LM675

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SNOSBP3E – MAY 1999 – REVISED MARCH 2013

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 circuit 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 voltages 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 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 LM675 is no exception. If the output of

the LM675 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 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 with

a 5

μH 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

possibility 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 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 device 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

resistance 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 improve the long-term reliability of any

power semiconductor.

Copyright © 1999–2013, Texas Instruments Incorporated

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