TPS2553QDBVRQ1 Datasheet, PDF (18/29 Pages) Texas Instruments – PRECISION ADJUSTABLE CURRENT-LIMITED POWER-DISTRIBUTION SWITCHES

TPS2553-Q1
SLVSBD0 – NOVEMBER 2012
www.ti.com
The constant-current device (TPS2553-Q1) operates during the initial onset of an overcurrent event, if the
overcurrent event lasts longer than the internal delay "deglitch" circuit (7.5-ms typ). The constant-current device
(TPS2553-Q1) asserts the FAULT flag after the deglitch period and continues 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.
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 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 r_DS(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 r_DS(on)
from the typical characteristics graph. Using this value, the power dissipation can be calculated by:
P_D = r_DS(on) × I_OUT ²
Where:
P_D = Total power dissipation (W)
r_DS(on) = Power switch on-resistance (Ω)
I_OUT = Maximum current-limit threshold (A)
This step calculates the total power dissipation of the N-channel MOSFET.
Finally, calculate the junction temperature:
T_J = P_D × θ_JA + T_A
Where:
T_A = Ambient temperature (°C)
θ_JA = Thermal resistance (°C/W)
P_D = 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" r_DS(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 θ_JA, and thermal resistance is highly dependent on the individual package and board
layout. The Thermal Information Table provides example thermal resistance for specific packages and board
layouts.
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