POWER DISSIPATION AND JUNCTION TEMPERATURE

TPS2552

TPS2553, TPS2552-1, TPS2553-1

While both the constant-current devices (TPS2552/53) and latch-off devices (TPS2552-1/53-1) operate identically

during the initial onset of an overcurrent event, they behave differently if the overcurrent event lasts longer than

the internal delay "deglitch" circuit (7.5-ms typ). The constant-current devices (TPS2552/53) assert the FAULT

flag after the deglitch period and continue to regulate the current to the current-limit threshold indefinitely. In

practical circuits, the power dissipation in the package will increase the die temperature above the

overtemperature shutdown threshold (135°C min), and the device will turn off until the die temperature decreases

by the hysteresis of the thermal shutdown circuit (10°C typ). The device will turn on and continue to thermal cycle

until the overload condition is removed. The constant-current devices resume normal operation once the

overload condition is removed. The latch-off devices (TPS2552-1/53-1) assert the FAULT flag after the deglitch

period and immediately turn off the device. The device remains off regardless of whether the overload condition

is removed from the output. The latch-off devices remain off and do not resume normal operation until the

surrounding system either toggles the enable or cycles power to the device.

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.

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

2

Where:

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

Finally, calculate the junction temperature:

Where:

θ

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

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

on thermal resistance

θ

layout. The Dissipating Rating Table provides example thermal resistances for specific packages and board

layouts.

Copyright © 2008–2009, Texas Instruments Incorporated

Submit Documentation Feedback

17

Product Folder Link(s): TPS2552 TPS2553 TPS2552-1 TPS2553-1