Datasheet

P =(V V )I-
D IN OUT OUT
TPS7A49xx
SBVS121B –AUGUST 2010–REVISED JANUARY 2010 www.ti.com
LAYOUT
PACKAGE MOUNTING at least +35°C above the maximum expected ambient
condition of your particular application. This
Solder pad footprint recommendations for the
configuration produces a worst-case junction
TPS7A49xx are available at the end of this product
temperature of +125°C at the highest expected
datasheet and at www.ti.com.
ambient temperature and worst-case load.
The internal protection circuitry of the TPS7A49xx
BOARD LAYOUT RECOMMENDATIONS TO
has been designed to protect against overload
IMPROVE PSRR AND NOISE PERFORMANCE
conditions. It was not intended to replace proper
To improve ac performance such as PSRR, output
heatsinking. Continuously running the TPS7A49xx
noise, and transient response, it is recommended that
into thermal shutdown degrades device reliability.
the board be designed with separate ground planes
for IN and OUT, with each ground plane connected
POWER DISSIPATION
only at the GND pin of the device. In addition, the
ground connection for the output capacitor should The ability to remove heat from the die is different for
connect directly to the GND pin of the device. each package type, presenting different
considerations in the PCB layout. The PCB area
Equivalent series inductance (ESL) and equivalent
around the device that is free of other components
series resistance (ESR) must be minimized to
moves the heat from the device to the ambient air.
maximize performance and ensure stability. Every
Performance data or JEDEC low- and high-K boards
capacitor (C
IN
, C
OUT
, C
NR/SS
, C
BYP
) must be placed as
are given in the Dissipation Ratings Table. Using
close as possible to the device and on the same side
heavier copper increases the effectiveness in
of the printed circuit board (PCB) as the regulator
removing heat from the device. The addition of plated
itself.
through-holes to heat dissipating layers also improves
the heatsink effectiveness.
Do not place any of the capacitors on the opposite
side of the PCB from where the regulator is installed.
Power dissipation depends on input voltage and load
The use of vias and long traces is strongly
conditions. Power dissipation (P
D
) is equal to the
discouraged because they may impact system
product of the output current times the voltage drop
performance negatively and even cause instability.
across the output pass element, as shown in
Equation 2:
If possible, and to ensure the maximum performance
denoted in this product datasheet, use the same
(2)
layout pattern used for TPS7A49 evaluation board,
available at www.ti.com.
SUGGESTED LAYOUT AND SCHEMATIC
THERMAL PROTECTION Layout is a critical part of good power-supply design.
There are several signal paths that conduct
Thermal protection disables the output when the
fast-changing currents or voltages that can interact
junction temperature rises to approximately +170°C,
with stray inductance or parasitic capacitance to
allowing the device to cool. When the junction
generate noise or degrade the power-supply
temperature cools to approximately +150°C, the
performance. To help eliminate these problems, the
output circuitry is enabled. Depending on power
IN pin should be bypassed to ground with a low ESR
dissipation, thermal resistance, and ambient
ceramic bypass capacitor with a X5R or X7R
temperature, the thermal protection circuit may cycle
dielectric.
on and off. This cycling limits the dissipation of the
regulator, protecting it from damage as a result of The GND pin should be tied directly to the PowerPAD
overheating. under the IC. The PowerPAD should be connected to
any internal PCB ground planes using multiple vias
Any tendency to activate the thermal protection circuit
directly under the IC.
indicates excessive power dissipation or an
inadequate heatsink. For reliable operation, junction It may be possible to obtain acceptable performance
temperature should be limited to a maximum of with alternate PCB layouts; however, the layout
+125°C. To estimate the margin of safety in a shown in Figure 30 and the schematic shown in
complete design (including heatsink), increase the Figure 31 have been shown to produce good results
ambient temperature until the thermal protection is and are meant as a guideline.
triggered; use worst-case loads and signal conditions.
For good reliability, thermal protection should trigger
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