Datasheet

TPS54120
www.ti.com SBVS180C –JANUARY 2012–REVISED JUNE 2012
OVERCURRENT PROTECTION
SWITCHER OVERCURRENT PROTECTION
The integrated switcher of the TPS54120 is protected from overcurrent conditions by using cycle-by-cycle current
limiting on both MOSFETs, the low-side and the high-side.
HIGH-SIDE MOSFET OVERCURRENT PROTECTION
High-side MOSFET overcurrent protection is achieved by an internal current comparator that monitors the current
in the high-side MOSFET on a cycle-by-cycle basis. If this current exceeds the current limit threshold, the high-
side MOSFET is turned off for the remainder of that switching cycle. During normal operation, the device
implements current mode control. Current mode control uses the COMP pin voltage to control the turn-off of the
high-side MOSFET and the turn-on of the low-side MOSFET on a cycle-by-cycle basis. Each cycle, the switch
current and the current reference generated by the COMP pin voltage are compared. When the peak switch
current intersects the current reference, the high-side switch is turned off.
LOW-SIDE MOSFET OVERCURRENT PROTECTION
While the low-side MOSFET is turned on, its conduction current is monitored by the internal circuitry. During
normal operation, the low-side MOSFET sources current to the load. At the end of every clock cycle, the low-side
MOSFET sourcing current is compared to the internally set low-side sourcing current limit. If the low-side
sourcing current is exceeded, the high-side MOSFET is not turned on and the low-side MOSFET stays on for the
next cycle. The high-side MOSFET is turned on again when the low-side current falls below the low-side sourcing
current limit at the start of a cycle. The low-side MOSFET may also sink current from the load. If the low-side
sinking current limit is exceeded, the low-side MOSFET is turned off immediately for the rest of that clock cycle.
In this scenario, both MOSFETs remain off until the start of the next cycle.
If an output overload condition (as measured by the COMP pin voltage) has lasted longer than the current-limit
protection mode wait time (programmed for 512 switching cycles), the device shuts down and restarts after the
current-limit protection mode time (set for 16384 cycles). The current-limit protection mode helps to reduce
device power dissipation under severe overcurrent conditions
LDO INTERNAL CURRENT LIMIT
In addition to the switcher overcurrent protection, the TPS54120 has an internal current limit on the integrated
LDO. The LDO internal current limit helps protect the LDO during fault conditions. During current limit, the output
sources a fixed amount of current that is largely independent of output voltage. For reliable operation, the device
should not be operated in a current-limit state for extended periods of time. The PMOS pass element in the
integrated LDO has a built-in body diode that conducts current when the voltage at OUT exceeds the voltage at
LDOIN. This current is not limited, so if extended reverse-voltage operation is anticipated, external limiting may
be required.
THERMAL INFORMATION
The internal protection circuitry of the device has been designed to protect against overload conditions. However,
this circuitry was not intended to replace proper heat sinking. Continuously running the device into thermal
shutdown degrades device reliability. The TPS54120 has thermal protection for both the switcher and the LDO,
and they operate independently of each other.
THERMAL PROTECTION OF THE SWITCHER
The internal thermal-shutdown circuitry of the switcher forces the device to stop switching if the junction
temperature exceeds +175°C, typically. The device turns back on when the junction temperature drops below
+165°C typically.
THERMAL PROTECTION OF THE LDO
Thermal protection of the integrated LDO disables the output of the TPS54120 when the junction temperature
rises to approximately +160°C, allowing the device to cool. When the junction temperature cools to approximately
+140°C, the output circuitry is enabled. Depending on power dissipation, thermal resistance, and ambient
temperature, the thermal protection circuit may cycle on and off. This cycling limits the dissipation of the
regulator, protecting it from damage because of overheating.
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