LMZ14202H_15 Datasheet, PDF (12/31 Pages) Texas Instruments – LMZ14202H SIMPLE SWITCHER® 6V to 42V, 2A High Output Voltage Power Module

LMZ14202
SNVS648J – JANUARY 2010 – REVISED OCTOBER 2015
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Feature Description (continued)
7.3.3 Current Limit
Current limit detection is carried out during the OFF-time by monitoring the current in the 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 2.6 A (typical) the
current limit comparator disables the start of the next ON-time period. The next switching cycle occurs only if the
FB input is less than 0.8 V and the inductor current has decreased below 2.6 A. Inductor current is monitored
during the period of time the synchronous MOSFET is conducting. While the inductor current exceeds 2.6 A,
further ON-time intervals for the top MOSFET do not occur. Switching frequency is lower during current limit due
to the longer OFF-time.
NOTE
Current limit is dependent on both duty cycle and temperature as illustrated in the graphs
in Typical Characteristics section.
7.3.4 Thermal Protection
Do not allow the junction temperature of the LMZ14202 device 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_O to fall, and
additionally the CSS capacitor is discharged to ground. Thermal protection helps prevent catastrophic failures for
accidental device overheating. When the junction temperature returns to below 145 °C (typical hysteresis = 20
°C) the SS pin is released, V_O rises smoothly, and normal operation resumes.
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 LMZ14202 starts 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. Figure 31
is a scope capture that shows proper behavior during this event.
OUTPUT
VOLTAGE
2V PRE-BIAS
3.3V OUTPUT
1.0 V/Div
OUTPUT
CURRENT
1.0 A/Div
ENABLE
2.0 V/Div
1 ms/Div
Figure 31. Prebiased Start-Up
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