LMZ14203

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SNVS632R – DECEMBER 2009 – REVISED OCTOBER 2015

Feature Description (continued)

7.3.4 Thermal Protection

The junction temperature of the LMZ14203 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

additionally the CSS capacitor is discharged to ground. Thermal protection helps prevent catastrophic failures for

accidental device overheating. When the junction temperature falls back below 145°C (typical hysteresis = 20 °C)

Applications requiring maximum output current especially those at high input voltage may require application

derating at elevated temperatures.

7.3.5 Zero Coil Current Detection

The current of the lower (synchronous) MOSFET is monitored by a zero coil current detection circuit which

inhibits the synchronous MOSFET when its current reaches zero until the next ON-time. This circuit enables the

DCM operating mode, which improves efficiency at light loads.

7.3.6 Prebiased Start-Up

The LMZ14203 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 23 is a scope capture that shows proper behavior during this event.

Figure 23. Prebiased Start-Up

7.4 Device Functional Modes

7.4.1 Discontinuous Conduction and Continuous Conduction Modes

At light-load, the regulator operates in discontinuous conduction mode (DCM). With load currents above the

critical conduction point, it operates in continuous conduction mode (CCM). When operating in DCM the

switching cycle begins at zero amps inductor current; increases up to a peak value, and then recedes back to

zero before the end of the OFF-time. During the period of time that inductor current is zero, all load current is

supplied by the output capacitor. The next ON-time period starts when the voltage on the FB pin falls below the

internal reference. The switching frequency is lower in DCM and varies more with load current as compared to

CCM. Conversion efficiency in DCM is maintained because conduction and switching losses are reduced with

the smaller load and lower switching frequency.

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