LMZ12003EXT

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SNVS663H – JUNE 2010 – REVISED AUGUST 2015

Feature Description (continued)

NOTE

Current limit is dependent on both duty cycle and temperature as illustrated in the graphs

in the Typical Characteristics section

7.3.4 Thermal Protection

The junction temperature of the LMZ12003EXT 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

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 LMZ12003EXT will properly start up into a prebiased output. This startup situation is common in multiple rail

logic applications where current paths may exist between different power rails during the startup sequence. The

following scope capture shows proper behavior during this event.

Figure 20. Prebiased Start-Up

7.4 Device Functional Modes

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 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 at 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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