Datasheet

SLVA037A
4 Designing a USB Power Distribution System Using the TPS2014 and TPS2015 Power Distribution Switches
The TPS2014 and TPS2015 provide overcurrent protection by switching to a constant current
source whenever the trip threshold current is exceeded. The controller is notified of a fault
condition when the overcurrent logic output is set to a low. The switch remains in this constant
current state until either the fault is removed or the switch is disabled.
Continuous overloads and short circuits increase the power dissipation in the switch and cause
the junction temperature to rise. To prevent overheating, a thermal protection circuit shuts the
switch off when the junction temperature exceeds its thermal limit.
3 USB Power Distribution
The USB power distribution requirements are designed to make 4 V the minimum voltage seen
by any downstream function. Power is supplied to the 5-V bus from either the host or a self-
powered hub and then distributed to downstream functions. A resistive voltage drop is
associated with each connection, wire, and component where power is distributed.
The USB specification includes power distribution requirements that must be met for each
component. The TPS2014 and TPS2015 power distribution switches meets the USB
requirements for voltage drops, continuous current, overcurrent protection, and fault notification.
3.1 Self-Powered Hub (Including Host)
Self-powered hubs (including host) contain an internal power supply that powers the hub’s
internal functions and output ports. A self-powered hub can include as many output ports as the
power supply can deliver. Typical designs have one, two, four, or seven output ports. Each
output port in a self-powered hub must be able to supply a continuous 500 mA within the
required voltage regulation.
Because power is supplied from an internal power supply, overcurrent protection is required on
the output ports for safety reasons. UL limits require ports to deliver no more than 5 A. This
requirement includes short circuit currents.
Power distribution for a self-powered hub can be designed either with one overcurrent protection
device per output port or by ganging multiple ports to one overcurrent protection device.
Although ganging multiple output ports to one overcurrent protection device appears to be a
more economical solution, the on-resistance required may not make it feasible (see subsection
3.1.2).
3.1.1 Overcurrent Protection Device
The following devices commonly provide overcurrent protection for the self-powered hub:
• Power distribution switches (such as the TPS2014 and TPS2015)
• Positive temperature coefficient (PTC) resistors (commonly known as polyfuses or
polyswitches)
• Fuses
The TPS2014 and TPS2015 offer several advantages over fuses and PTC resistors, including:
• Extremely fast trip time