Datasheet
SLVS333 − AUGUST 2001
18
www.ti.com
APPLICATION INFORMATION
thermal protection
Thermal protection prevents damage to the IC when heavy-overload or short-circuit faults are present for
extended periods of time. The faults force the TPS2145 and TPS2147 into constant-current mode at first, which
causes the voltage across the high-side switch to increase; under short-circuit conditions, the voltage across
the switch is equal to the input voltage. The increased dissipation causes the junction temperature to rise to high
levels.
The protection circuit senses the junction temperature of the switch and shuts it off. Hysteresis is built into the
thermal sense circuit, and after the device has cooled approximately 10 degrees, the switch turns back on. The
switch continues to cycle in this manner until the load fault or input power is removed.
The TPS2145 and TPS2147 implement a dual thermal trip to allow fully independent operation of the power
distribution switches. In an overcurrent or short-circuit condition, the junction temperature will rise. Once the die
temperature rises to approximately 120°C, the internal thermal-sense circuitry checks to determine which
power switch is in an overcurrent condition and turns that power switch off, thus isolating the fault without
interrupting operation of the adjacent power switch. Should the die temperature exceed the first thermal trip
point of 120°C and reach 155°C, the device will turn off.
undervoltage lockout (UVLO)
An undervoltage lockout ensures that the device (LDO and switches) is in the off state at power up. The UVLO
will also keep the device from being turned on until the power supply has reached the start threshold (see
undervoltage lockout table), even if the switches are enabled. The UVLO will also be activated whenever the
input voltage falls below the stop threshold as defined in the undervoltage lockout table. This function facilitates
the design of hot-insertion systems where it is not possible to turn off the power switches before input power
is removed. Upon reinsertion, the power switches will be turned on with a controlled rise time to reduce EMI and
voltage overshoots.
universal serial bus (USB) applications
The universal serial bus (USB) interface is a multiplexed serial bus operating at either 12-Mb/s, or 1.5-Mb/s for
USB 1.1, or 480 Mb/s for USB 2.0. The USB interface is designed to accommodate the bandwidth required by
PC peripherals such as keyboards, printers, scanners, and mice. The four-wire USB interface was conceived
for dynamic attach-detach (hot plug-unplug) of peripherals. Two lines are provided for differential data, and two
lines are provided for 5-V power distribution.
USB data is a 3.3-V level signal, but power is distributed at 5 V to allow for voltage drops in cases where power
is distributed through more than one hub across long cables. Each function must provide its own regulated 3.3-V
from the 5-V input or its own internal power supply.
The USB specification defines the following five classes of devices, each differentiated by power-consumption
requirements:
D Hosts/self-powered hubs (SPH)
D Bus-powered hubs (BPH)
D Low-power, bus-powered functions
D High-power, bus-powered functions
D Self-powered functions
The TPS2145 and TPS2147 are well suited for USB hub and peripheral applications. The internal LDO can be
used to provide the 3.3-V power needed by the controller while the dual switches distribute power to the
downstream functions.