LM4867

LM4867 Output-Transient-Free Dual 2.1W Audio Amplifier Plus No Coupling Capacitor Stereo Headphone Function August 2002 LM4867 Output-Transient-Free Dual 2.1W Audio Amplifier Plus No Coupling Capacitor Stereo Headphone Function General Description The LM4867 is a dual bridge-connected audio power amplifier which, when connected to a 5V supply, will deliver 2.1W to a 4Ω load (Note 1) or 2.4W to a 3Ω load (Note 2) with less than 1.0% THD+N. The LM4867 uses advanced, latest generation circuitry to eliminate all traces of clicks and pops when the supply voltage is first applied. The amplifier has a headphone-amplifier-select input pin. It is used to switch the amplifiers from bridge to single-ended mode for driving headphones. A new circuit topology eliminates headphone output coupling capacitors (patent pending). A MUX control pin allows selection between the two sets of stereo input signals. The MUX control can also be used to select between two different customer-specified closed-loop responses. Boomer audio power amplifiers are designed specifically to provide high quality output power from a surface mount package and require few external components. To simplify audio system design, the LM4867 combines dual bridge speaker amplifiers and stereo headphone amplifiers in one package. The LM4867 features an externally controlled power-saving micropower shutdown mode, a stereo headphone amplifier mode, and thermal shutdown protection. Note 1: An LM4867LQ or LM4867MTE that has been properly mounted to a circuit board will deliver 2.1W into 4Ω. The Mux control can also be used to select two different closed-loop responses. LM4867MT will deliver 1.1W into 8Ω. See the Application Information sections for further information concerning the LM4867LQ and the LM4867MT. Note 2: An LM4867LQ or LM4867MTE that has been properly mounted to a circuit board and forced-air cooled will deliver 2.4W into 3Ω. Key Specifications n PO at 1% THD+N n LM4867LQ, 3Ω load 2.4W (typ) n LM4867LQ, 4Ω load 2.1W (typ) n LM4867MTE, 4Ω 1.9W (typ) n LM4867MT, 8Ω 1.1W (typ) n Single-ended mode - THD+N at 75mW into 32Ω 0.5% (max) n Shutdown current 0.7µA (typ) Features n Advanced “click and pop” suppression circuitry n Eliminates headphone amplifier output coupling capacitors (patent pending) n Stereo headphone amplifier mode n Input mux control and two separate inputs per channel n Thermal shutdown protection circuitry n LLP, TSSOP, and exposed-DAP TSSOP packaging available Applications n Multimedia monitors n Portable and desktop computers n Portable audio systems Connection Diagrams 20001358 Top View Order Number LM4867MT, LM4867MTE See NS Package Number MTC20 for TSSOP See NS Package Number MXA20A for Exposed-DAP TSSOP Boomer ® is a registered trademark of National Semiconductor Corporation. © 2004 National Semiconductor Corporation DS200013 www.national.com LM4867 Connection Diagrams (Continued) 20001338 Top View Order Number LM4867LQ See NS Package Number LQA24A for Exposed-DAP LLP Typical Application 20001331 * Refer to the Application Information section titled PROPER SELECTION OF EXTERNAL COMPONENTS for details concerning the value of CB. FIGURE 1. Typical Audio Amplifier Application Circuit (Pin out shown for the 24-pin Exposed-DAP LLP package. Numbers in ( ) are for the 20-pin MTE and MT packages.) www.national.com 2 LM4867 Absolute Maximum Ratings (Note 3) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage Storage Temperature Input Voltage Power Dissipation (Note 4) ESD Susceptibility (Note 5) All pins except Pin 3 (MT, MTE), Pin 2 (LQ) Pin 3 (MT, MTE), Pin 2 (LQ) ESD Susceptibility (Note 6) Junction Temperature Solder Information Small Outline Package Vapor Phase (60 sec.) Infrared (15 sec.) 215˚C 220˚C 2000V 8000V 200V 150˚C 6.0V −65˚C to +150˚C −0.3V to VDD +0.3V Internally limited Thermal Resistance θJC (typ) — MTC20 θJA (typ) — MTC20 θJC (typ) — MXA20A θJA (typ) — MXA20A θJA (typ) — MXA20A θJA (typ) — MXA20A θJC (typ) — LQA24A θJA (typ) — LQA24A 20˚C/W 80˚C/W 2˚C/W 41˚C/W (Note 7) 51˚C/W (Note 8) 90˚C/W (Note 9) 3.0˚C/W 42˚C/W (Note 10) Operating Ratings Temperature Range TMIN ≤ TA ≤ TMAX Supply Voltage −40˚C ≤ TA ≤ 85˚C 2.0V ≤ VDD ≤ 5.5V See AN-450 “Surface Mounting and their Effects on Product Reliablilty” for other methods of soldering surface mount devices. Electrical Characteristics for Entire IC (Notes 3, 11) The following specifications apply for VDD= 5V unless otherwise noted. Limits apply for TA= 25˚C. Symbol Parameter Conditions LM4867 Typical (Note 12) VDD IDD ISD Supply Voltage Quiescent Power Supply Current Shutdown Current VIN = 0V, IO = 0A (Note 14), HP-IN = 0V VIN = 0V, IO = 0A (Note 14), HP-IN = 4V VDD applied to the SHUTDOWN pin 7.5 3.0 0.7 Limit (Note 13) 2 5.5 15 6 2 V (min) V (max) mA (max) mA (max) µA (max) Units (Limits) Electrical Characteristics for Bridged-Mode Operation (Notes 3, 11) The following specifications apply for VDD= 5V unless otherwise specified. Limits apply for TA= 25˚C. Symbol Parameter Conditions LM4867 Typical (Note 12) VOS PO Output Offset Voltage Output Power (Note 15) VIN = 0V THD = 1%, f = 1kHz (Note 16) LM4867MTE, RL = 3Ω LM4867LQ, RL = 3Ω LM4867MTE, RL = 4Ω LM4867LQ, RL = 4Ω LM4867, RL = 8Ω THD+N = 10%, f = 1kHz (Note 16) LM4867MTE, RL = 3Ω LM4867LQ, RL = 3Ω LM4867MTE, RL = 4Ω LM4867LQ, RL = 4Ω LM4867, RL = 8Ω THD+N = 1%, f = 1 kHz, RL = 32Ω 3.0 3.0 2.6 2.6 1.5 0.34 W W W W W W 2.2 2.4 1.9 2.1 1.1 1.0 W W W W W (min) 5 Limit (Note 13) 50 mV (max) Units (Limits) 3 www.national.com LM4867 Electrical Characteristics for Bridged-Mode Operation (Notes 3, 11) The following specifications apply for VDD= 5V unless otherwise specified. Limits apply for TA= 25˚C. Symbol Parameter Conditions Typical (Note 12) THD+N Total Harmonic Distortion+Noise 20Hz ≤ f ≤ 20kHz, AVD = 2 LM4867MTE, RL = 4Ω, PO = 2W LM4867LQ, RL = 4Ω, PO = 2W LM4867, RL = 8Ω, PO = 1W VDD = 5V, VRIPPLE = 200 mVRMS, RL = 8Ω, CB = 2.2µF f = 1 kHz, CB = 2.2µF VDD = 5V, PO = 1.1W, RL = 8Ω 0.3 0.3 0.3 67 80 97 (Continued) Units (Limits) LM4867 Limit (Note 13) % % % dB dB dB PSRR XTALK SNR Power Supply Rejection Ratio Channel Separation Signal To Noise Ratio Electrical Characteristics for Single-Ended Operation (Notes 3, 11) The following specifications apply for VDD= 5V unless otherwise specified. Limits apply for TA= 25˚C. Symbol Parameter Conditions LM4867 Typical (Note 12) VOS PO Output Offset Voltage Output Power VIN = 0V THD = 0.5%, f = 1kHz, RL = 32Ω THD+N = 1%, f = 1kHz, RL = 8Ω (Note 17) THD+N THD+N THD+N THD+N VOUT THD+N PSRR XTALK SNR Output Voltage Swing Total Harmonic Distortion+Noise Power Supply Rejection Ratio Channel Separation Signal To Noise Ratio = = = = 1%, f = 1kHz, RL = 16Ω 1%, f = 1kHz, RL = 32Ω 10%, f = 1kHz, RL = 16Ω 10%, f = 1kHz, RL = 32Ω 5 85 180 165 88 208 114 1 0.2 52 60 94 Limit (Note 13) 50 75 mV (max) mW (min) mW mW mW mW mW VP-P % dB dB dB Units (Limits) THD = 0.05%, RL = 5kΩ AV = −1, PO = 75mW, 20 Hz ≤ f ≤ 20kHz, RL = 32Ω CB = 2.2µF, VRIPPLE = 200mVRMS, f = 1kHz f = 1kHz, CB = 2.2µF VDD = 5V, PO = 340mW, RL = 8Ω Note 3: 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 operates within the Operating Ratings. Specifications are not guaranteed for parameters where no limit is given. The typical value however, is a good indication of device performance. Note 4: 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. For the LM4867, TJMAX = 150˚C. For the θJAs for different packages, please see the Application Information section or the Absolute Maximum Ratings section. Note 5: Human body model, 100pF discharged through a 1.5kΩ resistor. Note 6: Machine model, 220pF–240pF discharged through all pins. Note 7: The given θJA is for an LM4867 packaged in an MXA20A with the Exposed-DAP soldered to an exposed 2in2 area of 1oz printed circuit board copper. Note 8: The given θJA is for an LM4867 packaged in an MXA20A with the Exposed-DAP soldered to an exposed 1in2 area of 1oz printed circuit board copper. Note 9: The given θJA is for an LM4867 packaged in an MXA20A with the Exposed-DAP not soldered to printed circuit board copper. Note 10: The given θJA is for an LM4867 packaged in an LQA24A with the Exposed-DAP soldered to an exposed 2in2 area of 1oz printed circuit board copper. Note 11: All voltages are measured with respect to the ground (GND) pins, unless otherwise specified. Note 12: Typicals are measured at 25˚C and represent the parametric norm. Note 13: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level). Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis. Note 14: The quiescent power supply current depends on the offset voltage when a practical load is connected to the amplifier. Note 15: Output power is measured at the device terminals. Note 16: When driving 3Ω or 4Ω loads and operating on a 5V supply, the LM4867LQ and LM4867MTE must be mounted to a circuit board that has a minimum of 2.5in2 of exposed, uniterrupted copper area connected to the LLP or TSSOP package’s exposed DAP. Note 17: See Application Information section "Single-Ended Output Power Performance and Measurement Considerations" for more information. www.national.com 4 LM4867 Typical Performance Characteristics MTE- and LQ- Specific Characteristics LM4867MTE THD+N vs Output Power LM4867MTE THD+N vs Frequency 20001334 20001333 LM4867LQ THD+N vs Output Power LM4867LQ THD+N vs Frequency 20001353 20001354 LM4867MTE THD+N vs Output Power LM4867LQ THD+N vs Output Power 20001336 20001355 5 www.national.com LM4867 Typical Performance Characteristics MTE- and LQ- Specific Characteristics LM4867LQ, LM4867MTE Power Dissipation vs Power Output (Continued) LM4867MTE (Note 18) Power Derating Curve 20001361 20001359 LM4867LQ Power Derating Curve 20001369 Note 18: This curve shows the LM4867MTE’s thermal dissipation ability at different ambient temperatures given these conditions: 500LFPM + JEDEC board: The part is soldered to a 1S2P 20-lead exposed-DAP TSSOP test board with 500 linear feet per minute of forced-air flow across it. Board information - copper dimensions: 74x74mm, copper coverage: 100% (buried layer) and 12% (top/bottom layers), 16 vias under the exposed-DAP. 500LFPM + 2.5in2: The part is soldered to a 2.5in2, 1 oz. copper plane with 500 linear feet per minute of forced-air flow across it. 2.5in2: The part is soldered to a 2.5in2, 1oz. copper plane. Not Attached: The part is not soldered down and is not forced-air cooled. www.national.com 6 LM4867 Typical Performance Characteristics THD+N vs Frequency THD+N vs Frequency 20001303 20001304 THD+N vs Frequency THD+N vs Output Power 20001305 20001306 THD+N vs Output Power THD+N vs Output Power 20001307 20001308 7 www.national.com LM4867 Typical Performance Characteristics THD+N vs Output Power (Continued) THD+N vs Frequency 20001365 20001363 THD+N vs Output Power THD+N vs Frequency 20001366 20001364 Output Power vs Load Resistance Power Dissipation vs Supply Voltage 20001362 20001360 www.national.com 8 LM4867 Typical Performance Characteristics Output Power vs Supply Voltage (Continued) Output Power vs Supply Voltage 20001309 20001310 Output Power vs Supply Voltage Output Power vs Load Resistance 20001311 20001312 Output Power vs Load Resistance Power Dissipation vs Output Power 20001313 20001314 9 www.national.com LM4867 Typical Performance Characteristics Dropout Voltage vs Supply Voltage (Continued) Power Derating Curve 20001315 20001316 Power Dissipation vs Output Power Noise Floor 20001317 20001318 Channel Separation Channel Separation 20001319 20001320 www.national.com 10 LM4867 Typical Performance Characteristics Power Supply Rejection Ratio (Continued) Open Loop Frequency Response 20001321 20001322 Supply Current vs Supply Voltage 20001323 External Components Description Figure 1 Components 1. 2. Ri Ci Functional Description Inverting input resistance which sets the closed-loop gain in conjunction with Rf. This resistor also forms a high pass filter with C i at fc = 1/(2πRiCi). Input coupling capacitor which blocks the DC voltage at the amplifier’s input terminals. Also creates a highpass 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. Feedback resistance which sets the closed-loop gain in conjunction with Ri. 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. 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. 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 LM4867 eliminates these coupling capacitors. Amp2A is internally configured to apply VDD/2 to a stereo headphone jack’s sleeve. This voltage matches the quiescent voltage present on the Amp1A and Amp1B outputs that drive the headphones. The headphones operate in a manner very similar to a bridge-tied-load (BTL). The same DC voltage is 11 www.national.com 3. 4. 5. Rf Cs CB Application Information ELIMINATING OUTPUT COUPLING CAPACITORS Typical single-supply audio amplifiers that can switch between driving bridge-tied-load (BTL) speakers and singleended (SE) headphones use a coupling capacitor on each SE output. This capacitor blocks the half-supply voltage to LM4867 Application Information (Continued) 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. When operating as a headphone amplifier, the headphone jack sleeve is not connected to circuit ground. Using the headphone output jack as a line-level output will place the LM4867’s one-half supply voltage on a plug’s sleeve connection. Driving a portable notebook computer or audiovisual display equipment is possible. This presents no difficulty when the external equipment uses capacitively coupled inputs. For the very small minority of equipment that is DC-coupled, the LM4867 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 LM4867 and the external equipment. For more information, see the section titled "Single-Ended Output Power Performance and Measurement Considerations". OUTPUT TRANSIENT ("POPS AND CLICKS") ELIMINATED The LM4867 contains advanced circuitry that eliminates output transients ("pop and click"). This circuitry prevents all traces of transients when the supply voltage is first applied, when the part resumes operation after shutdown, or when switching between BTL speakers and SE headphones. Two circuits combine to eliminate pop and click. One circuit mutes the output when switching between speaker loads. Another circuit monitors the input signal. It maintains the muted condition until there is sufficient input signal magnitude ( > 60mVRMS, typ) to mask any remaining transient that may occur. Figure 2 shows the LM4867’s lack of transients in the differential signal (Trace B) across a BTL 8Ω load. The LM4867’s active-high SHUTDOWN pin is driven by the logic signal shown in Trace A. Trace C is the VOUT- output signal and trace D is the VOUT+ output signal. The shutdown signal frequency is 1Hz with a 50% duty cycle. Figure 3 is generated with the same conditions except that the output drives a 32Ω single-ended (SE) load. Again, no trace of output transients is seen. USING THE LM4867 TO UPGRADE LM4863 AND LM4873 DESIGNS The LM4867’s noise-free operation plus couplingcapacitorless headphone operation and functional compatibility with the LM4873 and the LM4863 simplifies upgrading systems using these parts. Upgrading older designs that use either the LM4863 or the LM4873 is easy. Simply remove and short the coupling capacitors located between the LM4873’s or LM4863’s Amp1A and Amp1B outputs and the headphone connections. Also remove the 1kΩ resistor between each headphone connection and ground. Finally, remove any resistors connected to the HP-IN pin (typically two 100kΩ resistors). Connect the HP-IN pin directly to the headphone jack control pin as shown in Figure 4. 20001356 FIGURE 2. Differential output signal (Trace B) is devoid of transients. The SHUTDOWN pin is driven by a shutdown signal (Trace A). The inverting output (Trace C) and the non-inverting output (Trace D) are applied across an 8Ω BTL load. The LM4867’s pin configuration simplifies the process of upgrading systems that use the LM4863. Except for its four MUX function pins, the LM4867’s pin configuration matches the LM4863’s pin configuration. If the LM4867’s MUX functionality is not needed when replacing an LM4863, connect the MUX CTRL pin to either VDD or ground. To ensure correct amplifier operation, unused MUX inputs must be tied to GND. As shown in Table 1, grounding the MUX CTRL pin selects stereo input 1 (−IN A1 and −IN B1), whereas applying VDD to the MUX CTRL pin selects stereo input 2 (−IN A2 and −IN B2). The LM4867’s unique headphone sense circuit requires a dual switch headphone jack. Replace the four-terminal headphone jack used with the LM4863 and LM4873 with the five-terminal headphone jack, such as the Switchcraft 35RAPC4BH3, shown in Figure 4. Connect the +OUT A (Amp2A) pin to the five-terminal headphone jack’s sleeve pin. 20001357 FIGURE 3. Single-ended output signal (Trace B) is devoid of transients. The SHUTDOWN pin is driven by a shutdown signal (Trace A). The inverting output (Trace C) and the VBYPASS output (Trace D) are applied across a 32Ω BTL load. www.national.com 12 LM4867 Application Information (Continued) 20001331 FIGURE 4. Typical Audio Amplifier Application Circuit (Pin out shown for the 24-pin Exposed-DAP LLP package. Numbers in ( ) are for the 20-pin MTE and MT packages.) STEREO-INPUT MULTIPLEXER (STEREO MUX) The LM4867 has two stereo inputs. The MUX CTRL Pin controls which stereo input is active. As shown in the Truth Table for Logic Inputs, applying 0V to the MUX CTRL input activates stereo input 1, whereas applying VDD to the MUX CTRL inputs activates stereo input 2. To ensure correct amplifier operation, unused MUX inputs must be tied to GND. Typical LM4867 applications use the MUX to switch between two stereo input signals. Each stereo channel’s gain can be tailored to produce the required output signal level by choosing the appropriate input and feedback resistor ratio. Another configuration uses the MUX to select two different gains or frequency compensated gains that amplify a single pair of stereo input signals. Figure 5 shows two different feedback networks, Network 1 and Network 2. Network 1 produces increasing gain as the input signal’s frequency decreases. This can be used to compensate a small, fullrange speaker’s low frequency response roll-off. Network 2 sets the gain for an alternate load such as headphones. The circuit in Figure 6 uses Network 1 when driving external speakers, switching to Network 2 when headphones are connected. The normally closed control switch in Figure 6’s headphone jack connects to the MUX CTRL pin. When headphones are connected, the LM4867’s internal pull-up that applies VDD to the HP-IN and the external 100kΩ resistor applies VDD to MUX CTRL pin. Simultaneously applying these control voltages automatically selects the amplifier (headphone or bridge) and switches the gain (MUX channel selection). Alternatively, leaving the MUX CTRL pin independently accessible allows a user to select bass boost as needed. This alternative user-selectable bass-boost scheme requires connecting equal ratio resistor feedback networks to each MUX input channel. The value of the resistor in the RC network is chosen to give a gain that is necessary to achieve the desired bass-boost. Switching between the MUX channels may change the input signal source or the feedback resistor network. During the channel switching transition, the average voltage level 13 www.national.com 20001370 FIGURE 5. Input MUX Example LM4867 Application Information (Continued) present on the internal amplifier’s input may change. This change can slew at a rate that may produce audible voltage transients or clicks in the amplifier’s output signal. Using the MUX to select between two vastly dissimilar gains is a typical transient-producing situation. As the MUX is switched, an audible click may occur as the gain suddenly changes. PIN OUT COMPATIBILITY WITH THE LM4863 The LM4867 pin out was designed to simplify replacing the LM4863: except for the four Pins(-IN A2, MUX CTRL, -IN B2, and NC) that implement the LM4867’s extra functionality, the LM4867MT/MTE and LM4863MT/MTE pin outs match. (Note 19) Note 19: If the LM4867 replaces an LM4863 and the input MUX circuitry is not being used, the LM4867 MUX CTRL pin must be tied to VDD or GND and the unused MUX inputs must be connected to GND. 20001339 FIGURE 6. As configured, connecting headphones to this jack automatically selects the stereo headphone amplifier and, with the additional NC switch, changes MUX channels (Network 2 in Figure 5 ) EXPOSED-DAP MOUNTING CONSIDERATIONS The LM4867’s exposed-DAP (die attach paddle) packages (MTE and LQ) provide a low thermal resistance between the die and the PCB to which the part is mounted and soldered. This allows rapid heat transfer from the die to the surrounding PCB copper area heatsink, copper traces, ground plane, and finally, surrounding air. The result is a low voltage audio power amplifier that produces 2.4W dissipation in a 4Ω load at ≤ 1% THD+N and over 3W in a 3Ω load at 10% THD+N. This high power is achieved through careful consideration of necessary thermal design. Failing to optimize thermal design may compromise the LM4867’s high power performance and activate unwanted, though necessary, thermal shutdown protection. The MTE and LQ packages must have their DAPs soldered to a copper pad on the PCB. The DAP’s PCB copper pad is then, ideally, connected to a large plane of continuous unbroken copper. This plane forms a thermal mass, heat sink, and radiation area. Place the heat sink area on either outside plane in the case of a two-sided or multi-layer PCB. (The heat sink area can also be placed on an inner layer of a multi-layer board. The thermal resistance, however, will be higher.) Connect the DAP copper pad to the inner layer or backside copper heat sink area with 32 (4 X 8) (MTE) or 6 (3 X 2) (LQ) vias. The via diameter should be 0.012in - 0.013in with a 1.27mm pitch. Ensure efficient thermal conductivity by plugging and tenting the vias with plating and solder mask, respectively. Best thermal performance is achieved with the largest practical copper heat sink area. If the heatsink and amplifier share the same PCB layer, a nominal 2.5in2 (min) area is necessary for 5V operation with a 4Ω load. Heatsink areas not placed on the same PCB layer as the LM4867 should be 5in2 (min) for the same supply voltage and load resistance. The last two area recommendations apply for 25˚C ambient temperature. Increase the area to compensate for ambient temperatures above 25˚C. In systems using cooling fans, the LM4867MTE can take advantage of forced air cooling. With an air flow rate of 450 linear-feet per minute and a 2.5in2 exposed copper or 5.0in2 inner layer copper plane heatsink, the LM4867MTE can continuously drive a 3Ω load to full power. The LM4867LQ achieves the same output power level without forced-air cooling. In all circumstances and under all conditions, the junction temperature must be held below 150˚C to prevent activating the LM4867’s thermal shutdown protection. The LM4867’s power de-rating curve in the Typical Performance Characteristics shows the maximum power dissipation versus temperature. Example PCB layouts for the exposed-DAP TSSOP and LQ packages are shown in the Demonstration Board Layout section. Further detailed and specific information concerning PCB layout and fabrication and mounting an LQ (LLP) is found in National Semiconductor’s AN1187. www.national.com 14 LM4867 Application Information (Continued) PCB LAYOUT AND SUPPLY REGULATION CONSIDERATIONS FOR DRIVING 3Ω AND 4Ω LOADS Power dissipated by a load is a function of the voltage swing across the load and the load’s impedance. As load impedance decreases, load dissipation becomes increasingly dependent on the interconnect (PCB trace and wire) resistance between the amplifier output pins and the load’s connections. Residual trace resistance causes a voltage drop, which results in power dissipated in the trace and not in the load as desired. For example, 0.1Ω trace resistance reduces the output power dissipated by a 4Ω load from 2.1W to 2.0W. The problem of decreased load dissipation is exacerbated as load impedance decreases. Therefore, to maintain the highest load dissipation and widest output voltage swing, PCB traces that connect the output pins to a load must be as wide as possible. Poor power supply regulation adversely affects maximum output power. A poorly regulated supply’s output voltage decreases with increasing load current. Reduced supply voltage causes decreased headroom, output signal clipping, and reduced output power. Even with tightly regulated supplies, trace resistance creates the same effects as poor supply regulation. Therefore, making the power supply traces as wide as possible helps maintain full output voltage swing. BRIDGE CONFIGURATION EXPLANATION As shown in Figure 4, the LM4867 consists of two pairs of operational amplifiers, forming a two-channel (channel A and channel B) stereo amplifier. (Though the following discusses channel A, it applies equally to channel B.) External resistors Rf and Ri set the closed-loop gain of Amp1A, whereas two internal 20kΩ resistors set Amp2A’s gain at -1. The LM4867 drives a load, such as a speaker, connected between the two amplifier outputs, -OUTA and +OUTA. Figure 4 shows that Amp1A’s output serves as Amp2A’s input. This results in both amplifiers producing signals identical in magnitude, but 180˚ out of phase. Taking advantage of this phase difference, a load is placed between -OUTA and +OUTA and driven differentially ("commonly referred to as bridge mode"). This results in a differential gain of AVD = 2 * (Rf/R i) (1) order to choose an amplifier’s closed-loop gain without causing excessive clipping, please refer to the Audio Power Amplifier Design section. Another advantage of the differential bridge output is no net DC voltage across the load. This is accomplished by biasing channel A’s and channel B’s outputs at half-supply. This eliminates the coupling capacitor that single supply, singleended amplifiers require. Eliminating an output coupling capacitor in a single-ended configuration forces a single-supply amplifier’s half-supply bias voltage across the load. This increases internal IC power dissipation and may permanently damage loads such as speakers. POWER DISSIPATION Power dissipation is a major concern when successful single-ended or bridged amplifier. states the maximum power dissipation point ended amplifier operating at a given supply driving a specified output load. PDMAX = (VDD)2/(2π2RL): designing a Equation (2) for a singlevoltage and Single-Ended (2) However, a direct consequence of the increased power delivered to the load by a bridge amplifier is higher internal power dissipation for the same conditions. The LM4867 has two operational amplifiers per channel. The maximum internal power dissipation per channel operating in the bridge mode is four times that of a single-ended amplifier. From Equation (3), assuming a 5V power supply and an 4Ω load, the maximum single channel power dissipation is 1.27W or 2.54W for stereo operation. PDMAX = 4 * (VDD)2/(2π2RL): Bridge Mode (3) The LM4867’s power dissipation is twice that given by Equation (2) or Equation (3) when operating in the single-ended mode or bridge mode, respectively. Twice the maximum power dissipation point given by Equation (3) must not exceed the power dissipation given by Equation (4): PDMAX’ = (TJMAX − TA)/θJA (4) Bridge mode amplifiers are different from single-ended amplifiers that drive loads connected between a single amplifier’s output and ground. For a given supply voltage, bridge mode has a distinct advantage over the single-ended configuration: its differential output doubles the voltage swing across the load. This produces four times the output power when 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 that the output signal is not clipped. To ensure minimum output signal clipping when choosing an amplifier’s closed-loop gain, refer to the Audio Power Amplifier Design section. A bridge amplifier design has a few distinct advantages over the single-ended configuration, as it provides differential drive to the load, thus doubling the 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 15 The LM4867’s TJMAX = 150˚C. In the LQ package soldered to a DAP pad that expands to a copper area of 5in2 on a PCB, the LM4867’s θJA is 20˚C/W. In the MTE package soldered to a DAP pad that expands to a copper area of 2in2 on a PCB, the LM4867’s θJA is 41˚C/W. At any given ambient temperature TA, use Equation (4) to find the maximum internal power dissipation supported by the IC packaging. Rearranging Equation (4) and substituting PDMAX for PDMAX’ results in Equation (5). This equation gives the maximum ambient temperature that still allows maximum stereo power dissipation without violating the LM4867’s maximum junction temperature. TA = TJMAX − 2 X PDMAX θJA (5) For a typical application with a 5V power supply and an 4Ω load, the maximum ambient temperature that allows maximum stereo power dissipation without exceeding the maximum junction temperature is approximately 99˚C for the LQ package and 45˚C for the MTE package. www.national.com LM4867 Application Information (Continued) (6) TJMAX = PDMAX θJA + TA Equation (6) gives the maximum junction temperature TJMAX. If the result violates the LM4867’s 150˚C, reduce the maximum junction temperature by reducing the power supply voltage or increasing the load resistance. Further allowance should be made for increased ambient temperatures. The above examples assume that a device is a surface mount part operating around the maximum power dissipation point. Since internal power dissipation is a function of output power, higher ambient temperatures are allowed as output power or duty cycle decreases. If the result of Equation (2) is greater than that of Equation (3), then decrease the supply voltage, increase the load impedance, or reduce the ambient temperature. If these measures are insufficient, a heat sink can be added to reduce θJA. The heat sink can be created using additional copper area around the package, with connections to the ground pin(s), supply pin and amplifier output pins. External, solder attached SMT heatsinks such as the Thermalloy 7106D can also improve power dissipation. When adding a heat sink, the θJA is the sum of θJC, θCS, and θSA. (θJC is the junction−to−case thermal impedance, θCS is the case−to−sink thermal impedance, and θSA is the sink−to−ambient thermal impedance.) Refer to the Typical Performance Characteristics curves for power dissipation information at lower output power levels. POWER SUPPLY BYPASSING As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply rejection. Applications that employ a 5V regulator typically use a 10µF in parallel with a 0.1µF filter capacitors to stabilize the regulator’s output, reduce noise on the supply line, and improve the supply’s transient response. However, their presence does not eliminate the need for a local 1.0µF tantalum bypass capacitance connected between the LM4867’s supply pins and ground. Do not substitute a ceramic capacitor for the tantalum. Doing so may cause oscillation. Keep the length of leads and traces that connect capacitors between the LM4867’s power supply pin and ground as short as possible. Connecting a 1µF capacitor, CB, between the BYPASS pin and ground improves the internal bias voltage’s stability and improves the amplifier’s PSRR. The PSRR improvements increase as the bypass pin capacitor value increases. Too large, however, increases turn−on time and can compromise the amplifier’s click and pop performance. The selection of bypass capacitor values, especially CB, depends on desired PSRR requirements, click and pop performance (as explained in the section, Proper Selection of External Components), system cost, and size constraints. MICRO−POWER SHUTDOWN The voltage applied to the SHUTDOWN pin controls the LM4867’s shutdown function. Activate micro−power shutdown by applying VDD to the SHUTDOWN pin. When active, the LM4867’s micro−power shutdown feature turns off the amplifier’s bias circuitry, reducing the supply current. The logic threshold is typically VDD/2. The low 0.7µA typical shutdown current is achieved by applying a voltage that is as near as VDD as possible to the SHUTDOWN pin. A voltage that is less than VDD may increase the shutdown current. Table 1 shows the logic signal levels that activate and deactivate micro−power shutdown and headphone amplifier operation. To ensure that the output signal remains transient−free, do not cycle the shutdown function faster than 1Hz. 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 closing the switch. Opening the switch connects the SHUTDOWN pin to VDD through the pull−up resistor, 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 a 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. Truth Table for Logic Inputs SHUTDOWN PIN Logic Low Logic Low Logic Low Logic Low Logic High HP-IN PIN = −OUTB signal = −OUTB signal ≠ −OUTB signal ≠ −OUTB signal X MUX CHANNEL INPUT SELECT Logic Low Logic High Logic Low Logic High X OPERATIONAL MODE (MUX INPUTCHANNEL #) Bridged amplifiers (1) Bridged amplifiers (2) Single-ended amplifiers (1) Single-ended amplifiers (2) Micro−power shutdown HEADPHONE (SINGLE-ENDED) AMPLIFIER OPERATION An internal pull−up circuit is connected to the HP−IN (pin 20) headphone amplifier control pin. When this pin is left unconnected, VDD is applied to the HP−IN. This turns off Amp2B and switches Amp2A’s input signal from an audio signal to the VDD/2 voltage present on pin 14. The result is muted bridge-connected loads. Quiescent current consumption is reduced when the IC is in this single−ended mode. Figure 8 shows the implementation of the LM4867’s headphone control function. An internal comparator with a nominal 400mV offset monitors the signal present at the −OUTB output. It compares this signal against the signal applied to the HP−IN pin. When these signals are equal, as is the case when a BTL is connected to the amplifier, the comparator forces the LM4867 to maintain bridged−amplifier operation. When the HP−IN pin is externally floated, such as when 16 www.national.com LM4867 Application Information (Continued) headphones are connected to the jack shown in Figure 8, and internal pull−up forces VDD on the internal comparator’s HP−IN inputs. This changes the comparator’s output state and enables the headphone function: it turns off Amp2B, switches Amp2A’s input signal from an audio signal to the VDD/2 voltage present on pin 14, and mutes the bridgeconnected loads. Amp1A and Amp1B drive the headphones. Figure 8 also shows the suggested headphone jack electrical connections. The jack is designed to mate with a three−wire plug. The plug’s tip and ring should each carry one of the two stereo output signals, whereas the sleeve provides the return to Amp2A. A headphone jack with one control pin contact is sufficient to drive the HP−IN pin when connecting headphones. A switch can replace the headphone jack contact pin. When a switch shorts the HP−IN pin to VDD, bridge−connected speakers are muted and Amp1A and Amp2A drive a pair of headphones. When a switch shorts the HP−IN pin to GND, the LM4867 operates in bridge mode. If headphone drive is not needed, short the HP−IN pin to the −OUTB pin. Figure 7 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). ESD PROTECTION As stated in the Absolute Maximum Ratings, pin 28 on the MT and MH packages have a maximum ESD susceptibility rating of 8000V. For higher ESD voltages, the addition of a PCDN042 dual transil (from California Micro Devices), as shown in Figure 7, will provide additional protection. 20001371 FIGURE 7. The PCDN042 provides additional ESD protection beyond the 8000V shown in the Absolute Maximum Ratings for the AMP2A output SINGLE-ENDED OUTPUT POWER PERFORMANCE AND MEASUREMENT CONSIDERATIONS The LM4867 delivers clean, low distortion SE output power into loads that are greater than 10Ω. As an example, output power for 16Ω and 32Ω loads are shown in the Typical Performance Characteristic curves. For loads less than 10Ω, the LM4876 can typically supply 180mW of low distortion power. However, when higher dissipation is desired in loads less than 10Ω, a dramatic increase in THD+N may occur. This is normal operation and does not indicate that proper functionality has ceased. When a jump from moderate to excessively high distortion is seen, simply reducing the output voltage swing will restore the clean, low distortion SE operation. The dramatic jump in distortion for loads less than 10Ω occurs when current limiting circuitry activates. During SE operation, AMP2A (refer to Figure 4) drives the headphone sleeve. An on-board circuit monitors this amplifier’s output current. The sudden increase in THD+N is caused by the current limit circuitry forcing AMP2A into a high−impedance output mode. When this occurs, the output waveform has discontinuities that produce large amounts of distortion. It has been observed that as the output power is steadily increased, the distortion may jump from 5% to greater than 35%. Indeed, 10% THD+N may not actually be achievable. USING THE SINGLE−ENDED OUTPUT FOR LINE LEVEL APPLICATIONS Some samples of the LM4867 may exhibit small amplitude, high frequency oscillation when the SE output is connected to a line-level input. This oscillation can be eliminated by connecting a 5%, 300Ω resistor between Amp2A’s output pin and each amplifier, AMP1A and AMP1B, output. 17 www.national.com LM4867 Application Information (Continued) 20001324 FIGURE 8. Headphone Circuit (Pin numbers in ( ) are for the 20-pin MTE and MT packages.) INPUT CAPACITOR VALUE SELECTION Amplifying the lowest audio frequencies requires high value input coupling capacitor (Ci in Figure 4). A high value capacitor can be expensive and may compromise space efficiency in portable designs. In many cases, however, the speakers used in portable systems, whether internal or external, have little ability to reproduce signals below 150Hz. Applications using speakers with this limited frequency response reap little improvement by using large input capacitor. Besides effecting system cost and size, Ci has an affect on the LM4867’s click and pop performance. When the supply voltage is first applied, a transient (pop) is created as the charge on the input capacitor changes from zero to a quiescent state. The magnitude of the pop is directly proportional to the input capacitor’s size. Higher value capacitors need more time to reach a quiescent DC voltage (usually VDD/2) when charged with a fixed current. The amplifier’s output charges the input capacitor through the feedback resistor, Rf. Thus, pops can be minimized by selecting an input capacitor value that is no higher than necessary to meet the desired −3dB frequency and is between 0.14CB and 0.20CB. A shown in Figure 4, the input resistor (RI) and the input capacitor, CI produce a −3dB high pass filter cutoff frequency that is found using Equation (7). f−3dB = 1/(2πRINCI) (7) 1.0µF along with a small value of Ci (in the range of 0.1µF to 0.39µF), produces a click-less and pop-less shutdown function. As discussed above, choosing Ci no larger than necessary for the desired bandwidth helps minimize clicks and pops. CB’s value should be in the range of 5 times to 7 times the value of Ci. This ensures that output transients are eliminated when power is first applied or the LM4867 resumes operation after shutdown. OPTIMIZING CLICK AND POP REDUCTION PERFORMANCE The LM4867 contains circuitry that eliminates turn-on and shutdown transients (“clicks and pops“) and transients that could occur when switching between BTL speakers and single-ended headphones. For this discussion, turn-on refers to either applying the power supply voltage or when the shutdown mode is deactivated. While the power supply is ramping to its final value, the LM4867’s internal amplifiers are configured as unity gain buffers and are disconnected from the -OUT and +OUT pins. An internal current source changes the voltage of the BYPASS pin in a controlled, linear manner. Ideally, the input and outputs track the voltage applied to the BYPASS pin. The gain of the internal amplifiers remains unity until the voltage on the bypass pin reaches 1/2 VDD. As soon as the voltage on the bypass pin is stable, the device becomes fully operational and the amplifier outputs are reconnected to the -OUT and +OUT pins. Although the BYPASS pin current cannot be modified, changing the size of CB alters the device’s turn-on time. There is a linear relationship between the size of CB and the turn-on time. Here are some typical turn-on times for various values of CB: CB 0.01µF 0.1µF 0.22µF 0.47µF 1.0µF 2.2µF TON 3ms 30ms 63ms 134ms 300ms 630ms As an example when using a speaker with a low frequency limit of 150Hz, Ci, using Equation (4) is 0.063µF. The 1.0µF Ci shown in Figure 4 allows the LM4867 to drive high efficiency, full range speaker whose response extends below 30Hz. BYPASS CAPACITOR VALUE SELECTION Besides minimizing the input capacitor size, careful consideration should be paid to value of CB, the capacitor connected to the BYPASS pin. Since CB determines how fast the LM4867 settles to quiescent operation, its value is critical when minimizing turn-on pops. The slower the LM4867’s outputs ramp to their quiescent DC voltage (nominally 1/2 VDD), the smaller the turn-on pop. Choosing CB equal to www.national.com 18 LM4867 Application Information (Continued) In order eliminate “clicks and pops“, all capacitors must be discharged before turn-on. Rapidly switching VDD may not allow the capacitors to fully discharge, which may cause “clicks and pops“. NO LOAD STABILITY The LM4867 may exhibit low level oscillation when the load resistance is greater than 10kΩ. This oscillation only occurs as the output signal swings near the supply voltages. Prevent this oscillation by connecting a 5kΩ between the output pins and ground. AUDIO POWER AMPLIFIER DESIGN Audio Amplifier Design: Driving 1W into an 8Ω Load The following are the desired operational parameters: Power Output: Load Impedance: Input Level: Input Impedance: Bandwidth: 1 WRMS 8Ω 1 VRMS 20 kΩ 100 Hz−20 kHz ± 0.25 dB Thus, a minimum gain of 2.83 allows the LM4867’s to reach full output swing and maintain low noise and THD+N performance. For this example, let AVD = 3. The amplifier’s overall gain is set using the input (Ri) and feedback (Ri) resistors. With the desired input impedance set at 20kΩ, the feedback resistor is found using Equation (11). (11) Rf/Ri = AVD/2 The value of Rf is 30kΩ. 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 (12) fL = 100Hz/5 = 20Hz and an fH = 20kHz x 5 = 100kHz (13) The design begins by specifying the minimum supply voltage necessary to obtain the specified output power. One way to find the minimum supply voltage is to use the Output Power vs Supply Voltage curve in the Typical Performance Characteristics section. Another way, using Equation (8), is to calculate the peak output voltage necessary to achieve the desired output power for a given load impedance. To account for the amplifier’s dropout voltage, two additional voltages, based on the Dropout Voltage vs Supply Voltage in the Typical Performance Characteristics curves, must be added to the result obtained by Equation (8). The result is Equation (9). 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 (12). (14) Ci≥ 1/(2πR ifL) The result is 1/(2π*20kΩ*20Hz) = 0.397µF Use a 0.39µF capacitor, the closest standard value. The product of the desired high frequency cutoff (100kHz in this example) and the differential gain AVD, determines the upper passband response limit. With AVD = 3 and fH = 100kHz, the closed-loop gain bandwidth product (GBWP) is 300kHz. This is less than the LM4867’s 3.5MHz GBWP. With this margin, the amplifier can be used in designs that require more differential gain while avoiding performance,restricting bandwidth limitations. RECOMMENDED PRINTED CIRCUIT BOARD LAYOUT Figures 8 through 12 show the recommended four-layer PC board layout that is optimized for the 24-pin LQ-packaged LM4867 and associated external components. Figures 13 through 17 show the recommended four-layer PC board layout that is optimized for the 24-pin MTE-packaged LM4867 and associated external components. Figures 18 through 20 show the recommended two-layer PC board layout that is optimized for the 20-pin MT-packaged LM4867 and associated external components. These circuits are designed for use with an external 5V supply and 4Ω speakers. These circuit boards are easy to use. Apply 5V and ground to the board’s VDD and GND pads, respectively. Connect 4Ω speakers between the board’s −OUTA and +OUTA and OUTB and +OUTB pads. (15) (8) VDD ≥ (VOUTPEAK+ (VODTOP + VODBOT)) (9) The Output Power vs Supply Voltage graph for an 8Ω load indicates a minimum supply voltage of 4.6V. This is easily met by the commonly used 5V supply voltage. The additional voltage creates the benefit of headroom, allowing the LM4867 to produce peak output power in excess of 1W without clipping or other audible distortion. The choice of supply voltage must also not create a situation that violates of maximum power dissipation as explained above in the Power Dissipation section. After satisfying the LM4867’s power dissipation requirements, the minimum differential gain needed to achieve 1W dissipation in an 8Ω load is found using Equation (10). (10) 19 www.national.com LM4867 Application Information (Continued) 20001340 FIGURE 9. Recommended LQ PC Board Layout: Component-Side Silkscreen 20001341 FIGURE 10. Recommended LQ PC Board Layout: Component-Side Layout www.national.com 20 LM4867 Application Information (Continued) 20001342 FIGURE 11. Recommended LQ PC Board Layout: Upper Inner-Layer Layout 20001343 FIGURE 12. Recommended LQ PC Board Layout: Lower Inner-Layer Layout 21 www.national.com LM4867 Application Information (Continued) 20001344 FIGURE 13. Recommended LQ PC Board Layout: Bottom-Side Layout 20001345 FIGURE 14. Recommended MTE PC Board Layout: Component-Side Silkscreen www.national.com 22 LM4867 Application Information (Continued) 20001346 FIGURE 15. Recommended MTE PC Board Layout: Component-Side Layout 20001347 FIGURE 16. Recommended MTE PC Board Layout: Upper Inner-Layer Layout 23 www.national.com LM4867 Application Information (Continued) 20001348 FIGURE 17. Recommended MTE PC Board Layout: Lower Inner-Layer Layout 20001349 FIGURE 18. Recommended MTE PC Board Layout: Bottom-Side Layout www.national.com 24 LM4867 Application Information (Continued) 20001350 FIGURE 19. Recommended MT PC Board Layout: Component-Side Silkscreen 20001351 FIGURE 20. Recommended MT PC Board Layout: Component-Side Layout 25 www.national.com LM4867 Application Information (Continued) 20001352 FIGURE 21. Recommended MT PC Board Layout: Bottom-Side Layout www.national.com 26 LM4867 Physical Dimensions unless otherwise noted inches (millimeters) 24-Lead MOLDED PKG, Leadless Leadframe Package LLP Order Number LM4867LQ NS Package Number LQA24A 20-Lead MOLDED PKG, TSSOP, JEDEC, 4.4mm BODY WIDTH Order Number LM4867MT NS Package Number MTC20 27 www.national.com LM4867 Output-Transient-Free Dual 2.1W Audio Amplifier Plus No Coupling Capacitor Stereo Headphone Function Physical Dimensions inches (millimeters) unless otherwise noted (Continued) 20-Lead MOLDED TSSOP, EXPOSED PAD, 6.5x4.4x0.9mm Order Number LM4867MTE NS Package Number MXA20A 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. For the most current product information visit us at www.national.com. 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