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THERMAL PROTECTION

UNDERVOLTAGE LOCKOUT

UNIVERSAL SERIAL BUS (USB) APPLICATIONS

BUS-POWERED AND SELF-POWER HUBS

TPS2014

TPS2015

SLVS159D – DECEMBER 1996 – REVISED SEPTEMBER 2007

Finally, calculate the junction temperature:

Where:

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 is provided to prevent damage to the IC when heavy-overload or a short-circuit fault is

present for an extended period of time. The fault forces the TPS20xx into constant current mode, 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 dangerously

high levels. The protection circuit senses the junction temperature of the switch and shuts it off. The switch

remains off until the junction temperature has dropped approximately 20C. The switch continues to cycle in this

manner until the load fault or the input power is removed.

An undervoltage lockout is provided to ensure that the power switch is in the off state at power up. Whenever the

input voltage falls below approximately 3.2 V, the power switch quickly turns off. This facilitates the design of

hot-insertion systems that may not have the ability to turn off the power switch before input power is removed.

Upon reapplication of the input voltage (if enabled), the power switch turns on with a controlled rise time to

reduce inrush current, EMI, and voltage overshoots.

For proper operation of the UVLO, the TPS20xx requires the voltage decay from 3 V to 2 V to take at least 200

s. Capacitance is added to the input or output of the TPS20xx to increase this decay rate. Capacitance is

generally added to the output to lower inrush current due to input capacitance.

The USB specification provides for five different classes of devices based on their power sourcing and sinking

requirements. These classes of devices are: bus-powered hub, self-powered hub, lower power bus-powered

function, high power bus-powered function, and self-powered functions. The TPS20xx can provide power

distribution solutions for many of these devices.

Hubs provide data and power for downstream functions through output ports. Self-power hubs have internal

power supplies that furnish power to downstream functions. Each port is required to supply 500 mA continuous to

a downstream function. Each port must have overcurrent protection to meet the requlatory safety limit that no

single port can deliver more than 5 A. The self-power hub must also have a method to detect and report an

overcurrent condition to the USB host. The TPS20xx provides the required current-limiting function and has an

overcurrent logic output to inform the hub controller of the fault condition. The on-state resistance of the TPS20xx

is low enough to meet all USB voltage regulation requirements. The switch also provides the capability to remove

power from a faulted port.

Bus-powered hubs distribute power and data from an input port to downstream ports. Each output port is

required to supply 100 mA continuous. A bus-powered hub is not required to provide overcurrent protection

because it is provided by the upstream port. In order to power up in a low power state, the self-powered hub

must be able to switch power to its output ports. The TPS20xx can also provide this function.

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