LT1970A
14
1970afc
For more information www.linear.com/LT1970A
applicaTions inForMaTion
When V
–
and V
+
are provided separately from V
CC
and V
EE
,
care must be taken to ensure that V
–
and V
+
are always
less than or equal to the main supplies in magnitude.
Protection Schottky diodes may be required to ensure this
in all cases, including power on/off transients.
Operation with reduced V
+
and V
–
supplies does not affect
any performance parameters except maximum output
swing. All DC accuracy and AC performance specifications
guaranteed with V
CC
= V
+
and V
EE
= V
–
are still valid with
the reduced output signal swing range.
Heat Sinking
The power dissipated in the LT1970A die must have a path
to the environment. With 100°C/W thermal resistance in
free air with no heat sink, the package power dissipation
is limited to only 1W. The 20-pin TSSOP package with
exposed copper underside is an efficient heat conductor
if it is effectively mounted on a PC board. Thermal resis
-
tances as low as 40°C/W can be obtained by soldering
the bottom of the package to a large copper pattern on
the PC board. For operation at 85°C, this allows up to
1.625W of power to be dissipated on the LT1970A. At
25°C operation, up to 3.125W of power dissipation can
be achieved. The PC board heat spreading copper area
must be connected to V
EE
.
Figure 5 shows examples of PCB metal being used for
heat spreading. These are provided as a reference for
what might be expected when using different combina
-
tions of metal area on different layers of a PCB. These
examples are with a 4-layer board using 1oz copper on
each layer. The most effective layers for spreading heat
are
those closest to the LT1970A junction. Soldering the
exposed thermal pad of the TSSOP package to the board
produces a thermal resistance from junction-to-case of
approximately 3°C/W.
As a minimum, the area directly beneath the package on
all PCB layers can be used for heat spreading. However,
limiting the area to that of the metal heat sinking pad is
not very effective. Expanding the area on various layers
significantly reduces the overall thermal resistance. The
addition of vias (small 13 mil holes which fill during PCB
plating) connecting all layers of metal also helps reduce
the operating temperature of the LT1970A. These are also
shown in Figure 5.
It is important to note that the metal planes used for heat
sinking are connecting electrically to V
EE
. These planes
must be isolated from any other power planes used in
the PCB design.
Another effective way to control the power amplifier operat
-
ing temperature is to use airflow over the board. Airflow
can significantly reduce the total thermal resistance as
also shown in Figure 5.
D
RIVING REACTIVE LOADS
Capacitive Loads
The LT1970A is much more tolerant of capacitive loading
than most operational amplifiers. In a worst-case con
-
figuration as a voltage follower, the circuit is stable for
cap
acitive loads less than 2.5nF. Higher gain configurations
improve the C
LOAD
handling. If very large capacitive loads
are to be driven, a resistive decoupling of the amplifier
from the capacitive load is effective in maintaining stability
and reducing peaking. The current sense resistor, usually
connected between the output pin and the load can serve
as a part of the decoupling resistance.
Inductive Loads
Load inductance is usually not a problem at the outputs of
operational amplifiers, but the LT1970A can be used as a
high output impedance current source. This condition may
be the main operating mode, or when the circuit enters
a protective current limit mode. Just as load capacitance
degrades the phase margin of normal op amps, load
inductance causes a peaking in the loop response of the
feedback controlled current source. The inductive load may
be caused by long lead lengths at the amplifier output. If
the amplifier will be driving inductive loads or long lead
lengths (greater than 4 inches) a 500pF capacitor from the
SENSE
–
pin to the ground plane will cancel the inductive
load and ensure stability.
