LM4892 1 Watt Audio Power Amplifier with Headphone Sense General Description Key Specifications The LM4892 is an audio power amplifier primarily designed for demanding applications in mobile phones and other portable communication device applications. It is capable of delivering 1 watt of continuous average power to an 8Ω BTL load with less than 1% distortion (THD+N) from a 5VDC power supply. Switching between bridged speaker mode and headphone (single-ended) mode is accomplished using the headphone sense pin. Boomer audio power amplifiers are designed specifically to provide high quality output power with a minimal amount of external components. The LM4892 does not require output coupling capacitors or bootstrap capacitors, and therefore is ideally suited for mobile phone and other low voltage applications where minimal power consumption is a primary requirement. The LM4892 features a low-power consumption shutdown mode, which is achieved by driving the shutdown pin with logic low. Additionally, the LM4892 features an internal thermal shutdown protection mechanism. The LM4892 contains advanced pop & click circuitry which eliminates noise which would otherwise occur during turn-on and turn-off transitions. The LM4892 is unity-gain stable and can be configured by external gain-setting resistors. j PSRR at 217Hz, VDD = 5V, 8Ω Load 62dB (typ) j Power Output at 5.0V & 1% THD 1.0W (typ) j Power Output at 3.3V & 1% THD 400mW (typ) j Shutdown Current 0.1µA (typ) Features n Available in space-saving packages: LLP, micro SMD, MSOP, and SOIC n Ultra low current shutdown mode n BTL output can drive capacitive loads up to 500pF n Improved pop & click circuitry eliminates noise during turn-on and turn-off transitions n 2.2 - 5.5V operation n No output coupling capacitors, snubber networks or bootstrap capacitors required n Thermal shutdown protection n Unity-gain stable n External gain configuration capability n Headphone amplifier mode Applications n Mobile Phones n PDAs n Portable electronic devices Typical Application 20012701 FIGURE 1. Typical Audio Amplifier Application Circuit (Pin #’s apply to M & MM packages) Boomer ® is a registered trademark of National Semiconductor Corporation. © 2002 National Semiconductor Corporation DS200127 www.national.com LM4892 1 Watt Audio Power Amplifier with Headphone Sense October 2002 LM4892 Connection Diagrams 8 Bump micro SMD Small Outline (SO) Package 20012735 Top View Order Number LM4892M See NS Package Number M08A 20012723 Top View Order Number LM4892IBP, LM4892IBPX See NS Package Number BPA08DDB Mini Small Outline (MSOP) Package micro SMD Marking 20012770 Top View Order Number LM4892MM See NS Package Number MUA08A Top View X - Date Code T - Die Traceability G - Boomer Family H - LM4892IBP SO Marking MSOP Marking 20012736 20012772 20012771 Top View XY - Date Code TT - Die Traceability Bottom 2 lines - Part Number Top View G - Boomer Family 92 - LM4892MM LLP Package 20012789 Top View Order Number LM4892LD See NS Package Number LDA10B www.national.com 2 θJC (MSOP) (Note 2) θJA (MSOP) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage See AN-1112 ’microSMD Wafers Level Chip Scale Package’. −0.3V to VDD +0.3V Power Dissipation (Note 3) Internally Limited ESD Susceptibility (Note 4) 2500V ESD Susceptibility (Note 5) 250V Junction Temperature See AN-1187 ’Leadless Leadframe Package (LLP)’. Operating Ratings Temperature Range 150˚C TMIN ≤ TA ≤ TMAX Thermal Resistance θJC (SOP) 35˚C/W θJA (SOP) 150˚C/W θJA (micro SMD) 220˚C/W 220˚C/W (Note 9) Soldering Information −65˚C to +150˚C Input Voltage 190˚C/W θJA (LLP) 6.0V Storage Temperature 56˚C/W −40˚C ≤ TA ≤ 85˚C 2.2V ≤ VDD ≤ 5.5V Supply Voltage Electrical Characteristics VDD = 5V (Notes 1, 2) The following specifications apply for VDD = 5V, AV = 2, and 8Ω load unless otherwise specified. Limits apply for TA = 25˚C. LM4892 Symbol Parameter IDD Quiescent Power Supply Current ISD Shutdown Current Po Output Power Conditions Typical Limit (Note 6) (Note 7) VIN = 0V, Io = 0A, HP sense = 0V 4 10 VIN = 0V, Io = 0A, HP sense = 5V 2.5 Vshutdown = GND (Note 8) µA (max) 1 THD = 1% (max), f = 1kHz, RL = 32Ω, HP Sense > 4V 90 HP Sense high input voltage VIL HP Sense low input voltage THD+N Total Harmonic Distortion+Noise Po = 0.4 Wrms; f = 1kHz 10Hz ≤ BW ≤ 80kHz PSSR Power Supply Rejection Ratio Vripple = 200mV sine p-p mA (max) mA (max) 0.1 THD = 2% (max), f = 1kHz, RL = 8Ω, HP Sense < 0.8V VIH Units (Limits) W mW 4 V (min) 0.8 V (max) 0.1 % 62 (f = 217Hz) 66 (f = 1kHz) dB Electrical Characteristics VDD = 3.3V (Notes 1, 2) The following specifications apply for VDD = 3.3V, AV = 2, and 8Ω load unless otherwise specified. Limits apply for TA = 25˚C. LM4892 Symbol Parameter IDD Quiescent Power Supply Current ISD Shutdown Current Po Output Power Conditions Typical Limit (Note 6) (Note 7) Units (Limits) VIN = 0V, Io = 0A, HP sense = 0V 3.5 mA (max) VIN = 0V, Io = 0A, HP sense = 3.3V 2.0 mA (max) Vshutdown = GND (Note 8) 0.1 µA (max) THD = 1% (max), f = 1kHz, RL = 8Ω, HP Sense < 0.8V 0.4 W THD = 1% (max), f = 1kHz, RL = 32Ω, HP Sense > 3V 35 mW VIH HP Sense high input voltage 2.6 V (min) VIL HP Sense low input voltage 0.8 V (max) THD+N Total Harmonic Distortion+Noise Po = 0.15 Wrms; f = 1kHz 10Hz ≤ BW ≤ 80kHz 3 0.1 % www.national.com LM4892 Absolute Maximum Ratings LM4892 Electrical Characteristics VDD = 3.3V (Notes 1, 2) The following specifications apply for VDD = 3.3V, AV = 2, and 8Ω load unless otherwise specified. Limits apply for TA = 25˚C. (Continued) LM4892 Symbol PSSR Parameter Power Supply Rejection Ratio Conditions Vripple = 200mV sine p-p Typical Limit (Note 6) (Note 7) 60(f = 217Hz) 62 (f = 1kHz) Units (Limits) dB Electrical Characteristics VDD = 2.6V (Notes 1, 2) The following specifications apply for VDD = 2.6V, AV = 2, and 8Ω load unless otherwise specified. Limits apply for TA = 25˚C. LM4892 Symbol Parameter IDD Quiescent Power Supply Current ISD Shutdown Current Po Output Power Conditions Typical Limit (Note 6) (Note 7) Units (Limits) VIN = 0V, Io = 0A, HP sense = 0V 2.6 mA (max) VIN = 0V, Io = 0A, HP sense = 2.6V 1.5 mA (max) Vshutdown = GND (Note 8) 0.1 µA (max) THD = 1% (max), f = 1kHz, RL = 8Ω, HP Sense < 0.8V 0.25 W THD = 1% (max), f = 1kHz, RL = 4Ω, HP Sense < 0.8V 0.28 W 20 mW THD = 1% (max), f = 1kHz, RL = 32Ω, HP Sense > 2.5V VIH HP Sense high input voltage 2.0 V (min) VIL HP Sense low input voltage 0.8 V (max) THD+N Total Harmonic Distortion+Noise Po = 0.1 Wrms; f = 1kHz 10Hz ≤ BW ≤ 80kHz PSSR Power Supply Rejection Ratio Vripple = 200mV sine p-p 0.1 % 44(f = 217Hz) 44 (f = 1kHz) dB Note 1: All voltages are measured with respect to the ground pin, unless otherwise specified. Note 2: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is functional, but do not guarantee specific performance limits. Electrical Characteristics state DC and AC electrical specifications under particular test conditions which guarantee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where no limit is given, however, the typical value is a good indication of device performance. Note 3: The maximum power dissipation must be derated at elevated temperatures and is dictated by TJMAX, θJA, and the ambient temperature TA. The maximum allowable power dissipation is PDMAX = (TJMAX–TA)/θJA or the number given in Absolute Maximum Ratings, whichever is lower. For the LM4892, see power derating currents for additional information. Note 4: Human body model, 100pF discharged through a 1.5kΩ resistor. Note 5: Machine Model, 220pF–240pF discharged through all pins. Note 6: Typicals are measured at 25˚C and represent the parametric norm. Note 7: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis. Note 8: For micro SMD only, shutdown current is measured in a Normal Room Environment. Exposure to direct sunlight will increase ISD by a maximum of 2µA. Note 9: The Exposed-DAP of the LDA10B package should be electrically connected to GND or an electrically isolated copper area. The LM4892LD demo board (views featured in the Application Information section) has the Exposed-DAP connected to GND with a PCB area of 353mils x 86.7mils (8.97mm x 2.20mm) on the copper top layer and 714.7mils x 368mils (18.15mm x 9.35mm) on the copper bottom layer. External Components Description (Figure 1) Components Functional Description 1. Ri Inverting input resistance which sets the closed-loop gain in conjunction with Rf. This resistor also forms a high pass filter with Ci at fC= 1/(2π RiCi). 2. Ci Input coupling capacitor which blocks the DC voltage at the amplifiers input terminals. Also creates a highpass filter with Ri at fc = 1/(2π RiCi). Refer to the section, Proper Selection of External Components, for an explanation of how to determine the value of Ci. 3. Rf Feedback resistance which sets the closed-loop gain in conjunction with Ri. www.national.com 4 Components LM4892 External Components Description (Figure 1) (Continued) Functional Description 4. CS Supply bypass capacitor which provides power supply filtering. Refer to the Power Supply Bypassing section for information concerning proper placement and selection of the supply bypass capacitor. 5. CB Bypass pin capacitor which provides half-supply filtering. Refer to the section, Proper Selection of External Components, for information concerning proper placement and selection of CB. 6. COUT This output coupling capacitor blocks DC voltage while coupling the AC audio signal to the headphone speaker. Combined with RL, the headphone impedance, it creates a high pass filter at fc = 1/(2πRLCOUT). Refer to the section, Proper Selection of External Components for an explanation of how to determine the value of COUT. 7. RPU This is the pull up resistor to activate headphone operation when a headphone plug is plugged into the headphone jack. 8. RS This is the current limiting resistor for the headphone input pin. 9. RPD This is the pull down resistor to de-activate headphone operation when no headphone is plugged into the headphone jack. Typical Performance Characteristics THD+N vs Frequency at VDD = 3.3V, 8Ω RL, and PWR = 150mW THD+N vs Frequency at VDD = 5V, 8Ω RL, and PWR = 250mW 20012737 20012738 THD+N vs Frequency at VDD = 2.6V, 4Ω RL, and PWR = 100mW THD+N vs Frequency at VDD = 2.6V, 8Ω RL, and PWR = 100mW 20012739 20012740 5 www.national.com LM4892 Typical Performance Characteristics (Continued) THD+N vs Power Out at VDD = 5V, 8Ω RL, 1kHz THD+N vs Power Out at VDD = 3.3V, 8Ω RL, 1kHz 20012784 20012742 THD+N vs Power Out at VDD = 2.6V, 4Ω RL, 1kHz THD+N vs Power Out at VDD = 2.6V, 8Ω RL, 1kHz 20012785 20012786 Power Supply Rejection Ratio (PSRR) vs Frequency at VDD = 5V, 8Ω RL Power Supply Rejection Ratio (PSRR) vs Frequency at VDD = 5V, 8Ω RL 20012745 20012773 Input terminated with 10Ω R www.national.com Input Floating 6 (Continued) Power Supply Rejection Ratio (PSRR) vs Frequency at VDD = 2.6V, 8Ω RL Power Supply Rejection Ratio (PSRR) vs Frequency at VDD = 3.3V, 8Ω RL 20012747 20012746 Input terminated with 10Ω R Input terminated with 10Ω R Power Dissipation vs Output Power VDD = 3.3V Power Dissipation vs Output Power VDD = 5V 20012749 20012748 Output Power vs Load Resistance Power Dissipation vs Output Power VDD = 2.6V 20012751 20012750 7 www.national.com LM4892 Typical Performance Characteristics LM4892 Typical Performance Characteristics (Continued) Supply Current vs Shutdown Voltage Clipping (Dropout) Voltage vs Supply Voltage 20012752 20012753 Open Loop Frequency Response VDD = 3V No Load Open Loop Frequency Response VDD = 5V No Load 20012787 20012782 Power Derating Curves Power Derating Curves vs for 8 Bump microSMD 20012788 www.national.com 20012783 8 LM4892 Typical Performance Characteristics (Continued) Frequency Response vs Input Capacitor Size Noise Floor 20012754 20012756 THD+N vs Power Out at VDD = 5V, RL = 32Ω, 1kHz, Headphone mode THD+N vs Frequency at VDD = 5V, RL = 32Ω, PWR = 70mW, Headphone mode 20012777 20012776 Output Power vs Supply Voltage RL = 16Ω Output Power vs Supply Voltage RL = 8Ω 20012778 20012779 9 www.national.com LM4892 Typical Performance Characteristics (Continued) Output Power vs Supply Voltage RL = 32Ω Output Power vs Supply Voltage Headphone Output, RL = 32Ω 20012781 20012780 Application Information BRIDGE CONFIGURATION EXPLANATION As shown in Figure 1, the LM4892 has two operational amplifiers internally, allowing for a few different amplifier configurations. The first amplifier’s gain is externally configurable, while the second amplifier is internally fixed in a unity-gain, inverting configuration. The closed-loop gain of the first amplifier is set by selecting the ratio of Rf to Ri while the second amplifier’s gain is fixed by the two internal 20kΩ resistors. Figure 1 shows that the output of amplifier one serves as the input to amplifier two which results in both amplifiers producing signals identical in magnitude, but out of phase by 180˚. Consequently, the differential gain for the IC is AVD= 2 *(Rf/Ri) By driving the load differentially through outputs Vo1 and Vo2, an amplifier configuration commonly referred to as “bridged mode” is established. Bridged mode operation is different from the classical single-ended amplifier configuration where one side of the load is connected to ground. A bridge amplifier design has a few distinct advantages over the single-ended configuration, as it provides differential drive to the load, thus doubling output swing for a specified supply voltage. Four times the output power is possible as compared to a single-ended amplifier under the same conditions. This increase in attainable output power assumes that the amplifier is not current limited or clipped. In order to choose an amplifier’s closed-loop gain without causing excessive clipping, please refer to the Audio Power Amplifier Design section. A bridge configuration, such as the one used in LM4892, also creates a second advantage over single-ended amplifiers. Since the differential outputs, Vo1 and Vo2, are biased at half-supply, no net DC voltage exists across the load. This eliminates the need for an output coupling capacitor which is required in a single supply, single-ended amplifier configuration. Without an output coupling capacitor, the half-supply bias across the load would result in both increased internal IC power dissipation and also possible loudspeaker damage. www.national.com POWER DISSIPATION Power dissipation is a major concern when designing a successful amplifier, whether the amplifier is bridged or single-ended. A direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in internal power dissipation. Since the LM4892 has two operational amplifiers in one package, the maximum internal power dissipation is 4 times that of a single-ended amplifier. The maximum power dissipation for a given application can be derived from the power dissipation graphs or from Equation 1. (1) PDMAX = 4*(VDD)2/(2π2RL) It is critical that the maximum junction temperature TJMAX of 150˚C is not exceeded. TJMAX can be determined from the power derating curves by using PDMAX and the PC board foil area. By adding additional copper foil, the thermal resistance of the application can be reduced from a free air value of 150˚C/W, resulting in higher PDMAX. Additional copper foil can be added to any of the leads connected to the LM4892. It is especially effective when connected to VDD, GND, and the output pins. Refer to the application information on the LM4892 reference design board for an example of good heat sinking. If TJMAX still exceeds 150˚C, then additional changes must be made. These changes can include reduced supply voltage, higher load impedance, or reduced ambient temperature. Internal power dissipation is a function of output power. Refer to the Typical Performance Characteristics curves for power dissipation information for different output powers and output loading. POWER SUPPLY BYPASSING As with any amplifier, proper supply bypassing is critical for low noise performance and high power supply rejection. The capacitor location on both the bypass and power supply pins should be as close to the device as possible. Typical applications employ a 5V regulator with 10µF tantalum or electrolytic capacitor and a ceramic bypass capacitor which aid in supply stability. This does not eliminate the need for bypassing the supply nodes of the LM4892. The selection of a bypass capacitor, especially CB, is dependent upon PSRR 10 A microprocessor or a switch can replace the headphone jack contact pin. When a microprocessor or switch applies a voltage greater than 4V to the HP Sense pin, a bridgedconnected speaker is muted and Amp1 drives the headphones. (Continued) requirements, click and pop performance (as explained in the section, Proper Selection of External Components), system cost, and size constraints. SHUTDOWN FUNCTION In order to reduce power consumption while not in use, the LM4892 contains a shutdown pin to externally turn off the amplifier’s bias circuitry. This shutdown feature turns the amplifier off when a logic low is placed on the shutdown pin. By switching the shutdown pin to ground, the LM4892 supply current draw will be minimized in idle mode. While the device will be disabled with shutdown pin voltages less than 0.5VDC, the idle current may be greater than the typical value of 0.1µA. (Idle current is measured with the shutdown pin grounded). In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry to provide a quick, smooth transition into shutdown. Another solution is to use a single-pole, single-throw switch in conjunction with an external pull-up resistor. When the switch is closed, the shutdown pin is connected to ground and disables the amplifier. If the switch is open, then the external pull-up resistor will enable the LM4892. This scheme guarantees that the shutdown pin will not float thus preventing unwanted state changes. 20012774 FIGURE 2. Headphone Circuit (Pin #’s apply to M & MM packages) PROPER SELECTION OF EXTERNAL COMPONENTS Proper selection of external components in applications using integrated power amplifiers is critical to optimize device and system performance. While the LM4892 is tolerant of external component combinations, consideration to component values must be used to maximize overall system quality. The LM4892 is unity-gain stable which gives the designer maximum system flexibility. The LM4892 should be used in low gain configurations to minimize THD+N values, and maximize the signal to noise ratio. Low gain configurations require large input signals to obtain a given output power. Input signals equal to or greater than 1 Vrms are available from sources such as audio codecs. Please refer to the section, Audio Power Amplifier Design, for a more complete explanation of proper gain selection. Besides gain, one of the major considerations is the closedloop bandwidth of the amplifier. To a large extent, the bandwidth is dictated by the choice of external components shown in Figure 1. The input coupling capacitor, Ci, forms a first order high pass filter which limits low frequency response. This value should be chosen based on needed frequency response for a few distinct reasons. Table 1. Logic Level Truth Table for Shutdown and HP Sense Operation Shutdown HP Sense Pin Operational Mode Logic High Logic Low Bridged Amplifier Logic High Logic High Single-Ended Amplifier Logic Low Logic Low Micro-Power Shutdown Logic Low Logic High Micro-Power Shutdown HP SENSE FUNCTION Applying a voltage between 4V and VCC to the LM4892’s HP-Sense headphone control pin turns off Amp2 and mutes a bridged-connected load. Quiescent current consumption is reduced when the IC is in the single-ended mode. Figure 2 shows the implementation of the LM4892’s headphone control function. With no headphones connected to the headphone jack, the R4-R6 voltage divider sets the voltage applied to the HP-Sense pin (pin3) at approximately 50mV. This 50mV enables the LM4892 and places it in bridged mode operation. While the LM4892 operates in bridged mode, the DC potential across the load is essentially 0V. Since the HP-Sense threshold is set at 4V, even in an ideal situation, the output swing can not cause a false single-ended trigger. Connecting headphones to the headphone jack disconnects the headphone jack contact pin from V01 and allows R4 to pull the HP Sense pin up to VCC. This enables the headphone function, turns off Amp2, and mutes the bridged speaker. The amplifier then drives the headphone whose impedance is in parallel with R6. Resistor R6 has negligible effect on output drive capability since the typical impedance of headphones is 32Ω. The output coupling capacitor blocks the amplifier’s half supply DC voltage, protecting the headphones. Selection Of Input Capacitor Size Large input capacitors are both expensive and space hungry for portable designs. Clearly, a certain sized capacitor is needed to couple in low frequencies without severe attenuation. But in many cases the speakers used in portable systems, whether internal or external, have little ability to reproduce signals below 100Hz to 150Hz. Thus, using a large input capacitor may not increase actual system performance. In addition to system cost and size, click and pop performance is effected by the size of the input coupling capacitor, Ci. A larger input coupling capacitor requires more charge to reach its quiescent DC voltage (nominally 1/2 VDD). This charge comes from the output via the feedback and is apt to 11 www.national.com LM4892 Application Information LM4892 Application Information (Continued) create pops upon device enable. Thus, by minimizing the capacitor size based on necessary low frequency response, turn-on pops can be minimized. Besides minimizing the input capacitor size, careful consideration should be paid to the bypass capacitor value. Bypass capacitor, CB, is the most critical component to minimize turn-on pops since it determines how fast the LM4892 turns on. The slower the LM4892’s outputs ramp to their quiescent DC voltage (nominally 1/2 VDD), the smaller the turn-on pop. Choosing CB equal to 1.0µF along with a small value of Ci (in the range of 0.1µF to 0.39µF), should produce a virtually clickless and popless shutdown function. While the device will function properly, (no oscillations or motorboating), with CB equal to 0.1µF, the device will be much more susceptible to turn-on clicks and pops. Thus, a value of CB equal to 1.0µF is recommended in all but the most cost sensitive designs. (2) 5V is a standard voltage in most applications, it is chosen for the supply rail. Extra supply voltage creates headroom that allows the LM4892 to reproduce peaks in excess of 1W without producing audible distortion. At this time, the designer must make sure that the power supply choice along with the output impedance does not violate the conditions explained in the Power Dissipation section. Once the power dissipation equations have been addressed, the required differential gain can be determined from Equation 3. (3) Rf/Ri = AVD/2 From Equation 3, the minimum AVD is 2.83; use AVD = 3. Since the desired input impedance was 20kΩ, and with a AVD of 3, a ratio of 1.5:1 of Rf to Ri results in an allocation of Ri = 20kΩ and Rf = 30kΩ. The final design step is to address the bandwidth requirements which must be stated as a pair of −3dB frequency points. Five times away from a −3dB point is 0.17dB down from passband response which is better than the required ± 0.25dB specified. fL = 100Hz/5 = 20Hz fH = 20kHz * 5 = 100kHz As stated in the External Components section, Ri in conjunction with Ci create a highpass filter. Ci ≥ 1/(2π*20 kΩ*20 Hz) = 0.397 µF; use 0.39 µF AUDIO POWER AMPLIFIER DESIGN A 1W/8Ω AUDIO AMPLIFIER Given: Power Output Load Impedance Input Level Input Impedance Bandwidth 1 Wrms 8Ω 1 Vrms 20 kΩ 100 Hz–20 kHz ± 0.25 dB A designer must first determine the minimum supply rail to obtain the specified output power. By extrapolating from the Output Power vs Supply Voltage graphs in the Typical Performance Characteristics section, the supply rail can be easily found. A second way to determine the minimum supply rail is to calculate the required Vopeak using Equation 2 and add the output voltage. Using this method, the minimum supply voltage would be (Vopeak + (VODTOP + VODBOT)), where VODBOT and VODTOP are extrapolated from the Dropout Voltage vs Supply Voltage curve in the Typical Performance Characteristics section. www.national.com The high frequency pole is determined by the product of the desired frequency pole, fH, and the differential gain, AVD. With a AVD = 3 and fH = 100kHz, the resulting GBWP = 150kHz which is much smaller than the LM4892 GBWP of 4 MHz. This figure displays that if a designer has a need to design an amplifier with a higher differential gain, the LM4892 can still be used without running into bandwidth limitations. 12 LM4892 Application Information (Continued) 20012724 FIGURE 3. Higher Gain Audio Amplifier nates possible high frequency oscillations. Care should be taken when calculating the -3dB frequency in that an incorrect combination of Rf and Cf will cause rolloff before 20kHz. A typical combination of feedback resistor and capacitor that will not produce audio band high frequency rolloff is Rf = 20kΩ and Cf = 25pF. These components result in a -3dB point of approximately 320 kHz. The LM4892 is unity-gain stable and requires no external components besides gain-setting resistors, an input coupling capacitor, and proper supply bypassing in the typical application. However, if a closed-loop differential gain of greater than 10 is required, a feedback capacitor (Cf) may be needed as shown in Figure 3 to bandwidth limit the amplifier. This feedback capacitor creates a low pass filter that elimi- 13 www.national.com LM4892 Application Information (Continued) 20012775 FIGURE 4. Reference Design Schematic For Demo Boards www.national.com 14 LM4892 Application Information LM4892 micro SMD BOARD ARTWORK (Continued) Silk Screen 20012757 Top Layer 20012758 Bottom Layer 20012759 15 www.national.com LM4892 Application Information LM4892 MSOP DEMO BOARD ARTWORK (Continued) Silk Screen LM4892 SO DEMO BOARD ARTWORK Silk Screen 20012765 Top Layer 20012762 Top Layer 20012766 Bottom Layer 20012763 Bottom Layer 20012767 20012764 www.national.com 16 LM4892 Application Information LM4892 LLP DEMO BOARD ARTWORK (Continued) Composite View Silk Screen 20012790 20012791 Top Layer Bottom Layer 20012792 20012793 17 www.national.com LM4892 Application Information (Continued) Mono LM4892 Reference Design Boards Bill of Material for all Demo Boards Part Description Qty Ref Designator LM4892 Audio Amplifier 1 U1 Tantalum Capacitor, 1µF 2 Cs, Cb Ceramic Capacitor, 0.39µF 1 Ci Capacitor, 100µF 1 Cout Resistor, 1kΩ, 1/10W 1 Rpd Resistor, 20kΩ, 1/10W 3 Ri, Rf, Rpu2 Resistor, 100kΩ, 1/10W 2 Rpu1, Rs Jumper Header Vertical Mount 2X1, 0.100’ spacing 1 J1 3.5mm Audio Jack (PC mount, w/o nut), PN# SJS-0357-B Shogyo International Corp. (www.shogyo.com) 1 J2 PCB LAYOUT GUIDELINES This section provides practical guidelines for mixed signal PCB layout that involves various digital/analog power and ground traces. Designers should note that these are only ’rule-of-thumb’ recommendations and the actual results will depend heavily on the final layout. Single-Point Power / Ground Connections The analog power traces should be connected to the digital traces through a single point (link). A ’Pi-filter’ can be helpful in minimizing High Frequency noise coupling between the analog and digital sections. It is further recommended to put digital and analog power traces over the corresponding digital and analog ground traces to minimize noise coupling. General Mixed Signal Layout Recommendation Placement of Digital and Analog Components All digital components and high-speed digital signal traces should be located as far away as possible from analog components and circuit traces. Power and Ground Circuits For 2 layer mixed signal design, it is important to isolate the digital power and ground trace paths from the analog power and ground trace paths. Star trace routing techniques (bringing individual traces back to a central point rather than daisy chaining traces together in a serial manner) can have a major impact on low level signal performance. Star trace routing refers to using individual traces to feed power and ground to each circuit or even device. This technique will require a greater amount of design time but will not increase the final price of the board. The only extra parts required will be some jumpers. www.national.com Avoiding Typical Design / Layout Problems Avoid ground loops or running digital and analog traces parallel to each other (side-by-side) on the same PCB layer. When traces must cross over each other do it at 90 degrees. Running digital and analog traces at 90 degrees to each other from the top to the bottom side as much as possible will minimize capacitive noise coupling and cross talk. 18 LM4892 Physical Dimensions inches (millimeters) unless otherwise noted Note: Unless otherwise specified. 1. Epoxy coating. 2. 63Sn/37Pb eutectic bump. 3. Recommend non-solder mask defined landing pad. 4. Pin 1 is established by lower left corner with respect to text orientation pins are numbered counterclockwise. 5. Reference JEDEC registration MO-211, variation BC. 8-Bump micro SMD Order Number LM4892IBP, LM4892IBPX NS Package Number BPA08DDB X1 = 1.361 ± 0.03 X2 = 1.361 ± 0.03 X3 = 0.850 ± 0.10 19 www.national.com LM4892 Physical Dimensions inches (millimeters) unless otherwise noted (Continued) MSOP Order Number LM4892MM NS Package Number MUA08A www.national.com 20 LM4892 Physical Dimensions inches (millimeters) unless otherwise noted (Continued) SO Order Number LM4892M NS Package Number M08A 21 www.national.com LM4892 1 Watt Audio Power Amplifier with Headphone Sense Physical Dimensions inches (millimeters) unless otherwise noted (Continued) LLP Order Number LM4892LD NS Package Number LDA10B 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 Americas Customer Support Center Email: [email protected] Tel: 1-800-272-9959 www.national.com National Semiconductor Europe Customer Support Center Fax: +49 (0) 180-530 85 86 Email: [email protected] 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 Support Center Fax: 65-6250 4466 Email: [email protected] Tel: 65-6254 4466 National Semiconductor Japan Customer Support Center Fax: 81-3-5639-7507 Email: [email protected] Tel: 81-3-5639-7560 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.