Datasheet

LMZ22010
SNVS687G –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
OUT
to fall.
CURRENT LIMIT
The LMZ22010 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 13A (typical) 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 duty cycle as
illustrated in the graph in the Typical Performance Characteristics section. The HS current limit monitors the
current of top side MOSFET. Once HS current limit is detected (16A 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.
THERMAL PROTECTION
The junction temperature of the LMZ22010 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
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 (typ Hyst = 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.
PRE-BIASED STARTUP
The LMZ22010 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 1.8V
pre-bias rising to 3.3V. Trace three is the SS voltage with a C
SS
= 0.47uF. Risetime determined by C
SS
.
Figure 56. 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). When operating in DCM, inductor
current is maintained to an average value equaling Iout . 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.
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