Application Hints

STABILITY

The LM675 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 LM675 can be made to oscillate

under certain conditions. These usually involve printed cir-

cuit board layout or output/input coupling.

When designing a printed circuit board 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 volt-

ages on the conductor which can effectively act as signals at

the input, resulting in high frequency oscillation or excessive

distortion. It is advisable to keep the output compensation

components and the 0.1 µF supply decoupling capacitors as

close as possible to the LM675 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 LM675 is no exception. If the output of the

LM675 is connected directly to a capacitor with no series re-

sistance, 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 os-

cillating, 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 LM675. 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.

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 operational 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 LM675 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 LM675, and needn’t be

added externally when standard reactive loads are driven.

THERMAL PROTECTION

The LM675 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 LM675 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. This circuitry is 100% tested without a heat sink.

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 operaton. 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.

POWER DISSIPATION AND HEAT SINKING

The LM675 should always be operated with a heat sink,

even though at idle worst case power dissipation will be only

1.8W (30 mA x 60V) which corresponds to a rise in die tem-

perature of 97˚C above ambient assuming

θ

a TO-220 package. This in itself will not cause the thermal

protection circuitry to shut down the amplifier when operating

at room temperature, but a mere 0.9W of additional power

dissipation will shut the amplifier down since T

crease from 122˚C (97˚C + 25˚C) to 170˚C.

In order to determine the appropriate heat sink for a given

application, the power dissipation of the LM675 in that appli-

cation must be known. When the load is resistive, the maxi-

mum average power that the IC will be required to dissipate

is approximately:

where V

S

is the total power supply voltage across the

LM675, R

power dissipation of the amplifier. The above equation is

only an approximation which assumes an “ideal” class B out-

put stage and constant power dissipation in all other parts of

the circuit. As an example, if the LM675 is operated on a 50V

power supply with a resistive load of 8

Ω, it can develop up to

19W of internal power dissipation. If the die temperature is to

remain below 150˚C for ambient temperatures up to 70˚C,

the total junction-to-ambient thermal resistance must be less

than

Using

θ

face thermal resistance and the heat-sink-to-ambient ther-

mal

resistance

must

be

less

than

2.2˚C/W.

The

case-to-heat-sink thermal resistance of the TO-220 package

varies with the mounting method used. A metal-to-metal in-

terface will be about 1˚C/W if lubricated, and about 1.2˚C/W

if dry. If a mica insulator is used, the thermal resistance will

be about 1.6˚C/W lubricated and 3.4˚C/W dry. For this ex-

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