LM7372IMAX

LM7372 High Speed, High Output Current, Dual Operational Amplifier February 2002 LM7372 High Speed, High Output Current, Dual Operational Amplifier General Description The LM7372 is a high speed dual voltage feedback amplifier that has the slewing characteristic of current feedback amplifiers; yet it can be used in all traditional voltage feedback amplifier configurations. The LM7372 is stable for gains as low as +2 or −1. It provides a very high slew rate at 3000V/µs and a wide gain bandwidth product of 120MHz, while consuming only 6.5mA/per amplifier of supply current. It is ideal for video and high speed signal processing applications such as xDSL and pulse amplifiers. With 150mA output current, the LM7372 can be used for video distribution, as a transformer driver or as a laser diode driver. Operation on ± 15V power supplies allows for large signal swings and provides greater dynamic range and signal-to-noise ratio. The LM7372 offers high SFDR and low THD, ideal for ADC/DAC systems. In addition, the LM7372 is specified for ± 5V operation for portable applications. The LM7372 is built on National’s Advance VIP™ III (Vertically integrated PNP) complementary bipolar process. Features n n n n n n n n −80dBc highest harmonic distortion @1MHz, 2VPP Very high slew rate: 3000V/µs Wide gain bandwidth product: 120MHz −3dB frequency @ AV = +2: 200MHz Low supply current: 13mA (both amplifiers) High open loop gain: 85dB High output current: 150mA Differential gain and phase: 0.01%, 0.02˚ Applications n n n n n n HDSL and ADSL Drivers Multimedia broadcast systems Professional video cameras CATV/Fiber optics signal processing Pulse amplifiers and peak detectors HDTV amplifiers Typical Application 20004903 FIGURE 1. Single Supply Application (SOIC-16) © 2002 National Semiconductor Corporation DS200049 www.national.com LM7372 Connection Diagrams 16-Pin SOIC 8-Contact LLP 20004902 Top View * Heatsink Pins. See note 4 20004901 Top View 8-Pin PSOP 20004929 Top View For PSOP SOIC-8 the exposed pad should be tied either to V− or left electrically floating. (die attach material is conductive and is internally tied to V−) Ordering Information Symbol Temperature Range −40˚C to +85˚C 16-Pin SOIC 8-Pin LLP 8-Pin PSOP LM7372IMA LM7372IMAX LM7372ILD LM7372ILDX LM7372MR LM7372MRX LM7372IMA LM7372IMA L7372 L7372 LM7372MR LM7372MR Rails 2.5k Units Tape and Reel 1k Units Tape and Reel 4.5k Units Tape and Reel Rails 2.5k Units Tape and Reel Package Markiing Transport Media NSC Drawing M16A LDC08A MRA08A www.national.com 2 LM7372 Absolute Maximum Ratings 3) (Notes 1, Input Voltage Maximum Junction Temperature (Note 4) V− to V+ 150˚C If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. ESD Tolerance Human Body Model Machine Model Suppy Voltage (V+−V−) Differential Input Voltage (VS = ± 15V) Output Short Circuit to Ground (Note 3) Storage Temp. Range Soldering Information Infrared or Convection Reflow (20 sec.) Wave Soldering Lead Temperature (10 sec.) 235˚C 260˚C 1.5kV (Note 2) 200V (Note 2) 36V Operating Ratings (Note 1) Supply Voltage Junction Temperature Range(TJ) LM7372 Thermal Resistance(θJA) 16-Pin SOIC See (Note 4) LLP-8 Package (See Application Section) 8-Pin PSOP (See Application Section) 106˚C/W 70˚C/W 40˚C/W 59˚C/W −40˚C ≤ TJ ≤ 85˚C 9V ≤ VS ≤ 36V ± 10V Continuous −65˚C to 150˚C ± 15V DC Electrical Characteristics Unless otherwise specified, all limits guaranteed for TJ = 25˚C, VCM = 0V and RL = 1kΩ. Boldface apply at the temperature extremes. Symbol VOS TC VOS IB IOS RIN RO CMRR PSRR VCM AV Parameter Input Offset Voltage Input Offset Voltage Average Drift Input Bias Current Input Offset Current Input Resistance Open Loop Output Resistance Common Mode Rejection Ratio Power Supply Rejection Ratio Input Common-Mode Voltage Range Large Signal Voltage Gain (Note 7) VCM = ± 10V VS = ± 15V to ± 5V CMRR > 60dB RL = 1kΩ RL = 100Ω VO Output Swing RL = 1kΩ 75 70 70 66 13 12.7 −13 −12.7 IOUT = − 150mA IOUT = 150mA ISC Output Short Circuit Current Sourcing Sinking 3 Conditions Min (Note 6) Typ (Note 5) 2.0 12 2.7 0.1 Max (Note 6) 8.0 10.0 10 12 4.0 6.0 Units mV µV/˚C µA µA MΩ MΩ Ω dB dB V dB dB V V V V mA mA Common Mode Differential Mode 75 70 75 70 40 3.3 15 93 90 ± 13 85 81 13.4 −13.3 12.4 −11.9 260 250 11.8 11.4 −11.2 −10.8 www.national.com LM7372 ± 15V DC Electrical Characteristics Symbol IS Parameter Supply Current (both Amps) (Continued) Unless otherwise specified, all limits guaranteed for TJ = 25˚C, VCM = 0V and RL = 1kΩ. Boldface apply at the temperature extremes. Conditions Min (Note 6) Typ (Note 5) 13 Max (Note 6) 17 19 Units mA ± 15V AC Electrical Characteristics Unless otherwise specified, all limits guaranteed for TJ = 25˚C, VCM = 0V and RL = 1kΩ. Boldface apply at the temperature extremes. Symbol SR Parameter Slew Rate (Note 8) Unity Bandwidth Product −3dB Frequency φm tS tP AD φD hd2 hd3 IMD Phase Margin Settling Time (0.1%) Propagation Delay Differential Gain (Note 9) Differential Phase (Note 9) Second Harmonic Distortion FIN = 1MHz, AV = +2 Third Harmonic Distortion FIN = 1MHz, AV = +2 Intermodulation Distortion VOUT = 2VP-P, RL = 100Ω VOUT = 16.8VP-P, RL = 100Ω VOUT = 2VP-P, RL = 100Ω VOUT = 16.8VP-P, RL = 100Ω Fin 1 = 75kHz, Fin 2 = 85kHz VOUT = 16.8VP-P, RL = 100Ω f = 10kHz f = 10kHz AV = +2 AVOL = 6dB AV = −1, AO = ± 5V, RL = 500Ω AV = −2, VIN = ± 5V, RL = 500Ω Conditions AV = +2, VIN 13VP-P AV = +2, VIN 10VP-P Min (Note 6) Typ (Note 5) 3000 2000 120 220 70 50 6.0 0.01 0.02 −80 −73 −91 −67 −87 MHz MHz deg ns ns % deg dBc dBc dBc dBc dBc Max (Note 6) Units V/µs en in Input-Referred Voltage Noise Input-Referred Current Noise 14 1.5 nV/ pA/ ± 5V DC Electrical Characteristics Unless otherwise specified, all limits guaranteed for TJ = 25˚C, VCM = 0V and RL = 1kΩ. Boldface apply at the temperature extremes. Symbol VOS TC VOS IB IOS RIN RO CMRR Parameter Input Offset Voltage Input Offset Voltage Average Drift Input Bias Current Input Offset Current Input Resistance Open Loop Output Resistance Common Mode Rejection Ratio VCM = ± 2.5V 70 65 Common Mode Differential Mode Conditions Min (Note 6) Typ (Note 5) 2.2 12 3.3 0.1 40 3.3 15 90 10 12 4 6 Max (Note 6) 8.0 10.0 Units mV µV/˚C µA µA MΩ MΩ Ω dB www.national.com 4 LM7372 ± 5V DC Electrical Characteristics Symbol PSRR VCM AV Parameter Power Supply Rejection Ratio Input Common-Mode Voltage Range Large Signal Voltage Gain (Note 7) (Continued) Unless otherwise specified, all limits guaranteed for TJ = 25˚C, VCM = 0V and RL = 1kΩ. Boldface apply at the temperature extremes. Conditions VS = ± 15V to ± 5V CMRR > 60dB RL = 1kΩ RL = 100Ω 70 65 64 60 3.2 3.0 −3.2 −3.0 IOUT = − 80mA IOUT = 80mA 2.5 2.2 −2.5 −2.2 Min (Note 6) 75 70 Typ (Note 5) 90 Max (Note 6) Units dB V dB dB V V V V mA mA 16 18 mA ±3 78 72 3.4 −3.4 2.8 −2.7 150 150 12.4 VO Output Swing RL = 1kΩ ISC IS Output Short Circuit Current Supply Current (both Amps) Sourcing Sinking ± 5V AC Electrical Characteristics Unless otherwise specified, all limits guaranteed for TJ = 25˚C, VCM = 0V and RL = 1kΩ. Boldface apply at the temperature extremes. Symbol SR Parameter Slew Rate (Note 8) Unity Bandwidth Product −3dB Frequency φm tS tP AD φD hd2 hd3 en in Phase Margin Settling Time (0.1%) Propagation Delay Differential Gain (Note 9) Differential Phase (Note 9) Second Harmonic Distortion FIN = 1MHz, AV = +2 Third Harmonic Distortion FIN = 1MHz, AV = +2 Input-Referred Voltage Noise Input-Referred Current Noise VOUT = 2VP-P, RL = 100Ω VOUT = 2VP-P, RL = 100Ω f = 10kHz f = 10kHz AV = −1, VO = ± 1V, RL = 500Ω AV = +2, VIN = ± 1V, RL = 500Ω AV = +2 Conditions AV = +2, VIN 3VP-P Min (Note 6) Typ (Note 5) 700 100 125 70 70 7 0.02 0.03 −84 −94 14 1.8 Max (Note 6) Units V/µs MHz MHz deg ns ns % deg dBc dBc nV/ pA/ Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is intended to be functional, but specific performance is not guaranteed. For guaranteed specifications and the test conditions, see the Electrical Characteristics. Note 2: For testing purposes, ESD was applied using human body model, 1.5kΩ in series with 100pF. Machine model, 0Ω in series with 200pF. Note 3: Applies to both single-supply and split-supply operation. Continuous short circuit operation at elevated ambient temperature can result in exceeding the maximum allowed junction temperature of 150˚C. 5 www.national.com LM7372 ± 5V AC Electrical Characteristics Note 5: Typical values represent the most likely parametic norm. Note 6: All limits are guaranteed by testing or statistical analysis. (Continued) Note 4: The maximum power dissipation is a function of T(JMAX), θJA, and TA. The maximum allowable power dissipation at any ambient temperature is PD = (T(JMAX) – TA)/θJA. All numbers apply for packages soldered directly into a PC board. The value for θJA is 106˚C/W for the SOIC 16 package. With a total area of 4sq. in of 1oz CU connected to pins 1,6,8,9 & 16, θJA for the SOIC 16 is decreased to 70˚C/W. Note 7: Large signal voltage gain is the total output swing divided by the input signal required to produce that swing. For VS = ± 15V, VOUT = ± 10V. For VS = ± 5V, VOUT = ± 2V Note 8: Slew Rate is the average of the rising and falling slew rates. Note 9: Differential gain and phase are measured with AV = +2, VIN = 1VPP at 3.58 MHz and output is 150Ω terminated. Typical Performance Characteristics Harmonic Distortion vs. Frequency Harmonic Distortion vs. Frequency 20004904 20004906 Harmonic Distortion vs. Frequency Harmonic Distortion vs. Frequency 20004905 20004907 www.national.com 6 LM7372 Typical Performance Characteristics Harmonic Distortion vs. Output Level (Continued) Harmonic Distortion vs. Output Level 20004908 20004909 Harmonic Distortion vs. Output Level Harmonic Distortion vs. Output Level 20004910 20004911 Harmonic Distortion vs. Load Resistance Harmonic Distortion vs. Load Resistance 20004912 20004913 7 www.national.com LM7372 Typical Performance Characteristics Harmonic Distortion vs. Load Resistance (Continued) Harmonic Distortion vs. Load Resistance 20004914 20004915 Frequency Response Frequency Response 20004916 20004917 Frequency Response Small Signal Pulse Response 20004920 20004918 www.national.com 8 LM7372 Typical Performance Characteristics Large Signal Pulse Response (Continued) Thermal Performance of 8ld-LLP 20004921 20004922 Harmonic Distortion vs. Frequency Input Bias Current (µA) vs. Temperature 20004923 20004927 Output Voltage vs. Output Current 20004924 9 www.national.com LM7372 Simplified Schematic Diagram 20004928 Application Notes The LM7372 is a high speed dual operational amplifier with a very high slew rate and very low distortion, yet like many other op amps, it is used in conventional voltage feedback amplifier applications. Also, again like many op amps, it has a class AB output stage in order to be able to deliver high currents to low impedance loads, yet draw very little quiescent supply current. For most op-amps in typical applications, this topology means that internal power dissipation is rarely an issue, even with the trend to smaller surface mount packages. However, the LM7372 has been designed for applications where significant levels of power dissipation will be encountered, and an effective means of removing the internal heat generated by this power dissipation is needed to maintain the semiconductor junction temperature at acceptable levels, particularly in environments with elevated ambient temperatures. Several factors contribute to power dissipation and consequently higher semiconductor junction temperatures, and these factors need to be well understood if the LM7372 is to perform to the desired specifications in a given application. Since different applications will have different dissipation levels and different compromises can be made between the ways these factors will contribute to the total junction temperature, this section will examine the typical application shown on the front page of this data sheet as an example, and offer suggestions for solutions where excessive junction temperatures are encountered. There are two major contributors to the internal power dissipation; the product of the supply voltage and the LM7372 quiescent current when no signal is being delivered to the external load, and the additional power dissipated while delivering power to the external load. The first of these components is easy to calculate simply by inspection of the data sheet. The LM7372 quiescent supply current is given as 6.5mA per amplifier, so with a 24Volt supply the power dissipation is PQ = VS x 2Iq (VS = VCC + VEE) = 24 x (6.5 x 10-3) = 312mW This is already a high level of internal power dissipation, and in a small surface mount package with a thermal resistance (θJA = 140˚C/Watt (a not unreasonable value for an SO-8 package) would result in a junction temperature 140˚C/W x 0.312W = 43.7˚C above the ambient temperature. A similar calculation using the worst case maximum current limit at an 85˚C ambient will yield a power dissipation of 456mW with a junction temperature of 149˚C, perilously close to the maximum permitted junction temperature of 150˚C! The second contributor to high junction temperature is the additional power dissipated internally when power is being delivered to the external load. This cause of temperature rise can be less amenable to calculation, even when the actual operating conditions are known. For a Class B output stage, one transistor of the output pair will conduct the load current as the output voltage swings positive, with the other transistor drawing no current, and hence dissipating no power. During the other half of the signal swing this situation is reversed, with the lower transistor sinking the load current and the upper transistor is cut off. The current in each transistor will be a half wave rectified version of the total load current. Ideally neither transistor will dissipate power when there is no signal swing, but will dissipate increasing power as the output current increases. However, as the signal voltage across the load increases with load current, the voltage across the output transistor (which is the difference voltage between the supply voltage and the instantaneous voltage across the load) will decrease and a point will be reached where the dissipation in the transistor will begin to decrease again. If the signal is driven into a square wave, ideally the transistor dissipation will fall all the way back to zero. www.national.com 10 LM7372 Application Notes (Continued) For each amplifier then, with an effective load each of RL and a sine wave source, integration over the half cycle with a supply voltage VS and a load voltage VL yields the average power dissipation PD = VSVL/πRL - VL2/2RL..........(1) Where VS is the supply voltage and VL is the peak signal swing across the load RL. For the package, the power dissipation will be doubled since there are two amplifiers in the package, each contributing half the swing across the load. The circuit in Figure 1 is using the LM7372 as the upstream driver in an ADSL application with Discrete MultiTone modulation. With DMT the upstream signal is spread into 32 adjacent channels each 4kHz wide. For transmission over POTS, the regular telephone service, this upstream signal from the CPE (Customer Premise Equipment) occupies a frequency band from around 20kHz up to a maximum frequency of 135kHz. At first sight, these relatively low transmission frequencies certainly do not seem to require the use of very high speed amplifiers with GBW products in the range of hundreds of megahertz. However, the close spacing of multiple channels places stringent requirements on the linearity of the amplifier, since non-linearities in the presence of multiple tones will cause harmonic products to be generated that can easily interfere with the higher frequency down stream signals also present on the line. The need to deliver 3rd Harmonic distortion terms lower than −75dBc is the reason for the LM7372 quiescent current levels. Each amplifier is running over 3mA in the output stage alone in order to minimize crossover distortion. xDSL signal levels are adjusted to provide a given power level on the line, and in the case of ADSL this is an average power of 13dBm. For a line with a characteristic impedance of 100Ω this is only 20mW. Because the transformer shown in Figure 1 is part of a transceiver circuit, two back-termination resistors are connected in series with each amplifier output. Therefore the equivalent RL for each amplifier is also 100Ω, and each amplifier is required to deliver 20mW to this load. Since VL2/2RL = 20mW then VL = 2V(peak). Using Equation (1) with this value for signal swing and a 24V supply, the internal power dissipation per amplifier is 132.8mW. Adding the quiescent power dissipation to the amplifier dissipation gives the total package internal power dissipation as PD(Total) = 312mW + (2 x 132.8mW) = 578mW This result is actually quite pessimistic because it assumes that the dissipation as a result of load current is simply added to the dissipation as a result of quiescent current. This is not correct since the AB bias current in the output stage is diverted to load current as the signal swing amplitude increases from zero. In fact with load currents in excess of 3.3mA, all the bias current is flowing in the load, consequently reducing the quiescent component of power dissipation. Also, it assumes a sine wave signal waveform when the actual waveform is composed of many tones of different phases and amplitudes which may demonstrate lower average power dissipation levels. The average current for a load power of 20mW is 14.1mA. Neglecting the AB bias current this appears as a full-wave rectified current waveform in the supply current with a peak value of 19.9mA. The peak to average ratio for a waveform of this shape is 1.57, so the total average load current is 11 12.7mA. Adding this to the quiescent current, and subtracting the power dissipated in the load gives the same package power dissipation level calculated above. Nevertheless, when the supply current peak swing is measured, it is found to be significantly lower because the AB bias current is contributing to the load current. The supply current has a peak swing of only 14mA (compared to 19.9mA) superimposed on the quiescent current, with a total average value of only 21mA. Therefore the total package power dissipation in this application is PD(Total) = (VS x Iavg) - Power in Load = (24 x 21)mW - 40mW = 464mW This level of power dissipation would not take the junction temperature in the SO-8 package over the absolute maximum rating at elevated ambient temperatures (barely), but there is no margin to allow for component tolerances or signal variances. To develop 20mW in a 100Ω requires each amplifier to deliver a peak voltage of only 2V, or 4V(P-P). This level of signal swing does not require a high supply voltage but the application uses a 24V supply. This is because the modulation technique uses a large number of tones to transmit the data. While the average power level is held to 20mW, at any time the phase and amplitude of individual tones will be such as to generate a combined signal with a higher peak value than 2V. For DMT this crest factor is taken to be around 5.33 so each amplifier has to be able to handle a peak voltage swing of VLpeak = 1.4 x 5.33 = 7.5V or 15V(P-P) If other factors, such as transformer loss or even higher peak to average ratios are allowed for, this means the amplifiers must each swing between 16 to 18V(P-P). The required signal swing can be reduced by using a step-up transformer to drive the line. For example a 1:2 ratio will reduce the peak swing requirement by half, and this would allow the supply to be reduced by a corresponding amount. This is not recommended for the LM7372 in this particular application for two reasons. Although the quiescent power contribution to the overall dissipation is reduced by about 150mW, the internal power dissipation to drive the load remains the same, since the load for each amplifier is now 25Ω instead of 100Ω. Furthermore, this is a transceiver application where downstream signals are simultaneously appearing at the transformer secondary. The down stream signals appear differentially across the back termination resistors and are now stepped down by the transformer turns ratio with a consequent loss in receiver sensitivity compared to using a 1:1 transformer. Any trade-off to reduce the supply voltage by an increase in turns ratio should bear these factors in mind, as well as the increased signal current levels required with lower impedance loads. At an elevated ambient temperature of 85˚C and with an average power dissipation of 464mW, a package thermal resistance between 60˚C/W and 80˚C/W will be needed to keep the maximum junction temperature in the range 110˚C to 120˚C. The PSOP or LLP package would be the package of choice here with ample board copper area to aid in heat dissipation (see table 2). For most standard surface mount packages, SO-8, SO-14, SO-16 etc, the only means of heat removal from the die is through the bond wires to external copper connecting to the leads. Usually it will be difficult to reduce the thermal resis- www.national.com LM7372 Application Notes (Continued) tance of these packages below 100˚C/W by these methods and several manufacturers, including National, offer package modifications to enhance the thermal characteristics. Improved removal of internal heat can be achieved by directly connecting bond wires to the lead frame inside the package. Since this lead frame supports the die attach paddle, heat is transferred directly from the substrate to the outside copper by these bond wires. For an 8 pin package, this enhancement is somewhat limited since only the V-bond wire can be used, because it is the only lead at the same voltage as the substrate and there is an electrical connection as well as a thermal connection. the small diameter bonding wires. Values of θJA in ˚C/W for the LLP package with various areas and weights of copper are tabulated below. TABLE 2. Thermal Resistance of LLP Package Copper Top Layer Only Bottom Layer Only Top And Bottom Area 0.5 oz 1.0 oz 2.0 oz 0.5 oz 1.0 oz 2.0 oz 0.5 oz 1.0 oz 2.0 oz 0.5 in2 115 91 74 102 92 85 83 71 63 1.0 in2 105 79 60 88 75 66 70 57 48 2.0 in2 102 72 52 81 65 54 63 47 37 20004925 FIGURE 2. Copper Heatsink Patterns The LM7372 is available in the SOIC-16 package. Since only 8 pins are needed for the two operational amplifiers, the remaining pins are used for heat sink purposes. Each of the end pins, 1,8,9 & 16 are internally bonded to the lead frame and form an effective means of transferring heat to external copper. This external copper can be either electrically isolated or be part of the topside ground plane in a single supply application. Figure 2. shows a copper pattern which can be used to dissipate internal heat from the LM7372. Table 1 gives some values of θJA for different values of L and H with 1oz copper. TABLE 1. Thermal Resistance with Area of Cu Package SOIC 16 SOIC 16 SOIC 16 L (in) 1 2 3 H (in) 0.5 1 1.5 θJA (˚C/W) 83 70 67 From Table 1 it is apparent that two areas of 1oz copper at each end of the package, each 2 in2 in area (for a total of 2600mm2) will be sufficient to hold the maximum junction temperature under 120˚C with an 85˚C ambient temperature. An even better package for removing internally generated heat is a package with an exposed die attach paddle. The LM7372 is also available in the 8 lead LLP and PSOP packages. For these packages the entire lower surface of the paddle is not covered with plastic, which would otherwise act as a thermal barrier to heat transfer. Heat is transferred directly from the die through the paddle rather than through Table 2 clearly demonstrates the superior thermal qualities of the exposed pad package. For example, using the topside copper only in the same way as shown for the SOIC package (Figure 2), with the L dimension held at 1 inch, the LLP requires half the area of 1 oz copper at each end of the package (1 in2, for a total of 1300mm2), for a comparable thermal resistance of 72˚C/Watt. This gives considerably more flexibility in the pcb layout aside from using less copper. The shape of the heat sink shown in Figure 2 is necessary to allow external components to be connected to the package pins. If thermal vias are used beneath the LLP to the bottom side ground plane, then a square pattern heat sink can be used and there is no restriction on component placement on the top side of the board. Even better thermal characteristics are obtained with bottom layer heatsinking. A 2 inch square of 0.5oz copper gives the same thermal resistance (81˚C/W) as a competitive thermally enhanced SO-8 package which needs two layers of 2 oz copper, each 4 in2 (for a total of 5000 mm2). With heavier copper, thermal resistances as low as 54˚C/W are possible with bottom side heatsinking only, substantially improving the long term reliability since the maximum junction temperature is held to less than 110˚C, even with an ambient temperature of 85˚C. If both top and bottom copper planes are used, the thermal resistance can be brought to under 40˚C/W. Power Supplies The LM7372 is fabricated on a high voltage, high speed process. Using high supply voltages ensures adequate headroom to give low distortion with large signal swings. In Figure 1, a single 24V supply is used. To maximize the output dynamic range the non-inverting inputs are biassed to half supply voltage by the resistive divider R1, R2. The input signals are AC coupled and the coupling capacitors (C1, C2) can be scaled with the bias resistors (R3, R4) to form a high pass filter if unwanted coupling from the POTS signal occurs. Supply decoupling is important at both low and high frequencies. The 10µF Tantalum and 0.1µF Ceramic capacitors should be connected close to the supply Pin 14. Note that the V− pin (pin 6), and the PCB area associated with the heatsink (Pins 1,8,9 & 16) are at the same potential. Any layout should avoid running input signal leads close to this ground plane, or unwanted coupling of high frequency supply currents may generate distortion products. Although this application shows a single supply, conversion to a split supply is straightforward. The half supply resistive 12 www.national.com LM7372 Application Notes (Continued) divider network is eliminated and the bias resistors at the non-inverting inputs are returned to ground, see Figure 3 (the pin numbers in Figure 3 are given for the LLP and PSOP packages, those in Figure 1 are for the SOIC package). With a split supply, note that the ground plane and the heatsink copper must be separate and are at different potentials, with the heatsink (pin 4 of the LLPand PSOP, pins 6,1,8,9 &16 of the SOIC) now at a negative potential (V−). In either configuration, the area under the input pins should be kept clear of copper (Whether ground plane copper or heatsink copper) to avoid parasitic coupling to the inputs. The LM7372 is stable with non inverting closed loop gains as low as +2. Typical of any voltage feedback operational amplifier, as the closed loop gain of the LM7372 is increased, there is a corresponding reduction in the closed loop signal bandwidth. For low distortion performance it is recommended to keep the closed loop bandwidth at least 10X the highest signal frequency. This is because there is less loop gain (the difference between the open loop gain and the closed loop gain) available at higher frequencies to reduce harmonic distortion terms. 20004926 FIGURE 3. Split Supply Application (LLP) Printed Circuit Board Layout and Evaluation Boards: Generally, a good high-frequency layout will keep power supply and ground traces away from the inverting input and output pins. Parasitic capacitance on these nodes to ground will cause frequency response peaking and possible circuit oscillations (see Application Note OA-15 for more information). National Semiconductor suggests the following evaluation boards as a guide for high frequency layout and as an aid in device testing and characterization: Device LM7372MA LM7372ILD LM7372MR Package 16-Pin SOIC 8-Pin LLP 8-Pin PSOP Evaluation Board PN None CLC730114 CLC730121 These free evaluation boards are shipped automatically when a device sample request is placed with National Semiconductor. The DAP (die attach paddle) on the LLP-8, and the PSOP should be tied to V−. It should not be tied to ground. See respective Evaluation Board documentation. 13 www.national.com LM7372 Physical Dimensions unless otherwise noted inches (millimeters) 16-Pin SOIC NS Package Number M16A 8-Pin LLP NS Package Number LDC08A www.national.com 14 LM7372 High Speed, High Output Current, Dual Operational Amplifier Physical Dimensions inches (millimeters) unless otherwise noted (Continued) 8-Pin PSOP NS Package Number MRA08A LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. National Semiconductor Corporation Americas Email: support@nsc.com National Semiconductor Europe Fax: +49 (0) 180-530 85 86 Email: europe.support@nsc.com Deutsch Tel: +49 (0) 69 9508 6208 English Tel: +44 (0) 870 24 0 2171 Français Tel: +33 (0) 1 41 91 8790 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. National Semiconductor Asia Pacific Customer Response Group Tel: 65-2544466 Fax: 65-2504466 Email: ap.support@nsc.com National Semiconductor Japan Ltd. Tel: 81-3-5639-7560 Fax: 81-3-5639-7507 www.national.com National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.