
SLVS931 – NOVEMBER 2009
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FAULT RESPONSE
The FAULT open-drain output is asserted (active low) during an overcurrent or overtemperature condition. The
TPS2556/57 asserts the FAULT signal until the fault condition is removed and the device resumes normal
operation. The TPS2556/57 is designed to eliminate false FAULTreporting by using an internal delay "deglitch"
circuit for overcurrent (9-ms typ) conditions without the need for external circuitry. This ensures that FAULTis not
accidentally asserted due to normal operation such as starting into a heavy capacitive load. The deglitch circuitry
delays entering and leaving current-limit induced fault conditions. The FAULTsignal is not deglitched when the
MOSFET is disabled due to an overtemperature condition but is deglitched after the device has cooled and
begins to turn on. This unidirectional deglitch prevents FAULT oscillation during an overtemperature event.
UNDERVOLTAGE LOCKOUT (UVLO)
The undervoltage lockout (UVLO) circuit disables the power switch until the input voltage reaches the UVLO
turn-on threshold. Built-in hysteresis prevents unwanted on/off cycling due to input voltage droop during turn on.
ENABLE (EN OR EN)
The logic enable controls the power switch and device supply current. The supply current is reduced to less than
2-
μA when a logic high is present on EN or when a logic low is present on EN. A logic low input on EN or a logic
high input on EN enables the driver, control circuits, and power switch. The enable input is compatible with both
TTL and CMOS logic levels.
THERMAL SENSE
The TPS2556/57 self protects by using two independent thermal sensing circuits that monitor the operating
temperature of the power switch and disable operation if the temperature exceeds recommended operating
conditions. The TPS2556/57 device operates in constant-current mode during an overcurrent conditions, which
increases the voltage drop across power switch. The power dissipation in the package is proportional to the
voltage drop across the power switch, which increases the junction temperature during an overcurrent condition.
The first thermal sensor (OTSD) turns off the power switch when the die temperature exceeds 135°C (min) and
the part is in current limit. Hysteresis is built into the thermal sensor, and the switch turns on after the device has
cooled approximately 20 °C.
The TPS2556/57 also has a second ambient thermal sensor (OTSD2). The ambient thermal sensor turns off the
power switch when the die temperature exceeds 155°C (min) regardless of whether the power switch is in
current limit and will turn on the power switch after the device has cooled approximately 20 °C. The TPS2556/57
continues to cycle off and on until the fault is removed.
APPLICATION INFORMATION
INPUT AND OUTPUT CAPACITANCE
Input and output capacitance improves the performance of the device; the actual capacitance should be
optimized for the particular application. For all applications, a 0.1
μF or greater ceramic bypass capacitor between
IN and GND is recommended as close to the device as possible for local noise decoupling. This precaution
reduces ringing on the input due to power-supply transients. Additional input capacitance may be needed on the
input to reduce voltage overshoot from exceeding the absolute-maximum voltage of the device during heavy
transient conditions. This is especially important during bench testing when long, inductive cables are used to
connect the evaluation board to the bench power supply.
Output capacitance is not required, but placing a high-value electrolytic capacitor on the output pin is
recommended when large transient currents are expected on the output.
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Copyright 2009, Texas Instruments Incorporated