TPS2149

TPS2159

SLVS401 – AUGUST 2001

14

www.ti.com

APPLICATION INFORMATION

Compare the calculated junction temperature with the initial estimate. If they do not agree within a few degrees,

repeat the calculation, using the calculated value as the new estimate. Two or three iterations are generally

sufficient to get a reasonable answer.

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 TPS2149 and TPS2159 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 TPS2149 and TPS2159 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 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 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 or 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: