LM4910 Output Capacitor-less Stereo 35mW Headphone Amplifier General Description Key Specifications The LM4910 is an audio power amplifier primarily designed for headphone applications in portable device applications. It is capable of delivering 35mW of continuous average power to a 32Ω load with less than 1% distortion (THD+N) from a 3.3VDC power supply. n PSRR at f = 217Hz 65dB (typ) n Power Output at VDD = 3.3V, RL = 32Ω, and THD ≤ 1% 35mW (typ) n Shutdown Current 0.1µA (typ) The LM4910 utilizes a new circuit topology that eliminates output coupling capacitors and half-supply bypass capacitors (patent pending). The LM4910 contains advanced pop & click circuitry which eliminates noises caused by transients that would otherwise occur during turn-on and turn-off. Features Boomer audio power amplifiers were designed specifically to provide high quality output power with a minimal amount of external components. Since the LM4910 does not require any output coupling capacitors, half-supply bypass capacitors, or bootstrap capacitors, it is ideally suited for low-power portable applications where minimal space and power consumption are primary requirements. The LM4910 features a low-power consumption shutdown mode, activated by driving the shutdown pin with logic low. Additionally, the LM4910 features an internal thermal shutdown protection mechanism. The LM4910 is also unity-gain stable and can be configured by external gain-setting resistors. n Eliminates headphone amplifier output coupling capacitors (patent pending) n Eliminates half-supply bypass capacitor (patent pending) n Advanced pop & click circuitry eliminates noises during turn-on and turn-off n Ultra-low current shutdown mode n Unity-gain stable n 2.2V - 5.5V operation n Available in space-saving MSOP, LLP, and SOIC packages Applications n n n n Mobile Phones PDAs Portable eletronics devices Portable MP3 players Typical Application 20030565 FIGURE 1. Typical Audio Amplifier Application Circuit Boomer ® is a registered trademark of National Semiconductor Corporation. © 2003 National Semiconductor Corporation DS200305 www.national.com LM4910 Output Capacitor-less Stereo 35mW Headphone Amplifier February 2003 LM4910 Connection Diagrams MSOP/SO Package 20030502 Top View Order Number LM4910MM or LM4910MA See NS Package Number MUA08A or M08A MSOP Marking 20030566 Top View G - Boomer Family C2 - LM4910MM SO Marking 20030567 Top View TT - Die Traceability Bottom 2 lines - Part Number LLP Package 20030595 Top View Order Number LM4910LQ See NS package Number LQB08A www.national.com 2 (Note 2) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage (Note 9) Storage Temperature θJC (MSOP) 56˚C/W θJA (MSOP) 190˚C/W θJC (SOP) 35˚C/W θJA (SOP) 150˚C/W 6.0V θJC (LQ) 57˚C/W −65˚C to +150˚C θJA (LQ) 140˚C/W -0.3V to VDD + 0.3V Input Voltage Power Dissipation (Note 3) Internally Limited ESD Susceptibility Pin 6 (Note 10) Operating Ratings 10kV Temperature Range ESD Susceptibility (Note 4) 2000V ESD Susceptibility (Note 5) 200V TMIN ≤ TA ≤ TMAX 150˚C Supply Voltage (VDD) Junction Temperature −40˚C ≤ T A ≤ 85˚C 2.2V ≤ VCC ≤ 5.5V Thermal Resistance Electrical Characteristics VDD = 3.3V (Notes 1, 2) The following specifications apply for VDD = 3.3V, AV = 1, and 32Ω load unless otherwise specified. Limits apply to TA = 25˚C. Symbol Parameter Conditions LM4910 Typ (Note 6) Limit (Notes 7, 8) Units (Limits) IDD Quiescent Power Supply Current VIN = 0V, 32Ω Load 3.5 6 mA (max) ISD Standby Current VSHUTDOWN = GND 0.1 1.0 µA (max) VOS Output Offset Voltage 5 30 mV (max) PO Output Power THD = 1% (max); f = 1kHz 35 30 mW (min) THD+N Total Harmonic Distortion + Noise PO = 30mWrms; f = 1kHz VRIPPLE = 200mVp-p sinewave Input terminated with 10Ω to ground 0.3 % 65 (f = 217Hz) 65 (f = 1kHz) dB PSRR Power Supply Rejection Ratio VIH Shutdown Input Voltage High 1.5 V (min) VIL Shutdown Input Voltage Low 0.4 V (max) Electrical Characteristics VDD = 3V (Notes 1, 2) The following specifications apply for VDD = 3V, AV = 1, and 32Ω load unless otherwise specified. Limits apply to TA = 25˚C. Symbol Parameter Conditions LM4910 Typ (Note 6) Limit (Notes 7, 8) Units (Limits) IDD Quiescent Power Supply Current VIN = 0V, 32Ω Load 3.3 6 ISD Standby Current VSHUTDOWN = GND 0.1 1.0 µA (max) VOS Output Offset Voltage 5 30 mV (max) 30 25 mW (min) PO Output Power THD+N Total Harmonic Distortion + Noise PO = 25mWrms; f = 1kHz THD = 1% (max); f = 1kHz VRIPPLE = 200mVp-p sinewave Input terminated with 10Ω to ground mA (max) 0.3 % 65 (f = 217 Hz) 65 (f = 1kHz) dB PSRR Power Supply Rejection Ratio VIH Shutdown Input Voltage High 1.5 V (min) VIL Shutdown Input Voltage Low 0.4 V (max) 3 www.national.com LM4910 Absolute Maximum Ratings LM4910 Electrical Characteristics VDD = 2.6V (Notes 1, 2) The following specifications apply for VDD = 2.6V, AV = 1, and 32Ω load unless otherwise specified. Limits apply to TA = 25˚C. Symbol Parameter Conditions LM4910 Typ (Note 6) Limit (Notes 7, 8) Units (Limits) IDD Quiescent Power Supply Current VIN = 0V, 32Ω Load 3.0 ISD Standby Current VSHUTDOWN = GND 0.1 µA (max) VOS Output Offset Voltage 5 mV (max) PO Output Power 13 mW THD+N Total Harmonic Distortion + Noise PO = 10mWrms; f = 1kHz 0.3 % 55 (f = 217Hz) 55 (f = 1kHz) dB PSRR Power Supply Rejection Ratio THD = 1% (max); f = 1kHz VRIPPLE = 200mVp-p sinewave Input terminated with 10Ω to ground mA (max) Note 1: All voltages are measured with respect to the GND 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 LM4910, see power derating currents for more 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: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level). Note 8: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis. Note 9: If the product is in shutdown mode and VDD exceeds 6V (to a max of 8V VDD) then most of the excess current will flow through the ESD protection circuits. If the source impedance limits the current to a max of 10ma then the part will be protected. If the part is enabled when VDD is above 6V circuit performance will be curtailed or the part may be permanently damaged. Note 10: Human body model, 100pF discharged through a 1.5kΩ resistor, Pin 6 to ground. External Components Description Components (Figure 1) 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/(2πRiCi). 2. CI Input coupling capacitor which blocks the DC voltage at the amplifier’s input terminals. Also creates a high-pass filter with Ri at fc = 1/(2πRiCi). 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. 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. www.national.com 4 LM4910 Typical Performance Characteristics THD+N vs Frequency THD+N vs Frequency 20030506 20030507 THD+N vs Frequency THD+N vs Frequency 20030508 20030509 THD+N vs Frequency THD+N vs Frequency 20030510 20030511 5 www.national.com LM4910 Typical Performance Characteristics (Continued) THD+N vs Output Power THD+N vs Output Power 20030516 20030517 THD+N vs Output Power THD+N vs Output Power 20030515 20030518 THD+N vs Output Power THD+N vs Output Power 20030519 www.national.com 20030520 6 LM4910 Typical Performance Characteristics (Continued) Output Power vs Load Resistance Output Power vs Load Resistance 20030523 20030578 Output Power vs Supply Voltage Output Power vs Supply Voltage 20030580 20030579 Output Power vs Supply Voltage Power Dissipation vs Output Power 20030581 20030530 7 www.national.com LM4910 Typical Performance Characteristics (Continued) Power Dissipation vs Output Power Power Dissipation vs Output Power 20030582 20030529 Channel Separation Power Supply Rejection Ratio 20030535 20030583 Power Supply Rejection Ratio Power Supply Rejection Ratio 20030584 www.national.com 20030585 8 LM4910 Typical Performance Characteristics (Continued) Open Loop Frequency Response Noise Floor 20030586 20030587 Frequency Response vs Input Capacitor Size Supply Current vs Supply Voltage 20030589 20030588 9 www.national.com LM4910 capacitance at the amplifier inputs. A more reliable way to lower gain or reduce power delivered to the load is to place a current limiting resistor in series with the load as explained in the Minimizing Output Noise / Reducing Output Power section. Application Information ELIMINATING OUTPUT COUPLING CAPACITORS Typical single-supply audio amplifiers that drive singleended (SE) headphones use a coupling capacitor on each SE output. This output coupling capacitor blocks the halfsupply voltage to which the output amplifiers are typically biased and couples the audio signal to the headphones. The signal return to circuit ground is through the headphone jack’s sleeve. The LM4910 eliminates these output coupling capacitors. Amp3 is internally configured to apply a bandgap referenced voltage (VREF = 1.58V) to a stereo headphone jack’s sleeve. This voltage matches the quiescent voltage present on the Amp1 and Amp2 outputs that drive the headphones. The headphones operate in a manner similar to a bridge-tiedload (BTL). The same DC voltage is applied to both headphone speaker terminals. This results in no net DC current flow through the speaker. AC current flows through a headphone speaker as an audio signal’s output amplitude increases on the speaker’s terminal. The headphone jack’s sleeve is not connected to circuit ground. Using the headphone output jack as a line-level output will place the LM4910’s bandgap referenced voltage on a plug’s sleeve connection. This presents no difficulty when the external equipment uses capacitively coupled inputs. For the very small minority of equipment that is DCcoupled, the LM4910 monitors the current supplied by the amplifier that drives the headphone jack’s sleeve. If this current exceeds 500mAPK, the amplifier is shutdown, protecting the LM4910 and the external equipment. 20030592 FIGURE 2. AMPLIFIER CONFIGURATION EXPLANATION As shown in Figure 1, the LM4910 has three operational amplifiers internally. Two of the amplifier’s have externally configurable gain while the other amplifier is internally fixed at the bias point acting as a unity-gain buffer. The closedloop gain of the two configurable amplifiers is set by selecting the ratio of Rf to Ri. Consequently, the gain for each channel of the IC is AV = -(Rf/Ri) ELIMINATING THE HALF-SUPPLY BYPASS CAPACITOR Typical single-supply audio amplifers are normally biased to 1/2VDD in order to maximize the output swing of the audio signal. This is usually achieved with a simple resistor divider network from VDD to ground that provides the proper bias voltage to the amplifier. However, this scheme requires the use of a half-supply bypass capacitor to improve the bias voltage’s stability and the amplifier’s PSRR performance. The LM4910 utilizes an internally generated, buffered bandgap reference voltage as the amplifier’s bias voltage. This bandgap reference voltage is not a direct function of VDD and therefore is less susceptible to noise or ripple on the power supply line. This allows for the LM4910 to have a stable bias voltage and excellent PSRR performance even without a half-supply bypass capacitor. By driving the loads through outputs VO1 and VO2 with VO3 acting as a buffered bias voltage the LM4910 does not require output coupling capacitors. The typical single-ended amplifier configuration where one side of the load is connected to ground requires large, expensive output coupling capacitors. A configuration such as the one used in the LM4910 has a major advantage over single supply, single-ended amplifiers. Since the outputs VO1, VO2, and VO3 are all biased at VREF = 1.58V, no net DC voltage exists across each load. This eliminates the need for output coupling capacitors that are required in a single-supply, single-ended amplifier configuration. Without output coupling capacitors in a typical singlesupply, single-ended amplifier, the bias voltage is placed across the load resulting in both increased internal IC power dissipation and possible loudspeaker damage. OUTPUT TRANSIENT (’CLICK AND POPS’) ELIMINATED The LM4910 contains advanced circuitry that virtually eliminates output transients (’clicks and pops’). This circuitry prevents all traces of transients when the supply voltage is first applied or when the part resumes operation after coming out of shutdown mode. The LM4910 remains in a muted condition until there is sufficient input signal magnitude ( > 5mVRMS, typ) to mask any remaining transient that may occur. Figure 2 shows the LM4910’s lack of transients in the differential signal (Trace B) across a 320 load. The LM4910’s active-low SHUTDOWN pin is driven by the logic signal shown in Trace A. Trace C is the VO1 output signal and Trace D is the VO3 output signal. To ensure optimal click and pop performance under low gain configurations (less than 0dB), it is critical to minimize the RC combination of the feedback resistor RF and stray input www.national.com POWER DISSIPATION Power dissipation is a major concern when designing a successful amplifier. A direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in internal power dissipation. The maximum power dissipation for a given application can be derived from the power dissipation graphs or from Equation 1. PDMAX = 4(VDD) 2 / (π2RL) (1) It is critical that the maximum junction temperature TJMAX of 150˚C is not exceeded. Since the typical application is for 10 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 and turn-on time. (Continued) headphone operation (32Ω impedance) using a 3.3V supply the maximum power dissipation is only 138mW. Therefore, power dissipation is not a major concern. SELECTION OF INPUT CAPACITOR SIZE Amplifiying the lowest audio frequencies requires a high value input coupling capacitor, Ci. A high value capacitor can be expensive and may compromise space efficiency in portable designs. In many cases, however, the headphones used in portable systems have little ability to reproduce signals below 60Hz. Applications using headphones with this limited frequency response reap little improvement by using a high value input capacitor. POWER SUPPLY BYPASSING As with any amplifier, proper supply bypassing is important for low noise performance and high power supply rejection. The capacitor location on the power supply pins should be as close to the device as possible. Typical applications employ a 3.3V 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 LM4910. A bypass capacitor value in the range of 0.1µF to 1µF is recommended for CS. In addition to system cost and size, turn-on time is affected by the size of the input coupling capacitor Ci. A larger input coupling capacitor requires more charge to reach its quiescent DC voltage. This charge comes from the output via the feedback Thus, by minimizing the capacitor size based on necessary low frequency response, turn-on time can be minimized. A small value of Ci (in the range of 0.1µF to 0.39µF), is recommended. MICRO POWER SHUTDOWN The voltage applied to the SHUTDOWN pin controls the LM4910’s shutdown function. Activate micro-power shutdown by applying a logic-low voltage to the SHUTDOWN pin. When active, the LM4910’s micro-power shutdown feature turns off the amplifier’s bias circuitry, reducing the supply current. The trigger point is 0.4V(max) for a logic-low level, and 1.5V(min) for a logic-high level. The low 0.1µA(typ) shutdown current is achieved by applying a voltage that is as near as ground as possible to the SHUTDOWN pin. A voltage that is higher than ground may increase the shutdown current. There are a few ways to control the micro-power shutdown. These include using a single-pole, single-throw switch, a microprocessor, or a microcontroller. When using a switch, connect an external 100kΩ pull-up resistor between the SHUTDOWN pin and VDD. Connect the switch between the SHUTDOWN pin and ground. Select normal amplifier operation by opening the switch. Closing the switch connects the SHUTDOWN pin to ground, activating micro-power shutdown. The switch and resistor guarantee that the SHUTDOWN pin will not float. This prevents unwanted state changes. In a system with a microprocessor or microcontroller, use a digital output to apply the control voltage to the SHUTDOWN pin. Driving the SHUTDOWN pin with active circuitry eliminates the pull-up resistor. USING EXTERNAL POWERED SPEAKERS The LM4910 is designed specifically for headphone operation. Often the headphone output of a device will be used to drive external powered speakers. The LM4910 has a differential output to eliminate the output coupling capacitors. The result is a headphone jack sleeve that is connected to VO3 instead of GND. For powered speakers that are designed to have single-ended signals at the input, the click and pop circuitry will not be able to eliminate the turn-on/turn-off click and pop. Unless the inputs to the powered speakers are fully differential the turn-on/turn-off click and pop will be very large. AUDIO POWER AMPLIFIER DESIGN A 30mW/32Ω Audio Amplifier Given: Power Output Load Impedance Input Level 30mWrms 32Ω 1Vrms Input Impedance 20kΩ 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. Since 3.3V is a standard supply voltage in most applications, it is chosen for the supply rail in this example. Extra supply voltage creates headroom that allows the LM4910 to reproduce peaks in excess of 30mW without producing audible distortion. At this time, the designer must make sure that the power supply choice along with the output impedance does no 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 2. SELECTING EXTERNAL COMPONENTS Selecting proper external components in applications using integrated power amplifiers is critical to optimize device and system performance. While the LM4910 is tolerant of external component combinations, consideration to component values must be used to maximize overall system quality. The LM4910 is unity-gain stable which gives the designer maximum system flexibility. The LM4910 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 1Vrms are available from sources such as audio codecs. Very large values should not be used for the gain-setting resistors. Values for Ri and Rf should be less than 1MΩ. 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 (2) 11 www.national.com LM4910 Application Information LM4910 Application Information (Continued) As mentioned in the Selecting Proper External Components section, Ri and Ci create a highpass filter that sets the amplifier’s lower bandpass frequency limit. Find the coupling capacitor’s value using Equation (3). From Equation 2, the minimum AV is 0.98; use AV = 1. Since the desired input impedance is 20kΩ, and with AV equal to 1, a ratio of 1:1 results from Equation 1 for Rf to Ri. The values are chosen with Ri = 20kΩ and Rf = 20kΩ. Ci≥ 1/(2πR ifL) The last step in this design example is setting the amplifier’s −3dB frequency bandwidth. To achieve the desired ± 0.25dB pass band magnitude variation limit, the low frequency response must extend to at least one-fifth the lower bandwidth limit and the high frequency response must extend to at least five times the upper bandwidth limit. The gain variation for both response limits is 0.17dB, well within the ± 0.25dB desired limit. The results are an fL = 100Hz/5 = 20Hz (3) fH = 20kHz x 5 = 100kHz (4) (5) The result is 1/(2π*20kΩ*20Hz) = 0.397µF Use a 0.39µF capacitor, the closest standard value. The high frequency pole is determined by the product of the desired frequency pole, fH, and the differential gain, AV. With an AV = 1 and fH = 100kHz, the resulting GBWP = 100kHz which is much smaller than the LM4910 GBWP of 11MHz. This figure displays that if a designer has a need to design an amplifier with higher differential gain, the LM4910 can still be used without running into bandwidth limitations. and an MINIMIZING OUTPUT NOISE / REDUCING OUTPUT POWER 20030568 FIGURE 3. Figure 4 shows an optional resistor connected between the amplifier output that drives the headphone jack sleeve and ground. This resistor provides a ground path that supressed power supply hum. This hum may occur in applications such as notebook computers in a shutdown condition and connected to an external powered speaker. The resistor’s 100Ω value is a suggested starting point. Its final value must be determined based on the tradeoff between the amount of noise suppression that may be needed and minimizing the additional current drawn by the resistor (25mA for a 100Ω resistor and a 5V supply). Output noise delivered to the load can be minimized with the use of an external resistor, RSERIES, placed in series with each load as shown in Figure 3. RSERIES forms a voltage divider with the impedance of the headphone driver RL. As a result, output noise is attenuated by the factor RL / (RL + RSERIES). Figure 4 illustrates the relationship between output noise and RSERIES for different loads. RSERIES also decreases output power delivered to the load by the factor RL / (RL + RSERIES)2. However, this may not pose a problem since most headphone applications require less than 10mW of output power. Figure 5 illustrates output power (@1% THD+N) vs RSERIES for different loads. www.national.com 12 LM4910 Application Information (Continued) ESD PROTECTION As stated in the Absolute Maximum Ratings, pin 6 (Vo3) on the LM4910 has a maximum ESD susceptibility rating of 10kV. For higher ESD voltages, the addition of a PCDN042 dual transil (from California Micro Devices), as shown in Figure 4, will provide additional protection. 20030594 FIGURE 4. The PCDN042 provides additional ESD protection beyond the 10kV shown in the Absolute Maximum Ratings for the Vo3 output Output Noise vs RSERIES 20030590 FIGURE 5. 13 www.national.com LM4910 Application Information (Continued) Output Power vs RSERIES 20030591 FIGURE 6. HIGHER GAIN AUDIO AMPLIFIER 20030593 FIGURE 7. www.national.com 14 feedback capacitor creates a low pass filter that eliminates possible high frequency oscillations. Care should be taken when calculating the -3dB frequency in that an incorrect combination of Rf and Cf will cause frequency response roll off before 20kHz. A typical combination of feedback resistor and capacitor that will not produce audio band high frequency roll off is Rf = 20kΩ and Cf = 25pF. These components result in a -3dB point of approximately 320kHz. (Continued) The LM4910 is unity-gain stable and requires no external components besides gain-setting resistors, input coupling capacitors, and proper supply bypassing in the typical application. However, if a very large closed-loop differential gain is required, a feedback capacitor (Cf) may be needed as shown in Figure 6 to bandwidth limit the amplifier. This REFERENCE DESIGN BOARD and LAYOUT GUIDELINES MSOP & SO BOARDS 20030569 FIGURE 8. (Note: RPU2 is not required. It is used for test measurement purposes only.) 15 www.national.com LM4910 Application Information LM4910 Application Information (Continued) LM4910 SO DEMO BOARD ARTWORK Composite View Silk Screen 20030571 20030570 Top Layer Bottom Layer 20030573 20030572 www.national.com 16 LM4910 Application Information (Continued) LM4910 MSOP DEMO BOARD ARTWORK Composite View Silk Screen 20030575 20030574 Top Layer Bottom Layer 20030577 20030576 17 www.national.com LM4910 Application Information (Continued) LM4910 LLP DEMO BOARD ARTWORK Composite View Silk Screen 20030598 20030597 Top Layer Bottom Layer 20030599 www.national.com 20030596 18 LM4910 Application Information (Continued) LM4910 Reference Design Boards Bill of Materials Part Description Qty LM4910 Mono Reference Design Board 1 Ref Designator LM4910 Audio AMP 1 U1 Tantalum Cap 1µF 16V 10 1 Cs Ceramic Cap 0.39µF 50V Z50 20 2 Ci Resistor 20kΩ 1/10W 5 4 Ri, Rf Resistor 100kΩ 1/10W 5 1 Rpu Jumper Header Vertical Mount 2X1, 0.100 1 J1 PCB LAYOUT GUIDELINES greatly enhance 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 may be some jumpers. 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. Further, place digital and analog power traces over the corresponding digital and analog ground traces to minimize noise coupling. Minimization of THD PCB trace impedance on the power, ground, and all output traces should be minimized to achieve optimal THD performance. Therefore, use PCB traces that are as wide as possible for these connections. As the gain of the amplifier is increased, the trace impedance will have an ever increasing adverse affect on THD performance. At unity-gain (0dB) the parasitic trace impedance effect on THD performance is reduced but still a negative factor in the THD performance of the LM4910 in a given application. 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. GENERAL MIXED SIGNAL LAYOUT RECOMMENDATION 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. Power and Ground Circuits For two 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 19 www.national.com LM4910 Physical Dimensions inches (millimeters) unless otherwise noted MSOP Order Number LM4910MM NS Package Number MUA08A www.national.com 20 LM4910 Physical Dimensions inches (millimeters) unless otherwise noted (Continued) SO Order Number LM4910MA NS Package Number M08A LQ Order Number LM4910LQ NS Package Number LQB08A 21 www.national.com LM4910 Output Capacitor-less Stereo 35mW Headphone Amplifier Notes 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.