Datasheet

LMZ22003

SNVS658H –MARCH 2011–REVISED OCTOBER 2013

www.ti.com

OUTPUT OVER-VOLTAGE PROTECTION

If the voltage at FB is greater than a 0.86V internal reference, the output of the error amplifier is pulled toward

ground, causing V

to fall.

CURRENT LIMIT

The LMZ22003 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 value, the current limit comparator disables the start of the next switching period. Switching cycles

are prohibited until current drops below the limit. It should also be noted that d.c. current limit is dependent on

both duty cycle as illustrated in the graph in the typical performance section. 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

to fall. Typical behavior of exceeding LS current limit is that f

drops to 1/2 of the operating frequency.

THERMAL PROTECTION

The junction temperature of the LMZ22003 should not be allowed to exceed its maximum ratings. Thermal

protection is implemented by an internal Thermal Shutdown circuit which activates at 165 °C (typ) causing the

device to enter a low power standby state. In this state the main MOSFET remains off causing V

to fall, and

additionally the C

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

accidental device overheating. When the junction temperature falls back below 150 °C (typ Hyst = 15°C) the SS

pin is released, V

rises smoothly, and normal operation resumes.

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

derating at elevated temperatures.

PRE-BIASED STARTUP

The LMZ22003 will properly start up into a pre-biased 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 in this mode. Trace one is Enable going high. Trace two is 1.5V

pre-bias rising to 3.3V. Rise-time determined by C

, trace three.

Figure 49. Pre-Biased Startup

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 Iout. Inductor current exhibits normal behavior for the

emulated current mode control method used. Output voltage ripple typically increases during this mode of

operation.

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

Product Folder Links: LMZ22003