdrive the threshold by a factor of 2, so direct drive from 3.3-V logic via a 1.5-kΩ or less Thevenin source resistance is recommended. Applying a sustained logic 1 corresponds to 0-Hz PWM frequency and will cause the module to stop switching.

7.3.2 Current Sharing

When a load current higher than 8A is required by the application, the LMZ22008 can be configured to share the load between multiple devices. To share the load current between the devices, connect the SH pin of all current sharing LMZ22008 modules. One device should be configured as the master by connecting FB normally. All other devices should be configured as slaves by leaving their respective FB pins floating. The modules should be synchronized by a clock signal to avoid beat frequencies in the output voltage caused by small differences in the internal 359 kHz clock. If the modules are not synchronized, the magnitude of the ripple voltage will depend on the phase relationship of the internal clocks. The external synchronizing clocks can be in phase for all modules, or out of phase to reduce the current stress on the input and output capacitors. As an example, two modules can be run 180 degrees out of phase, and three modules can be run 120 degrees out of phase. The VIN, VOUT, PGND, and AGND pins should also be connected with low impedance paths. It is particularly important to pay close attention to the layout of AGND and SH, as offsets in grounding or noise picked up from other devices will be seen as a mismatch in current sharing and could cause noise issues.

Current sharing modules can be configured to share the same set of bulk input and output capacitors, while each having their own local input and output bypass capacitors. A C_{IN_BYP} >= 30 µF is still recommended for each module that is connected in a current sharing configuration. A C_{OUT_BYP} consisting of 47-nF X7R ceramic capacitor in parallel with a 22-µF ceramic capacitor is recommended to locally bypass the output voltage for each module. These capacitors will provide local bypassing of high frequency switched currents.

In a current sharing system using two or more modules, the slaves have their error amp circuitry disconnected. The master over-rides the error amplifier outputs of the slaves. This signal is then compared to each module’s individual current sense circuitry. Due to this, the current sense gain of the entire system increases according to the number of modules slaved to the master. To compensate for this and ensure good stability, the total output capacitance has to be increased. For example, two modules configured to provide 1.2 V_OUT and 16 amps have a required total bulk output capacitance of C_{OUT_BULK} = 2 × 450 µF (ESR 25 mΩ). This is a thirty six percent increase in the required output capacitance of a stand alone module. Up to 6 modules can be connected in parallel for loads up to 48 A. For more information on current sharing refer to AN-2093 (SNVA460).

Figure 45. Current-Sharing Example Schematic

Figure 46. Output Voltage Ripple of Two Modules With Synchronization Clocks in Phase

Figure 47. Output Voltage Ripple of Two Modules With Synchronization Clocks 180 Degrees Out of Phase

7.3.3 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_OUT to fall.

7.3.4 Current Limit

The LMZ22008 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 13 A (typical) 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 (16 A typical) , the HS MOSFET is shutoff immediately, until the next cycle. Exceeding HS current limit causes V_OUT to fall. Typical behavior of exceeding LS current limit is that f_SW drops to 1/2 of the operating frequency.

7.3.5 Thermal Protection

The junction temperature of the LMZ22008 should 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_OUT 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_OUT rises smoothly, and normal operation resumes.

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

7.3.6 Prebiased Start-Up

The LMZ22008 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. The following scope capture shows proper behavior in this mode. Trace one is Enable going high. Trace two is 1.8-V prebias rising to 3.3 V. Trace three is the SS voltage with a C_SS= 0.47 µF. Rise-time determined by C_SS.

Figure 48. 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). When operating in DCM, inductor current is maintained to an average value equaling I_OUT. In DCM the low-side switch will turn off when the inductor current falls to zero, this causes the inductor current to resonate. Although it is in DCM, the current is allowed to go slightly negative to charge the bootstrap capacitor.

In CCM, current flows through the inductor through the entire switching cycle and never falls to zero during the OFF-time.

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

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

Figure 49. CCM and DCM Operating Modes