LM1875TLB05 National Semiconductor, LM1875TLB05 Datasheet - Page 6

• Manufacturer Part Number: LM1875TLB05
• Description: 20W Audio Power Amplifier
• Manufacturer: National Semiconductor

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STABILITY
The LM1875 is designed to be stable when operated at a
closed-loop gain of 10 or greater, but, as with any other
high-current amplifier, the LM1875 can be made to oscillate
under certain conditions. These usually involve printed cir-
cuit board layout or output/input coupling.
Proper layout of the printed circuit board is very important.
While the LM1875 will be stable when installed in a board
similar to the ones shown in this data sheet, it is sometimes
necessary to modify the layout somewhat to suit the physical
requirements of a particular application. When designing a
different layout, it is important to return the load ground, the
output compensation ground, and the low level (feedback
and input) grounds to the circuit board ground point through
separate paths. Otherwise, large currents flowing along a
ground conductor will generate voltages on the conductor
which can effectively act as signals at the input, resulting in
high frequency oscillation or excessive distortion. It is advis-
able to keep the output compensation components and the
0.1 µF supply decoupling capacitors as close as possible to
the LM1875 to reduce the effects of PCB trace resistance
and inductance. For the same reason, the ground return
paths for these components should be as short as possible.
Occasionally, current in the output leads (which function as
antennas) can be coupled through the air to the amplifier in-
put, resulting in high-frequency oscillation. This normally
happens when the source impedance is high or the input
leads are long. The problem can be eliminated by placing a
small capacitor (on the order of 50 pF to 500 pF) across the
circuit input.
Most power amplifiers do not drive highly capacitive loads
well, and the LM1875 is no exception. If the output of the
LM1875 is connected directly to a capacitor with no series
resistance, the square wave response will exhibit ringing if
the capacitance is greater than about 0.1 µF. The amplifier
can typically drive load capacitances up to 2 µF or so without
oscillating, but this is not recommended. If highly capacitive
loads are expected, a resistor (at least 1 ) should be placed
in series with the output of the LM1875. A method commonly
employed to protect amplifiers from low impedances at high
frequencies is to couple to the load through a 10 resistor in
parallel with a 5 µH inductor.
DISTORTION
The preceding suggestions regarding circuit board ground-
ing techniques will also help to prevent excessive distortion
levels in audio applications. For low THD, it is also neces-
sary to keep the power supply traces and wires separated
from the traces and wires connected to the inputs of the
LM1875. This prevents the power supply currents, which are
large and nonlinear, from inductively coupling to the LM1875
inputs. Power supply wires should be twisted together and
separated from the circuit board. Where these wires are sol-
dered to the board, they should be perpendicular to the
plane of the board at least to a distance of a couple of
inches. With a proper physical layout, THD levels at 20 kHz
with 10W output to an 8
and less than 0.02% at 1 kHz.
CURRENT LIMIT AND SAFE OPERATING AREA (SOA)
PROTECTION
A power amplifier’s output transistors can be damaged by
excessive applied voltage, current flow, or power dissipation.
The voltage applied to the amplifier is limited by the design of
load should be less than 0.05%,
6
the external power supply, while the maximum current
passed by the output devices is usually limited by internal
circuitry to some fixed value. Short-term power dissipation is
usually not limited in monolithic audio power amplifiers, and
this can be a problem when driving reactive loads, which
may draw large currents while high voltages appear on the
output transistors. The LM1875 not only limits current to
around 4A, but also reduces the value of the limit current
when an output transistor has a high voltage across it.
When driving nonlinear reactive loads such as motors or
loudspeakers with built-in protection relays, there is a possi-
bility that an amplifier output will be connected to a load
whose terminal voltage may attempt to swing beyond the
power supply voltages applied to the amplifier. This can
cause degradation of the output transistors or catastrophic
failure of the whole circuit. The standard protection for this
type of failure mechanism is a pair of diodes connected be-
tween the output of the amplifier and the supply rails. These
are part of the internal circuitry of the LM1875, and needn’t
be added externally when standard reactive loads are
driven.
THERMAL PROTECTION
The LM1875 has a sophisticated thermal protection scheme
to prevent long-term thermal stress to the device. When the
temperature on the die reaches 170˚C, the LM1875 shuts
down. It starts operating again when the die temperature
drops to about 145˚C, but if the temperature again begins to
rise, shutdown will occur at only 150˚C. Therefore, the de-
vice is allowed to heat up to a relatively high temperature if
the fault condition is temporary, but a sustained fault will limit
the maximum die temperature to a lower value. This greatly
reduces the stresses imposed on the IC by thermal cycling,
which in turn improves its reliability under sustained fault
conditions.
Since the die temperature is directly dependent upon the
heat sink, the heat sink should be chosen for thermal resis-
tance low enough that thermal shutdown will not be reached
during normal operation. Using the best heat sink possible
within the cost and space constraints of the system will im-
prove the long-term reliability of any power semiconductor
device.
POWER DISSIPATION AND HEAT SINKING
The LM1875 must always be operated with a heat sink, even
when it is not required to drive a load. The maximum idling
current of the device is 100 mA, so that on a 60V power sup-
ply an unloaded LM1875 must dissipate 6W of power. The
54˚C/W junction-to-ambient thermal resistance of a TO-220
package would cause the die temperature to rise 324˚C
above ambient, so the thermal protection circuitry will shut
the amplifier down if operation without a heat sink is at-
tempted.
In order to determine the appropriate heat sink for a given
application, the power dissipation of the LM1875 in that ap-
plication must be known. When the load is resistive, the
maximum average power that the IC will be required to dis-
sipate is approximately:
where V
LM1875, R
power dissipation of the amplifier. The above equation is
only an approximation which assumes an “ideal” class B out-
S
L
is the total power supply voltage across the
is the load resistance, and P
Q
is the quiescent