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TPS25221

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SLVSDT3D – JANUARY 2018 – REVISED DECEMBER 2019

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Copyright © 2018–2019, Texas Instruments Incorporated

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 classifies two different classes of

devices depending on its maximum current draw. A device classified as low-power can draw up to 100 mA as

defined by the standard. A device classified as high-power can draw up to 500 mA. It is important that the

minimum current-limit threshold of the current-limiting power-switch exceed the maximum current-limit draw of

the intended application. The latest USB standard must always be referenced when considering the current-limit

threshold

The USB specification defines two types of devices as hubs and functions. A USB hub is a device that contains

multiple ports for different USB devices to connect and can be self-powered (SPH) or bus-powered (BPH). A

function is a USB device that is able to transmit or receive data or control information over the bus. A USB

function can be embedded in a USB hub. A USB function can be one of three types included in the list below.

•

Low-power, bus-powered function

•

High-power, bus-powered function

•

Self-powered function

SPHs and BPHs distribute data and power to downstream functions. The TPS25221 has higher current capability

than required for a single USB port allowing it to power multiple downstream ports.

11 Power Supply Recommendations

11.1 Self-Powered and Bus-Powered Hubs

A SPH has a local power supply that powers embedded functions and downstream ports. This power supply

must provide between 4.75 V to 5.25 V to downstream facing devices under full-load and no-load conditions.

SPHs are required to have current-limit protection and must report over-current conditions to the USB controller.

Typical SPHs are desktop PCs, monitors, printers, and stand-alone hubs.

A BPH obtains all power from an upstream port and often contains an embedded function. It must power up with

less than 100 mA. The BPH usually has one embedded function, and power is always available to the controller

of the hub. If the embedded function and hub require more than 100 mA on power up, keep the power to the

embedded function off until enumeration is completed. This can be accomplished by removing power or by

shutting off the clock to the embedded function. Power-switching the embedded function is not necessary if the

aggregate power draw for the function and controller is less than 100 mA. The total current drawn by the bus-

powered device is the sum of the current to the controller, the embedded function, and the downstream ports,

and it is limited to 500 mA from an upstream port.

11.2 Low-Power Bus-Powered and High-Power Bus-Powered Functions

Both low-power and high-power bus-powered functions obtain all power from upstream ports. Low-power

functions always draw less than 100 mA; high-power functions must draw less than 100 mA at power up and can

draw up to 500 mA after enumeration. If the load of the function is more than the parallel combination of 44 Ω

and 10 µF at power up, the device must implement inrush current limiting.

11.3 Power Dissipation and Junction Temperature

The low ON-resistance of the N-channel MOSFET allows small surface-mount packages to pass large currents.

It is required design practice to determine power dissipation and junction temperature. The below analysis gives

an approximation for calculating junction temperature based on the power dissipation in the package. However, it

is important to note that thermal analysis is strongly dependent on additional system level factors. Such factors

include air flow, board layout, copper thickness and surface area, and proximity to other devices dissipating

power. Good thermal design practice must include all system level factors in addition to individual component

analysis.

Begin by determining the rDS(on) of the N-channel MOSFET relative to the input voltage and operating

temperature. As an initial estimate, use the highest operating ambient temperature expected and read rDS(on)

from the typical characteristics graph. Using this value, the power dissipation can be calculated using Equation 7: