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LM6584 Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM6584 Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 16 page Application Notes CIRCUIT DESCRIPTION GENERAL & SPEC The LM6584 is a bipolar process operational amplifier. It has an exceptional output current capability of 330mA. The part has both rail to rail inputs and outputs. It has a −3dB band- width of 24MHz. The part has input voltage noise of 23nV/ , and 2 nd and 3rd harmonic distortion of −53dB and −40dB respectively. INPUT SECTION The LM6584 has rail to rail inputs and thus has an input range over which the device may be biased of V − minus 0.5V, and V + plus V. The ultimate limit on input voltage excursion is the ESD protection diodes on the input pins. The most important consideration in Rail-to-Rail input op amps is to understand the input structure. Most Rail-to-Rail input amps use two differential input pairs to achieve this function. This is how the LM6584 works. A conventional PNP differential transistor pair provides the input gain from 0.5V below the negative rail to about one volt below the positive rail. At this point internal circuitry activates a differential NPN transistor pair that allows the part to function from 1 volt below the positive rail to 0.5V above the positive rail. The effect on the inputs pins is as if there were two different op amps connected to the inputs. This has several unique implications. • The input offset voltage will change, sometimes from positive to negative as the inputs transition between the two stages at about a volt below the positive rail. this effect is seen in the V OS vs. VCM chart in the Typical Performance Characteristics section of this datasheet. • The input bias currents can be either positive or negative. Do not expect a consistent flow in or out of the pins. • The part will have different specifications depending on whether the NPN or PNP stage is operating. • There is a little more input capacitance then a single stage input although the ESD diodes often swamp out the added base capacitance. • Since the input offset voltages can change from positive to negative the output may not be monotonic when the inputs are transitioning between the two stages and the part is in a high gain configuration. It should be remembered that swinging the inputs across the input stage transition may cause output distortion and accu- racy anomalies. It is also worth noting that anytime any amps inputs are swung near the rails THD and other specs are sure to suffer. OUTPUT SECTION Current Rating The LM6584 has an output current rating, sinking or sourc- ing, of 300mA. The LM6584 is ideally suited to loads that require a high value of peak current but only a reduced value of average current. This condition is typical of driving the gate of a MOSFET. While the output drive rating is 300mA peak, and the output structure supports rail-to-rail operation, the attainable output current is reduced when the gain and drive conditions are such that the output voltage approaches either rail. Output Power Because of the increased output drive capability, internal heat dissipation must be held to a level that does not in- crease the junction temperature above its maximum rated value of 150˚ C. Power Requirements The LM6584 operates from a voltage supply, of V + and ground, or from a V − and V+ split supply. Single-ended voltage range is +5V to +13V and split supply range is ±2.5V to ±6.5V. 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+, fol- lowed 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 be- tween 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 de- sign 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 sud- denly drawing current or suddenly stopping a current draw. The design engineer should insure that there are no damag- ing 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 deliver- ing to cause oscillations, instability, overshoot and distortion. A ground plane is the most effective way of insuring the www.national.com 9 |
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