Datasheet

160
140
120
100
80
60
40
20
0
q
JA
( C/W)
°
0 1 2 3
4 5
6
7
8 9 10
Board Copper Area ( )in
2
DRV
DRB
R
qJA
+
(
)125
O
C * T
A
)
P
D
P
D
+
ǒ
V
IN
*V
OUT
Ǔ
@ I
OUT
TPS735xx
SBVS087K –JUNE 2008–REVISED AUGUST 2013 www.ti.com
approximately 50μA. In typical applications, the Power dissipation depends on input voltage and load
junction cannot reach high temperatures at light loads conditions. Power dissipation is equal to the product
because there is no appreciable dissipated power. of the output current time the voltage drop across the
The specified ground current would then be valid at output pass element, as shown in Equation 2:
no load in most applications.
(2)
Thermal Information
Note: When the device is used in a condition of
higher input and lower output voltages with the DRV
and DRB packages, P
D
exceeds the package rating
Thermal Protection
at room temperature. This equation shows an
example of the DRB package:
Thermal protection disables the output when the
junction temperature rises to approximately +165°C,
P
D
= (6.5V – 1.0V) × 500mA = 2.75W, which is
allowing the device to cool. When the junction
greater than 2.5W at +25°C.
temperature cools to approximately +145°C the
Power dissipation can be minimized and greater
output circuitry is again enabled. Depending on power
efficiency can be achieved by using the lowest
dissipation, thermal resistance, and ambient
possible input voltage necessary to achieve the
temperature, the thermal protection circuit may cycle
required output voltage regulation.
on and off. This cycling limits the dissipation of the
regulator, protecting it from damage as a result of
On both SON (DRB) and SON (DRV) packages, the
overheating.
primary conduction path for heat is through the
exposed pad to the printed circuit board (PCB). The
Any tendency to activate the thermal protection circuit
pad can be connected to ground or be left floating;
indicates excessive power dissipation or an
however, it should be attached to an appropriate
inadequate heatsink. For reliable operation, junction
amount of copper PCB area to ensure the device
temperature should be limited to +125°C maximum.
does not overheat. The maximum junction-to-ambient
To estimate the margin of safety in a complete design
thermal resistance depends on the maximum ambient
(including heatsink), increase the ambient
temperature, maximum device junction temperature,
temperature until the thermal protection is triggered;
and power dissipation of the device and can be
use worst-case loads and signal conditions. For good
calculated using Equation 3:
reliability, thermal protection should trigger at least
+35°C above the maximum expected ambient
condition of your particular application. This
(3)
configuration produces a worst-case junction
Knowing the maximum R
θJA
, the minimum amount of
temperature of +125°C at the highest expected
PCB copper area needed for appropriate heatsinking
ambient temperature and worst-case load.
can be estimated using Figure 22.
The internal protection circuitry of the TPS735xx has
been designed to protect against overload conditions.
It was not intended to replace proper heatsinking.
Continuously running the TPS735xx into thermal
shutdown degrades device reliability.
Package Mounting
Solder pad footprint recommendations for the
TPS735xx are available from the Texas Instruments
web site at www.ti.com.
Power Dissipation
The ability to remove heat from the die is different for
each package type, presenting different
considerations in the PCB layout. The PCB area
around the device that is free of other components
Note: θ
JA
value at board size of 9in
2
(that is, 3in ×
moves the heat from the device to the ambient air.
3in) is a JEDEC standard.
Performance data for JEDEC low- and high-K boards
Figure 22. θ
JA
vs Board Size
are given in the Thermal Information table. Using
heavier copper increases the effectiveness in
removing heat from the device. The addition of plated
through-holes to heat-dissipating layers also
improves the heatsink effectiveness.
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