TPS65257 is a power management IC with three step-down buck converters. Both high-side
and low-side MOSFETs are integrated to provide fully synchronous conversion with higher efficiency.
The converters are designed to simplify its application while giving the designer the option to
optimize their usage according to the target application.
The converters can operate in 5-, 9-, 12- or 15-V systems. The output voltage can be set
externally using a resistor divider to any value between 0.8 V and the input supply minus the
resistive drops on the converter path. Each converter features enable pin that allows a delayed
start-up for sequencing purposes, soft start pin that allows adjustable soft-start time by choosing
the soft-start capacitor, and a current limit (RLIM) pin that enables designer to adjust current
limit by selecting an external resistor and optimize the choice of inductor. All converters operate
in &145;hiccup mode: Once an over-current lasting more than 10 ms is sensed in any of the converters,
they will shut down for 10 ms and then the start-up sequencing will be tried again. If the overload
has been removed, the converter will ramp up and operate normally. If this is not the case the
converter will see another over-current event and shuts down again repeating the cycle (hiccup)
until the failure is cleared. If an overload condition lasts for less than 10 ms, only the relevant
converter affected will shut-down and re-start and no global hiccup mode will occur.
The switching frequency of the converters is set by an external resistor connected to
ROSC pin. The switching regulators are designed to operate from 300 kHz to 2.2 MHz. The converters
operate with 180° phase between then to minimize the input filter requirements.
All converters have peak current mode control which simplifies external frequency
compensation. The device has a built-in slope compensation ramp. The slope compensation can prevent
sub harmonic oscillations in peak current mode control. A traditional type II compensation network
can stabilize the system and achieve fast transient response. Moreover, an optional capacitor in
parallel with the upper resistor of the feedback divider provides one more zero and makes the
crossover frequency over 100 kHz.
All converters feature an automatic low power pulse PFM skipping mode which improves
efficiency during light loads and standby operation, while guaranteeing a very low output ripple,
allowing for a value of less than 2% at low output voltages.
The device incorporates an overvoltage transient protection circuit to minimize voltage
overshoot. The OVP feature minimizes the output overshoot by implementing a circuit to compare the
FB pin voltage to OVP threshold which is 109% of the internal voltage reference. If the FB 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 FB voltage drops lower than
the OVP lower threshold which is 107%, the high side MOSFET is allowed to turn on the next clock
TPS65257 features a supervisor circuit which monitors each buck’s output and the PGOOD
pin is asserted once sequencing is done. The PGOOD pin is an open drain output. The PGOOD pin is
pulled low when any buck converter is pulled below 85% of the nominal output voltage. The PGOOD is
pulled up when all converter outputs are more than 90% of its nominal output voltage. The default
reset time is 100 ms. The polarity of the PGOOD is active high.
The push button operation has been designed to allow for automatic system start when the
input supply is applied or to provide an integrated ON/OFF system management without the need of
additional external components. The behavior of the device will depend on the status of the INT pin
(see start-up signals).
The USB switch provides up to 1-A of current as required by downstream USB devices. When
the output load exceeds the current-limit threshold or a short is present, the PMU limits the
output current to a safe level by switching into a constant-current mode and pulling the over
current logic output low. When continuous heavy overloads and short circuits increase the power
dissipation in the switch, causing the junction temperature to rise, a thermal warning protection
circuit shuts off the USB switch and allows the buck converters to carry on operating.
The device implements an internal thermal shutdown to protect itself if the junction
temperature exceeds 160°C. The thermal shutdown forces the device to stop operating when the
junction temperature exceeds thermal trip threshold. Once the die temperature decreases below
140°C, the device reinitiates the power up sequence. The thermal shutdown hysteresis is 20°C.
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