Application Hints

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

input, 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

Ω) 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

Ω resistor in

parallel witha5µH 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

soldered 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

Ω load should be less than 0.05%,

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

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

between 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

improve 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

supply 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

attempted.

In order to determine the appropriate heat sink for a given

application, the power dissipation of the LM1875 in that

application must be known. When the load is resistive, the

maximum average power that the IC will be required to

dissipate is approximately:

where V

S

is the total power supply voltage across the

LM1875, R

power dissipation of the amplifier. The above equation is

only an approximation which assumes an “ideal” class B

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