LM6584

SNOSA44C – MAY 2003 – REVISED MARCH 2013

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APPLICATION HINTS

POWER SUPPLIES

Sequencing

Best practice design technique for operational amplifiers includes careful attention to power sequencing.

Although the LM6584 is a bipolar op amp, recommended op amp turn on power sequencing of ground (or V−),

followed by V+, followed by input signal should be observed. Turn off power sequence should be the reverse of

the turn-on sequence. Depending on how the amp is biased the outputs may swing to the rails on power-on or

power-off. Due to the high output currents and rail to rail output stage in the LM6584 the output may oscillate

very slightly if the power is slowly raised between 2V and 4V The part is unconditionally stable at 5V. Quick turn-

off and turn-on times will eliminate oscillation problems.

PSRR and Noise

Care should be taken to minimize the noise in the power supply rails. The figure of merit for an op amp’s ability

to keep power supply noise out of the signal is called Power Supply Rejection Ratio (PSRR). Observe from the

PSRR charts in the Typical Performance Characteristics section that the PSRR falls of dramatically as the

frequency of the noise on the power supply line goes up. This is one of the reasons switching power supplies

can cause problems. It should also be noticed from the charts that the negative supply pin is far more susceptible

to power noise. The design engineer should determine the switching frequencies and ripple voltages of the

power supplies in the system. If required, a series resistor or in the case of a high current op amp like the

LM6584, a series inductor can be used to filter out the noise.

Transients

In addition to the ripple and noise on the power supplies there are also transient voltage changes. This can be

caused by another device on the same power supply suddenly drawing current or suddenly stopping a current

draw. The design engineer should insure that there are no damaging transients induced on the power supply

lines when the op amp suddenly changes current delivery.

LAYOUT

Ground Planes

Do not assume the ground (or more properly, the common or return) of the power supply is an ocean of zero

impedance. The thinner the trace, the higher the resistance. Thin traces cause tiny inductances in the power

lines. These can react against the large currents the LM6584 is capable of delivering to cause oscillations,

instability, overshoot and distortion. A ground plane is the most effective way of insuring the ground is a uniform

low impedance. If a four layer board cannot be used, consider pouring a plane on one side of a two layer board.

If this cannot be done be sure to use as wide a trace as practicable and use extra decoupling capacitors to

minimize the AC variations on the ground rail.

Decoupling

A high-speed, high-current amp like the LM6584 must have generous decoupling capacitors. They should be as

close to the power pins as possible. Putting them on the back side opposite the power pins may give the tightest

layout. If ground and power planes are available, the placement of the decoupling caps are not as critical.

Breadboards

The high currents and high frequencies the LM6584 operates at, as well as thermal considerations, require that

prototyping of the design be done on a circuit board as opposed to a “Proto-Board” style breadboard.

STABILITY

General

High speed parts with large output current capability require special care to insure lack of oscillations. Keep the

“+” pin isolated from the output to insure stability. As noted above care should be take to insure the large output

currents do not appear in the ground or ground plane and then get coupled into the “+” pin. As always, good tight

layout is essential as is adequate use of decoupling capacitors on the power supplies.

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