TPS2550DBVT datasheet(19/28 Pages) TI

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POWER DISSIPATION AND JUNCTION TEMPERATURE

UNIVERSAL SERIAL BUS (USB) POWER-DISTRIBUTION REQUIREMENTS

TPS2550

TPS2551

SLVS736 – FEBRUARY 2008

Select the closest 1% resistor greater than the calculated value: RILIM = 21.5 kΩ. This sets the maximum

current-limit threshold at 1.25 A . Use the IOS equations, Figure 21, and the previously calculated value for RILIM

to calculate the minimum resulting current-limit threshold.

• R

ILIM = 21.5 kΩ

• I

OS(min) (mA) = (39700 V) / (RILIM (kΩ))

1.342

• I

OS(min) (mA) = (39700 V) / (21.5 (kΩ))

1.342

• I

OS(min) = 647 mA

The resulting minimum current-limit threshold is 647 mA with a 21.5 k

Ω resistor.

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

is good design practice to estimate 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 of interest and read rDS(on)

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

PD = rDS(on) × IOUT

Where:

PD = Total power dissipation (W)

rDS(on) = Power switch on-resistance (Ω)

IOUT = Maximum current-limit threshold (A)

This step calculates the total power dissipation of the N-channel MOSFET.

Finally, calculate the junction temperature:

TJ = PD × RΘJA + TA

Where:

TA = Ambient temperature (°C)

RΘJA = Thermal resistance (°C/W)

PD = Total power dissipation (W)

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

the calculation using the "refined" rDS(on) from the previous calculation as the new estimate. Two or three

iterations are generally sufficient to achieve the desired result. The final junction temperature is highly dependent

on thermal resistance RθJA, and thermal resistance is highly dependent on the individual package and board

layout. The "Dissipating Rating Table" at the begginng of this document provides example thermal resistances for

specific packages and board layouts.

One application for this device is for current-limiting in universal serial bus (USB) applications. The original USB

interface was a 12-Mb/s or 1.5-Mb/s, multiplexed serial bus designed for low-to-medium bandwidth PC

peripherals (e.g., keyboards, printers, scanners, and mice). As the demand for more bandwidth increased, the

USB 2.0 standard was introduced increasing the maximum data rate to 480-Mb/s. The four-wire USB interface is

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 classifies two different classes of

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