Datasheet

SYNCClock=2V/div
PH=2V/div
Time=500nsec/div
TPS54318
Clock
Source
PLL
R
T
RT/CLK
TPS54318
www.ti.com SLVS975A –SEPTEMBER 2009–REVISED SEPTEMBER 2013
REVERSE OVERCURRENT PROTECTION
The TPS54318 implements low-side current protection by detecting the voltage across the low-side MOSFET.
When the converter sinks current through its low-side FET, the control circuit turns off the low-side MOSFET if
the reverse current is more than 1.3 A. By implementing this additional protection scheme, the converter is able
to protect itself from excessive current during power cycling and start-up into pre-biased outputs.
SYNCHRONIZE USING THE RT/CLK PIN
The RT/CLK pin is used to synchronize the converter to an external system clock. See Figure 29. To implement
the synchronization feature in a system, connect a square wave to the RT/CLK pin with an on-time of at least 75
ns. If the pin is pulled above the PLL upper threshold, a mode change occurs and the pin becomes a
synchronization input. The internal amplifier is disabled and the pin is a high impedance clock input to the
internal PLL. If clocking edges stop, the internal amplifier is re-enabled and the mode returns to the frequency set
by the resistor. The square wave amplitude at this pin must transition lower than 0.6 V and higher than 1.6 V
typically. The recommended synchronization frequency range is 300 kHz to 2000 kHz. The rising edge of the PH
is synchronized to the falling edge of RT/CLK pin. If the external system clock is to be removed, TI recommends
that it be removed on the falling edge of the clock signal.
Figure 29. Synchronizing to a System Clock Figure 30. Plot of Synchronizing to System Clock
POWER GOOD (PWRGD PIN)
The PWRGD pin output is an open-drain MOSFET. The output is pulled low when the VSENSE voltage enters
the fault condition by falling below 91% or rising above 107% of the nominal internal reference voltage. There is
a 2% hysteresis on the threshold voltage, so when the VSENSE voltage rises to the good condition above 93%
or falls below 105% of the internal voltage reference the PWRGD output MOSFET is turned off. TI recommends
to use a pullup resistor between the values of 1 kΩ and 100 kΩ to a voltage source that is 6 V or less. The
PWRGD is in a valid state once the VIN input voltage is greater than 1.2 V.
OVERVOLTAGE-TRANSIENT PROTECTION
The TPS54318 incorporates an overvoltage-transient-protection (OVTP) circuit to minimize voltage overshoot
when recovering from output fault conditions or strong unload transients. The OVTP feature minimizes the output
overshoot by implementing a circuit to compare the VSENSE pin voltage to the OVTP threshold which is 109%
of the internal voltage reference. If the VSENSE pin voltage is greater than the OVTP threshold, the high-side
MOSFET is disabled preventing current from flowing to the output and minimizing output overshoot. When the
VSENSE voltage drops lower than the OVTP threshold the high-side MOSFET is allowed to turn on the next
clock cycle.
THERMAL SHUTDOWN
The device implements an internal thermal shutdown to protect itself if the junction temperature exceeds 175°C.
The thermal shutdown forces the device to stop switching when the junction temperature exceeds the thermal
trip threshold. Once the die temperature decreases below 160°C, the device reinitiates the power-up sequence
by discharging the SS pin to 0 V. The thermal shutdown hysteresis is 15°C.
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