DETAILED DESCRIPTION
OVERVIEW
OVERCURRENT
REVERSE-VOLTAGE PROTECTION
FAULT RESPONSE
UNDERVOLTAGE LOCKOUT (UVLO)
www.ti.com ........................................................................................................................................ SLVS736B – FEBRUARY 2008 – REVISED NOVEMBER 2008
The TPS2550/51 are current-limited, power distribution switches using N-channel MOSFETs for applications
where short-circuits or heavy capacitive loads will be encountered. These devices allow the user to program the
current-limit threshold between 100 mA and 1.1 A via an external resistor. Additional device shutdown features
include overtemperature protection and reverse-voltage protection. The device incorporates an internal charge
pump and gate drive circuitry necessary to drive the N-channel MOSFET. The charge pump supplies power to
the driver circuit and provides the necessary voltage to pull the gate of the MOSFET above the source. The
charge pump operates from input voltages as low as 2.5 V and requires little supply current. The driver controls
the gate voltage of the power switch. The driver incorporates circuitry that controls the rise and fall times of the
output voltage to limit large current and voltage surges and provide built-in soft-start functionality.
The TPS2550/51 responds to an overcurrent condition by limiting its output current to the IOC and IOS levels
shown in
Figure 21. Three response profiles are possible depending on the loading conditions and are
One response profile occurs if the TPS2550/51 is enabled into a short-circuit. The output voltage is held near
zero potential with respect to ground and the TPS2550/51 ramps the output current to IOS (see Figure 7). A second response profile occurs if a short is applied to the output after the TPS2550/51 is enabled. The device
responds to the overcurrent condition within time tIOS (see Figure 3). The current-sense amplifier is over-driven during this time and momentarily disables the internal current-limit MOSFET. The current-sense amplifier
gradually recovers and limits the output current to IOS.
A third response profile occurs if the load current gradually increases. The device first limits the load current to
IOC. If the load demands a current greater than IOC, the TPS2550/51 folds back the current to IOS and the output
voltage decreases to IOS x RLOAD for a resistive load, which is shown in Figure 4. The TPS2550/51 thermal cycles if an overload condition is present long enough to activate thermal limiting in any
of the above cases. The device turns off when the junction temperature exceeds 135°C (typ). The device
remains off until the junction temperature cools 15°C (typ) and then restarts. The TPS2550/51 cycles on/off until
The reverse-voltage protection feature turns off the N-channel MOSFET whenever the output voltage exceeds
the input voltage by 135 mV (typical) for 4-ms. This prevents damage to devices on the input side of the
TPS2550/51 by preventing significant current from sinking into the input capacitance. The N-channel MOSFET is
allowed to turn-on once the output voltage goes below the input voltage for the same 4-ms deglitch time. The
reverse-voltage comparator also asserts the FAULT output (active-low) after 4-ms.
The FAULT open-drain output is asserted (active low) during an overcurrent, overtemperature or reverse-voltage
condition. The output remains asserted until the fault condition is removed. The TPS2550/51 is designed to
eliminate false FAULT reporting by using an internal delay "deglitch" circuit for overcurrent (7.5-ms) and
reverse-voltage (4-ms) conditions without the need for external circuitry. This ensures that FAULT is not
accidentally asserted due to normal operation such as starting into a heavy capacitive load. The deglitch circuitry
delays entering and leaving fault conditions. Overtemperature conditions are not deglitched and assert the
FAULT signal immediately.
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 drop from large
current surges.
Copyright 2008, Texas Instruments Incorporated
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