LMZ14202EXT Datasheet, PDF (16/21 Pages) Texas Instruments – 2A SIMPLE SWITCHER® Power Module with 42V Maximum Input Voltage for Military and Rugged Applications

LMZ14202EXT
Additional Features
OUTPUT OVER-VOLTAGE COMPARATOR
The voltage at FB is compared to a 0.92V internal reference. If FB rises above 0.92V the on-time is immediately terminated. This
condition is known as over-voltage protection (OVP). It can occur if the input voltage is increased very suddenly or if the output
load is decreased very suddenly. Once OVP is activated, the top MOSFET on-times will be inhibited until the condition clears.
Additionally, the synchronous MOSFET will remain on until inductor current falls to zero.
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 will occur only if the FB input is less than 0.8V and the inductor current has decreased below
2.6 A. Inductor current is monitored during the period of time the synchronous MOSFET is conducting. So long as inductor current
exceeds 2.6A, further on-time intervals for the top MOSFET will not occur. Switching frequency is lower during current limit due to
the longer off-time. It should also be noted that current limit is dependent on both duty cycle and temperature as illustrated in the
graphs in the typical performance section.
THERMAL PROTECTION
The junction temperature of the LMZ14202EXT should not be allowed to exceed its maximum ratings. Thermal protection is im-
plemented 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_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 falls back
below 145 °C (typ Hyst = 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.
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.
PRE-BIASED STARTUP
The LMZ14202EXT 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 during this event.
Pre-Biased Startup
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