LM6584

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SNOSA44C – MAY 2003 – REVISED MARCH 2013

Unity Gain

The unity gain or voltage-follower configuration is the most subject to oscillation. If a part is stable at unity gain it

is almost certain to work in other configurations. In certain applications where the part is setting a reference

voltage or is being used as a buffer greater stability can be achieved by configuring the part as a gain of

−1 or −2

or +2.

Phase Margin

The phase margin of an op amps gain-phase plot is an indication of the stability of the amp. It is desirable to

have at least 45°C of phase margin to insure stability in all cases. The LM6584 has 60°C of phase margin even

with it’s large output currents and Rail-to-Rail output stage, which are generally more prone to stability issues.

Capacitive Load

The LM6584 can withstand 30pF of capacitive load in a unity gain configuration before stability issues arise. At

very large capacitances, the load capacitor will attenuate the gain like any other heavy load and the part

becomes stable again. The LM6584 will be stable at 330nF and higher load capacitance. Refer to the chart in the

Typical Performance Characteristics section.

OUTPUT

Swing vs. Current

The LM6584 will get to about 25mV or 30mV of either rail when there is no load. The LM6584 can sink or source

hundreds of milliamperes while remaining less then 0.5V away from the rail. It should be noted that if the outputs

are driven to the rail and the part can no longer maintain the feedback loop, the internal circuitry will deliver large

base currents into the huge output transistors, trying to get the outputs to get past the saturation voltage. The

base currents will approach 16 milliamperes and this will appear as an increase in power supply current.

Operating at this power dissipation level for extended periods will damage the part, especially in the higher

thermal resistance TSSOP package. Because of this phenomenon, unused parts should not have the inputs

strapped to either rail, but should have the inputs biased at the midpoint or at least a diode drop (0.6V) within the

rails.

Self Heating

As discussed above the LM6584 is capable of significant power by virtue of its 300mA current handling

capability. A TSSOP package cannot sustain these power levels for more then a brief period.

TFT Display Application

INTRODUCTION

In today’s high-resolution TFT displays, op amps are used for the following three functions:

1. VCOM Driver

2. Gamma Buffer

3. Panel Repair Buffer

All of these functions utilize op amps as non-inverting, unity-gain buffers. The VCOM Driver and Gamma Buffer

are buffers that supply a well regulated DC voltage. A Panel Repair Buffer, on the other hand, provides a high

frequency signal that contains part of the display’s visual image.

In an effort to reduce production costs, display manufacturers use a minimum variety of different parts in their

TFT displays. As a result, the same type of op amp will be used for the VCOM Driver, Gamma Buffer, and Panel

Repair Buffer. To perform all these functions, such an op amp must have the following characteristics:

1. Large output current drive

2. Rail to rail input common mode range

3. Rail to rail output swing

4. Medium speed gain bandwidth and slew rate

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