Datasheet

P =(V V )I-
D IN OUT OUT
TPS7A33
www.ti.com SBVS169C –DECEMBER 2011–REVISED FEBRUARY 2013
LAYOUT
POWER DISSIPATION
The primary TPS7A33 application is to provide ultralow noise voltage rails to high-performance analog circuitry in
order to maximize system accuracy and precision. The high-current and high-voltage characteristics of this
regulator means that, often enough, high power (heat) is dissipated from the device itself. This heat, if dissipated
into the PCB (as is the case with SMT packages), creates a temperature gradient in the surrounding area that
causes nearby components to react to this temperature change (drift). In high-performance systems, such drift
may degrade overall system accuracy and precision.
Compared to surface-mount packages, the TO-220 (KC) package allows for an external heatsink to be used to
maximize thermal performance and keep heat from dissipating into the PCB.
Power dissipation depends on input voltage and load conditions. Power dissipation (P
D
) is equal to the product of
the output current times the voltage drop across the output pass element, as shown in Equation 2:
(2)
THERMAL PERFORMANCE AND HEATSINK SELECTION
Heat flows from the device to the ambient air through many paths, each of which represents resistance to the
heat flow; this is called thermal resistance.
The total thermal resistance of a system is defined by: θ
JA
= (T
J
– T
A
)/P
D
; where: θ
JA
is the thermal resistance (in
°C/W), T
J
is the allowable juntion temperature of the device (in °C), T
A
is the maximum temperature of the
ambient cooling air (in °C), and P
D
is the amount of power (heat) dissipated by the device (in W).
Whenever a heatsink is installed, the total thermal resistance (θ
JA
) is the sum of all the individual resistances
from the device, going through its case and heatsink to the ambient cooling air (θ
JA
= θ
JC
+ θ
CS
+ θ
SA
).
Reallistically, only two resistances can be controlled: θ
CS
and θ
SA
. Therefore, for a device with a known θ
JC
, θ
CS
and θ
SA
become the main design variables in selecting a heat sink.
The thermal interface between the case and the heat sink (θ
CS
) is controlled by selecting the correct heat-
conducting material. Once the θ
CS
is selected, the required thermal resistance from the heatsink to ambient is
calculated by the following equation: θ
SA
= [(T
J
– T
A
)/P
D
] – [θ
JC
+ θ
CS
]. This information allows the the most
appropriate heatsink to be selected for any particular application.
PACKAGE MOUNTING
The TO-220 (KC) 7-lead, straight-formed package lead spacing poses a challenge when creating a suitable PCB
footprint without bending the leads. Component forming pliers, such as Excelta's Q-6482, can be used to
manually bend the package leads into a 7-lead stagger pattern with increased lead spacing that can be more
easily used.
The TPS7A33 evaluation board layout can be used as a guideline on suitable PCB footprints, available at
www.ti.com
BOARD LAYOUT RECOMMENDATIONS TO IMPROVE PSRR AND NOISE PERFORMANCE
To improve ac performance such as PSRR, output noise, and transient response, it is recommended that the
board be designed with separate planes for IN, OUT, and GND. The IN and OUT planes should be isolated from
each other by a GND plane section. In addition, the ground connection for the output capacitor should connect
directly to the GND pin of the device.
Equivalent series inductance (ESL) and equivalent series resistance (ESR) must be minimized in order to
maximize performance and ensure stability. Every capacitor (C
IN
, C
OUT
, C
NR/SS
, C
FF
) must be placed as close as
possible to the device and on the same side of the printed circuit board (PCB) as the regulator itself.
Do not place any of the capacitors on the opposite side of the PCB from where the regulator is installed. The use
of vias and long traces is strongly discouraged because they may impact system performance negatively and
even cause instability.
If possible, and to ensure the maximum performance specified in this product datasheet, use the same layout
pattern used for the TPS7A33 evaluation board, available at www.ti.com.
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