19-0515; Rev 0; 5/06 1.2W, Low-EMI, Filterless, Class D Audio Amplifier Features The MAX9746 mono Class D audio power amplifier provides Class AB amplifier performance with Class D efficiency, conserving board space and extending battery life. The MAX9746 is designed specifically for systems using 1.8V logic interface. Using a Class D architecture, the MAX9746 delivers 1.2W into an 8Ω load while offering efficiencies near 90%. A low-EMI modulation scheme renders the traditional Class D output filter unnecessary. The MAX9746 offers two modulation schemes: a fixedfrequency (FFM) mode, and a spread-spectrum (SSM) mode that reduces EMI-radiated emissions due to the modulation frequency. Furthermore, the MAX9746 oscillator can be synchronized to an external clock through the SYNC input, allowing the switching frequency to be user defined. The SYNC input also allows multiple MAX9746s to be cascaded and frequency locked, minimizing interference due to clock intermodulation. The device utilizes a fully differential architecture, a fullbridged output, and comprehensive click-and-pop suppression. The gain of the MAX9746 is set internally further reducing external component count. The MAX9746 features high 72dB PSRR, a low 0.1% THD+N, and SNR in excess of 90dB. Short-circuit and thermal-overload protection prevent the device from damage during a fault condition. The MAX9746 is available in a 12-bump UCSP™ (1.5mm ✕ 2mm ✕ 0.6mm) package. The MAX9746 is specified over the extended -40°C to +85°C temperature range. ♦ Filterless Amplifier Passes FCC Radiated Emissions Standards with 100mm of Cable ♦ Unique Spread-Spectrum Mode Offers 5dB Emissions Improvement Over Conventional Methods ♦ Optional External SYNC Input ♦ Simple Master-Slave Setup for Stereo Operation ♦ 88% Efficiency ♦ 1.2W into 8Ω ♦ Low 0.1% THD+N ♦ High PSRR (72dB at 217Hz) ♦ Integrated Click-and-Pop Suppression ♦ 1.8V Logic-Compatible ♦ Low Quiescent Current (4mA) ♦ Low-Power Shutdown Mode (0.1µA) ♦ Short-Circuit and Thermal-Overload Protection ♦ Available in Thermally Efficient, Space-Saving 12-Bump UCSP Package (1.5mm x 2mm x 0.6mm) Ordering Information PART TEMP RANGE PIN-PACKAGE MAX9746BEBC+T -40oC to +85oC 12 UCSP-12 +Denotes lead-free package. Applications Cellular Phones MP3 Players PDAs Portable Audio Block Diagram Pin Configuration TOP VIEW (BUMP SIDE DOWN) 1 VDD MAX9746 2 VDD DIFFERENTIAL AUDIO INPUT MODULATOR AND H-BRIDGE 3 4 SYNC OUT+ PGND PVDD A IN+ SHDN IN- GND B SYNC INPUT OSCILLATOR MAX9746 OUT- C UCSP UCSP is a trademark of Maxim Integrated Products, Inc. ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MAX9746 General Description MAX9746 1.2W, Low-EMI, Filterless, Class D Audio Amplifier ABSOLUTE MAXIMUM RATINGS VDD to GND..............................................................................6V PVDD to PGND .........................................................................6V GND to PGND .......................................................-0.3V to +0.3V All Other Pins to GND.................................-0.3V to (VDD + 0.3V) Continuous Current Into/Out of PVDD/PGND/OUT_ ........±600mA Continuous Input Current (all other pins)..........................±20mA Duration of OUT_ Short Circuit to GND or PVDD ........Continuous Duration of Short Circuit Between OUT+ and OUT- ..Continuous Continuous Power Dissipation (TA = +70°C) 12-Bump UCSP (derate 6.1mW/°C above +70°C)........484mW Junction Temperature ......................................................+150°C Operating Temperature Range ...........................-40°C to +85°C Storage Temperature Range .............................-65°C to +150°C Lead Temperature (soldering, 10s) .................................+300°C Bump Temperature (soldering) Reflow ..........................................................................+235°C Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (VDD = PVDD = SHDN = 3.3V, GND = PGND = 0V, SYNC = GND (FFM), RL = 8Ω, RL connected between OUT+ and OUT-, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.) (Notes 1, 2) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS GENERAL Supply Voltage Range VDD 5.5 V Quiescent Current IDD Inferred from PSRR test 2.5 4 5.2 mA Shutdown Current 10 ISHDN 0.1 Turn-On Time tON 30 ms Input Resistance RIN TA = +25°C 12 20 kΩ VBIAS Either input 0.73 0.83 Input Bias Voltage Voltage Gain Output Offset Voltage Common-Mode Rejection Ratio AV VOS CMRR TA = +25°C ±11 TMIN ≤ TA ≤ TMAX fIN = 1kHz, input referred Output Power Total Harmonic Distortion Plus Noise PSRR POUT THD+N SNR fOSC fRIPPLE = 20kHz 55 THD+N = 1% RL = 8Ω 530 mW fIN = 1kHz, either FFM or SSM RL = 8Ω, POUT = 125mW 0.1 % VOUT = 2VRMS FFM 89 SSM 87 FFM 92 SSM dB dB 90 SYNC = GND 980 SYNC = float 1280 SYNC = VDD (SSM mode) 2 dB 70 72 A-weighted Oscillator Frequency mV fRIPPLE = 217Hz 200mVP-P ripple BW = 22Hz to 22kHz Signal-to-Noise Ratio ±80 72 50 V dB ±120 VDD = 2.5V to 5.5V, TA = +25°C Power-Supply Rejection Ratio (Note 3) 0.93 12 µA 1100 1220 1450 1620 1220 ±120 _______________________________________________________________________________________ kHz 1.2W, Low-EMI, Filterless, Class D Audio Amplifier (VDD = PVDD = SHDN = 3.3V, GND = PGND = 0V, SYNC = GND (FFM), RL = 8Ω, RL connected between OUT+ and OUT-, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.) (Notes 1, 2) PARAMETER SYMBOL CONDITIONS MIN SYNC Frequency Lock Range Efficiency TYP 800 η POUT = 500mW, fIN = 1kHz MAX UNITS 2000 kHz 94 % DIGITAL INPUTS (SHDN, SYNC) VIH SHDN Input Threshold 1.4 VIL 0.4 SHDN Input Leakage Current ±1 VIH External clock 0.4 SSM mode Internal clock µA 1.4 VIL SYNC Input Threshold V 1.9 Tie to VDD FFM mode (1.1MHz) Tie to GND FFM mode (1.45MHz) Float SYNC Input Current V 0.4 ±5 µA ELECTRICAL CHARACTERISTICS (VDD = PVDD = SHDN = 5V, GND = PGND = 0V, SYNC = GND (FFM), RL = 8Ω, RL connected between OUT+ and OUT-, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.) (Notes 1, 2) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS Quiescent Current IDD 5.2 mA Shutdown Current ISHDN 0.1 µA Common-Mode Rejection Ratio CMRR 72 dB f = 1kHz, input referred f = 217Hz 72 f = 20kHz 55 Power-Supply Rejection Ratio PSRR 200mVP-P ripple Output Power POUT THD+N = 1% RL = 8Ω f = 1kHz, either FFM or SSM RL = 8Ω, POUT = 125mW Total Harmonic Distortion Plus Noise Signal-to-Noise Ratio THD+N SNR VOUT = 3VRMS BW = 22Hz to 22kHz A-weighted dB 1200 mW 0.1 % FFM 92.5 SSM 90.5 FFM 95.5 SSM 93.5 dB Note 1: All devices are 100% production tested at +25°C. All temperature limits are guaranteed by design. Note 2: Testing performed with a resistive load in series with an inductor to simulate an actual speaker load. For RL = 8Ω, L = 68µH. Note 3: PSRR is specified with the amplifier inputs connected to GND through CIN. _______________________________________________________________________________________ 3 MAX9746 ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (VDD = 3.3V, SYNC = VDD (SSM), TA = +25°C, unless otherwise noted.) VDD = +3.3V RL = 8Ω POUT = 125mW 10 0.1 0.01 SSM (VDD) 0.1 1k 10k 100k 0.001 10 100 1k 10k TOTAL HARMONIC DISTORTION PLUS NOISE vs. OUTPUT POWER MAX9746 toc04 fIN = 1kHz 1 fIN = 10kHz 0.1 10 1 100 SYNC = FLOAT 0.1 VDD = 3.3V RL = 8Ω 0.1 SYNC = 800kHz 0.001 0.001 0.001 0 0.1 0.2 0.3 0.4 0.5 0.6 OUTPUT POWER (W) 0.7 0.8 0 0.1 0.3 0.4 0.2 OUTPUT POWER (W) 0 0.6 0.5 1 0.1 MAX9746 toc08 90 80 EFFICIENCY (%) THD+N (%) 10 0.3 0.4 0.2 OUTPUT POWER (W) 100 MAX9746 toc07 VDD = 3.3V fIN = 1kHz POUT = 500mW RL = 8Ω DIFFERENTIAL INPUT 0.1 EFFICIENCY vs. OUTPUT POWER TOTAL HARMONIC DISTORTION PLUS NOISE vs. COMMON-MODE VOLTAGE 100 SYNC = 1.4MHz 0.01 SYNC = GND 0.01 70 60 50 40 30 20 0.01 VDD = 3.3V fIN = 1kHz RL = 8Ω 10 0 0.001 0 4 0.5 1.0 1.5 2.0 COMMON-MODE VOLTAGE (V) 1.5 SYNC = 2MHz 1 SYNC = VDD 0.01 1.3 0.5 0.8 1.0 OUTPUT POWER (W) 10 THD+N (%) 10 THD+N (%) fIN = 100Hz VDD = 3.3V RL = 8Ω 0.3 TOTAL HARMONIC DISTORTION PLUS NOISE vs. OUTPUT POWER MAX9746 toc05 TOTAL HARMONIC DISTORTION PLUS NOISE vs. OUTPUT POWER 100 0 100k FREQUENCY (Hz) VDD = 3.3V RL = 8Ω f = 10kHz f = 1kHz FREQUENCY (Hz) 100 0.1 0.01 0.001 100 f = 100Hz FFM (GND) 0.01 0.001 1 MAX9746 toc06 THD+N (%) POUT = 300mW 10 VDD = 5V RL = 8Ω 1 POUT = 125mW THD+N (%) 1 100 MAX9746 toc02 VDD = +3.3V RL = 8Ω THD+N (%) 10 MAX9746 toc01 10 TOTAL HARMONIC DISTORTION PLUS NOISE vs. OUTPUT POWER TOTAL HARMONIC DISTORTION PLUS NOISE vs. FREQUENCY MAX9746 toc03 TOTAL HARMONIC DISTORTION PLUS NOISE vs. FREQUENCY THD+N (%) MAX9746 1.2W, Low-EMI, Filterless, Class D Audio Amplifier 2.5 0 0.1 0.2 0.3 0.4 OUTPUT POWER (W) _______________________________________________________________________________________ 0.5 0.6 0.5 0.6 1.2W, Low-EMI, Filterless, Class D Audio Amplifier EFFICIENCY vs. SUPPLY VOLTAGE 90 80 EFFICIENCY (%) 70 60 50 40 30 70 70 60 50 40 30 20 20 VDD = 5.0V fIN = 1kHz RL = 8Ω 10 0.4 0.2 0.6 0.8 1.0 30 VDD = 3.3V fIN = 1kHz THD+N = 1% RL = 8Ω 0 3.0 3.5 4.0 4.5 5.0 5.5 800 1000 1200 1400 1600 1800 2000 SUPPLY VOLTAGE (V) SYNC FREQUENCY (kHz) OUTPUT POWER vs. SUPPLY VOLTAGE OUTPUT POWER vs. LOAD RESISTANCE COMMON-MODE REJECTION RATIO vs. FREQUENCY THD+N = 10% 1.0 0.8 THD+N = 1% 0.6 0.4 5.0V 3.3V 4.5 5.0 MAX9746 TOC14 -60 -100 0 4.0 -50 -90 0 3.5 -40 -80 fIN = 1kHz RL = 8Ω 3.0 -20 -70 0.5 0.2 INPUT REFERRED VIN = 200mVP-P -30 1.5 1.0 0 -10 CMRR (dB) 2.0 OUTPUT POWER (W) 1.4 fIN = 1kHz ZLOAD = 3.3µH IN SERIES WITH RL THD+N = 1% MAX9746 toc13 2.5 MAX9746 toc12 1.8 5.5 1 SUPPLY VOLTAGE (V) 10 100 1000 10 100 1k 10k 100k FREQUENCY (Hz) LOAD RESISTANCE (Ω) POWER-SUPPLY REJECTION RATIO vs. FREQUENCY GSM POWER-SUPPLY REJECTION MAX9746 toc16 MAX9746 TOC15 0 OUTPUT REFERRED INPUTS AC GROUNDED VDD = 3.3V -10 -20 500mV/div VDD -30 PSRR (dB) 2.5 40 OUTPUT POWER (W) 1.6 1.2 50 10 0 0 60 20 fIN = 1kHz THD+N = 1% RL = 8Ω 10 0 OUTPUT POWER (W) 80 EFFICIENCY (%) 80 90 MAX9746 toc10 90 EFFICIENCY (%) 100 MAX9746 toc09 100 EFFICIENCY vs. SYNC FREQUENCY MAX9746 toc11 EFFICIENCY vs. OUTPUT POWER -40 -50 -60 -70 MAX9746 OUTPUT -80 -90 100µV/div -100 10 100 1k FREQUENCY (Hz) 10k 100k f = 217Hz INPUT LOW = 3V INPUT HIGH = 3.5V 2ms/div DUTY CYCLE = 88% RL = 8Ω _______________________________________________________________________________________ 5 MAX9746 Typical Operating Characteristics (continued) (VDD = 3.3V, SYNC = VDD (SSM), TA = +25°C, unless otherwise noted.) Typical Operating Characteristics (continued) (VDD = 3.3V, SYNC = VDD (SSM), TA = +25°C, unless otherwise noted.) OUTPUT FREQUENCY SPECTRUM OUTPUT FREQUENCY SPECTRUM -60 -80 -100 -120 -40 -80 -100 -120 -140 -140 0 5k 10k 15k FREQUENCY (Hz) 20k 0 20k -60 -80 -100 RBW = 10kHz -10 -20 OUTPUT AMPLITUDE (dB) SSM MODE VOUT = -60dBV f = 1kHz RL = 8Ω A-WEIGHTED -40 10k 15k FREQUENCY (Hz) 0 MAX9746 toc19 0 -20 5k WIDEBAND OUTPUT SPECTRUM (FFM MODE) OUTPUT FREQUENCY SPECTRUM OUTPUT MAGNITUDE (dBV) MAX9746 toc18 -60 MAX9746 toc20 OUTPUT MAGNITUDE (dBV) -40 SSM MODE VOUT = -60dBV f = 1kHz RL = 8Ω UNWEIGHTED -20 OUTPUT MAGNITUDE (dBV) FFM MODE VOUT = -60dBV f = 1kHz RL = 8Ω UNWEIGHTED -20 0 MAX9746 toc17 0 -30 -40 -50 -60 -70 -80 -120 -90 -100 -140 5k 10k 15k FREQUENCY (Hz) 1M 20k 100M RBW = 10kHz -20 MAX9746 toc21 TURN-ON/TURN-OFF RESPONSE 0 SHDN 3V -30 -40 -50 0V -60 -70 MAX9746 OUTPUT -80 250mV/div -90 -100 1M 10M 100M FREQUENCY (Hz) 6 1G FREQUENCY (Hz) WIDEBAND OUTPUT SPECTRUM (SSM MODE) -10 10M MAX9746 toc22 0 OUTPUT AMPLITUDE (dB) MAX9746 1.2W, Low-EMI, Filterless, Class D Audio Amplifier 1G f = 1kHz RL = 8Ω 10ms/div _______________________________________________________________________________________ 1.2W, Low-EMI, Filterless, Class D Audio Amplifier TA = +85°C 5.0 TA = +25°C 4.5 TA = -40°C 4.0 TA = +85°C 0.14 SUPPLY CURRENT (µA) SUPPLY CURRENT (mA) 5.5 0.16 MAX9746 toc23 6.0 MAX9746 toc24 SHUTDOWN SUPPLY CURRENT vs. SUPPLY VOLTAGE SUPPLY CURRENT vs. SUPPLY VOLTAGE 0.12 0.10 TA = +25°C 0.08 0.06 0.04 3.5 TA = -40°C 0.02 0 3.0 2.5 3.0 3.5 4.0 4.5 SUPPLY VOLTAGE (V) 5.0 2.5 5.5 3.0 3.5 4.0 4.5 5.0 5.5 SUPPLY VOLTAGE (V) Functional Diagram VDD 1µF B2 SHDN A1 B4 A3 VDD PVDD SYNC UVLO/POWER MANAGEMENT CLICK-AND-POP SUPPRESSION OSCILLATOR PVDD 1µF 1µF B1 IN+ CLASS D MODULATOR C1 IN- OUT+ A4 PGND PVDD OUT- C4 MAX9746 PGND PGND B3 GND C2 _______________________________________________________________________________________ 7 MAX9746 Typical Operating Characteristics (continued) (VDD = 3.3V, SYNC = VDD (SSM), TA = +25°C, unless otherwise noted.) 1.2W, Low-EMI, Filterless, Class D Audio Amplifier MAX9746 Pin Description BUMP NAME A1 VDD Analog Power Supply FUNCTION B1 IN+ Noninverting Audio Input C1 IN- Inverting Audio Input C2 GND Analog Ground B2 SHDN Active-Low Shutdown Input. Connect to VDD for normal operation. A3 SYNC Frequency Select and External Clock Input. SYNC = GND: Fixed-frequency mode with fS = 1100kHz. SYNC = Float: Fixed-frequency mode with fS = 1450kHz. SYNC = VDD: Spread-spectrum mode with fS = 1220kHz ±120kHz. SYNC = Clocked: Fixed-frequency mode with fS = external clock frequency. B3 PGND Power Ground A4 OUT+ Amplifier-Output Positive Phase C4 OUT- Amplifier-Output Negative Phase B4 PVDD H-Bridge Power Supply Detailed Description The MAX9746 filterless, Class D audio power amplifier features several improvements to switch-mode amplifier technology. The MAX9746 offers Class AB performance with Class D efficiency, while occupying minimal board space. A unique filterless modulation scheme, synchronizable switching frequency, and SSM mode create a compact, flexible, low-noise, efficient audio power amplifier. The differential input architecture reduces common-mode noise pickup, and can be used without input-coupling capacitors. The device can also be configured as a single-ended input amplifier. Comparators monitor the MAX9746 inputs and compare the complementary input voltages to the sawtooth waveform. The comparators trip when the input magnitude of the sawtooth exceeds their corresponding input voltage. Both comparators reset at a fixed time after the rising edge of the second comparator trip point, generating a minimum-width pulse tON(MIN) at the output of the second comparator (Figure 1). As the input voltage increases or decreases, the duration of the pulse at one output increases (the first comparator to trip) while the other output pulse duration remains at tON(MIN). This causes the net voltage across the speaker (VOUT+ VOUT-) to change. Operating Modes Fixed-Frequency Modulation (FFM) Mode The MAX9746 features two FFM modes. The FFM modes are selected by setting SYNC = GND for a 1.1MHz 8 switching frequency, and SYNC = FLOAT for a 1.45MHz switching frequency. In FFM mode, the frequency spectrum of the Class D output consists of the fundamental switching frequency and its associated harmonics (see the Wideband Output Spectrum graph in the Typical Operating Characteristics). The MAX9746 allows the switching frequency to be changed by +32%, should the frequency of one or more of the harmonics fall in a sensitive band. This can be done at any time and does not affect audio reproduction. Spread-Spectrum Modulation (SSM) Mode The MAX9746 features a unique spread-spectrum mode that flattens the wideband spectral components, improving EMI emissions that may be radiated by the speaker and cables by 5dB. Proprietary techniques ensure that the cycle-to-cycle variation of the switching period does not degrade audio reproduction or efficiency (see the Typical Operating Characteristics). Select SSM mode by setting SYNC = VDD. In SSM mode, the switching frequency varies randomly by ±120kHz around the center frequency (1.22MHz). The modulation scheme remains the same, but the period of the sawtooth waveform changes from cycle to cycle (Figure 2). Instead of a large amount of spectral energy present at multiples of the switching frequency, the energy is now spread over a bandwidth that increases with frequency. Above a few megahertz, the wideband spectrum looks like white noise for EMI purposes. _______________________________________________________________________________________ 1.2W, Low-EMI, Filterless, Class D Audio Amplifier MAX9746 tSW VIN- VIN+ OUT- OUT+ tON(MIN) VOUT+ - VOUT- Figure 1. MAX9746 Outputs with an Input Signal Applied Table 1. Operating Modes SYNC INPUT MODE GND FFM with fS = 1100kHz FLOAT FFM with fS = 1450kHz VDD Clocked SSM with fS = 1220kHz ±120kHz FFM with fS = external clock frequency External Clock Mode The SYNC input allows the MAX9746 to be synchronized to a system clock (allowing a fully synchronous system), or allocating the spectral components of the switching harmonics to insensitive frequency bands. Applying an external TTL clock of 800kHz to 2MHz to SYNC synchronizes the switching frequency of the MAX9746. The period of the SYNC clock can be randomized, enabling the MAX9746 to be synchronized to another MAX9746 operating in SSM mode. _______________________________________________________________________________________ 9 MAX9746 1.2W, Low-EMI, Filterless, Class D Audio Amplifier tSW tSW tSW tSW VIN- VIN+ OUT- OUT+ tON(MIN) VOUT+ - VOUT- Figure 2. MAX9746 Output with an Input Signal Applied (SSM Mode) Filterless Modulation/Common-Mode Idle The MAX9746 uses Maxim’s unique modulation scheme that eliminates the LC filter required by traditional Class D amplifiers, improving efficiency, reducing component count, and conserving board space and system cost. Conventional Class D amplifiers output a 50% dutycycle square wave when no signal is present. With no filter, the square wave appears across the load as a DC voltage, resulting in finite load current, increasing power consumption. When no signal is present at the input of the MAX9746, the outputs switch as shown in Figure 3. Because the MAX9746 drives the speaker differentially, the two outputs cancel each other, resulting in no net idle mode voltage across the speaker, minimizing power consumption. VIN = 0V OUT- OUT+ VOUT+ - VOUT- = 0V Figure 3. MAX9746 Outputs with No Input Signal 10 ______________________________________________________________________________________ 1.2W, Low-EMI, Filterless, Class D Audio Amplifier The theoretical best efficiency of a linear amplifier is 78%; however, that efficiency is only exhibited at peak output powers. Under normal operating levels (typical music reproduction levels), efficiency falls below 30%, whereas the MAX9746 still exhibits near 90% efficiencies under the same conditions (Figure 4). Shutdown The MAX9746 has a shutdown mode that reduces power consumption and extends battery life. Driving SHDN low places the MAX9746 in a low-power (0.1µA) shutdown mode. Connect SHDN to VDD for normal operation. Click-and-Pop Suppression The MAX9746 features comprehensive click-and-pop suppression that eliminates audible transients on startup and shutdown. While in shutdown, the H-bridge is in a high-impedance state. During startup or power-up, the input amplifiers are muted and an internal loop sets the modulator bias voltages to the correct levels, preventing clicks and pops when the H-bridge is subsequently enabled. For 35ms following startup, a soft-start function gradually unmutes the input amplifiers. EFFICIENCY vs. OUTPUT POWER 100 90 EFFICIENCY (%) 80 70 MAX9746 60 Applications Information Filterless Operation Traditional Class D amplifiers require an output filter to recover the audio signal from the amplifier’s output. The filters add cost, increase the solution size of the amplifier, and can decrease efficiency. The traditional PWM scheme uses large differential output swings (2 x VDD peak-to-peak) and causes large ripple currents. Any parasitic resistance in the filter components results in a loss of power, lowering the efficiency. The MAX9746 does not require an output filter. The device relies on the inherent inductance of the speaker coil and the natural filtering of both the speaker and the human ear to recover the audio component of the square-wave output. Eliminating the output filter results in a smaller, less costly, more efficient solution. Because the frequency of the MAX9746 output is well beyond the bandwidth of most speakers, voice coil movement due to the square-wave frequency is very small. Although this movement is small, a speaker not designed to handle the additional power can be damaged. For optimum results, use a speaker with a series inductance >10µH. Typical 8Ω speakers exhibit series inductances in the 20µH to 100µH range. Power-Conversion Efficiency Unlike a Class AB amplifier, the output offset voltage of a Class D amplifier does not noticeably increase quiescent current draw when a load is applied. This is due to the power conversion of the Class D amplifier. For example, an 8mV DC offset across an 8Ω load results in 1mA extra current consumption in a Class AB device. In the Class D case, an 8mV offset into 8Ω equates to an additional power drain of 8µW. Due to the high efficiency of the Class D amplifier, this represents an additional quiescent-current draw of 8µW/(VDD/100η), which is in the order of a few microamps. Input Amplifier 50 CLASS AB 40 30 VDD = 3.3V f = 1kHz RL = 8Ω 20 10 0 0 0.2 0.4 0.6 OUTPUT POWER (W) Figure 4. MAX9746 Efficiency vs. Class AB Efficiency Differential Input The MAX9746 features a differential input structure, making it compatible with many CODECs, and offering improved noise immunity over a single-ended input amplifier. In devices such as cellular phones, high-frequency signals from the RF transmitter can be picked up by the amplifier’s input traces. The signals appear at the amplifier’s inputs as common-mode noise. A differential input amplifier amplifies the difference of the two inputs; any signal common to both inputs is canceled. ______________________________________________________________________________________ 11 MAX9746 Efficiency Efficiency of a Class D amplifier is attributed to the region of operation of the output stage transistors. In a Class D amplifier, the output transistors act as currentsteering switches and consume negligible additional power. Any power loss associated with the Class D output stage is mostly due to the I ✕ R loss of the MOSFET on-resistance, and quiescent current overhead. MAX9746 1.2W, Low-EMI, Filterless, Class D Audio Amplifier Single-Ended Input The MAX9746 can be configured as a single-ended input amplifier by capacitively coupling either input to GND and driving the other input (Figure 5). DC-Coupled Input The input amplifier can accept DC-coupled inputs that are biased within the amplifier’s common-mode range (see the Typical Operating Characteristics). DC coupling eliminates the input-coupling capacitors, reducing component count to potentially one external component (see the System Diagram). However, the low-frequency rejection of the capacitors is lost, allowing low-frequency signals to feedthrough to the load. Component Selection Input Filter An input capacitor, CIN, in conjunction with the input impedance of the MAX9746 forms a highpass filter that removes the DC bias from an incoming signal. The ACcoupling capacitor allows the amplifier to bias the signal to an optimum DC level. Assuming zero source impedance, the -3dB point of the highpass filter is given by: f−3dB = 1 2πRINCIN Choose CIN so f-3dB is well below the lowest frequency of interest. Setting f-3dB too high affects the low-frequency response of the amplifier. Use capacitors whose dielectrics have low-voltage coefficients, such as tantalum or aluminum electrolytic. Capacitors with high-voltage coefficients, such as ceramics, may result in increased distortion at low frequencies. Other considerations when designing the input filter include the constraints of the overall system and the actual frequency band of interest. Although high-fidelity audio calls for a flat gain response between 20Hz and 20kHz, portable voice-reproduction devices such as cellular phones and two-way radios need only concen- 12 1µF SINGLE-ENDED AUDIO INPUT IN+ MAX9746 IN1µF Figure 5. Single-Ended Input trate on the frequency range of the spoken human voice (typically 300Hz to 3.5kHz). In addition, speakers used in portable devices typically have a poor response below 150Hz. Taking these two factors into consideration, the input filter may not need to be designed for a 20Hz to 20kHz response, saving both board space and cost due to the use of smaller capacitors. Output Filter The MAX9746 does not require an output filter. The device passes FCC emissions standards with 100mm of unshielded speaker cables. However, output filtering can be used if a design is failing radiated emissions due to board layout or cable length, or the circuit is near EMI-sensitive devices. Use an LC filter when radiated emissions are a concern, or when long leads are used to connect the amplifier to the speaker. Supply Bypassing/Layout Proper power-supply bypassing ensures low-distortion operation. For optimum performance, bypass VDD to GND and PVDD to PGND with separate 0.1µF capacitors as close to each pin as possible. A low-impedance, high-current power-supply connection to PVDD is assumed. Additional bulk capacitance should be added as required depending on the application and power-supply characteristics. GND and PGND should be star connected to system ground. Refer to the MAX9746 evaluation kit for layout guidance. ______________________________________________________________________________________ 1.2W, Low-EMI, Filterless, Class D Audio Amplifier Two MAX9746s can be configured as a stereo amplifier (Figure 6). Device U1 is the master amplifier; its unfiltered output drives the SYNC input of the slave device (U2), synchronizing the switching frequencies of the two devices. Synchronizing two MAX9746s ensures that no beat frequencies occur within the audio spectrum. This configuration works when the master device is in either FFM or SSM mode. There is excellent THD+N performance and minimal crosstalk between devices due to the SYNC connection (Figures 7 and 8). U2 locks onto only the frequency present at SYNC, not the pulse width. The internal feedback loop of device U2 ensures that the audio component of U1’s output is rejected. 1µF RIGHT-CHANNEL DIFFERENTIAL AUDIO INPUT VDD PVDD IN+ MAX9746 OUT+ U1 IN- OUTSYNC Designing with Volume Control The MAX9746 can easily be driven by single-ended sources (Figure 5), but extra care is needed if the source impedance “seen” by each differential input is unbalanced, such as the case in Figure 9, where the MAX9746 is used with an audio taper potentiometer acting as a volume control. Functionally, this configuration works well, but can suffer from click-pop transients at power-up (or coming out of shutdown) depending on the volume-control setting. As shown, the click-pop performance is fine for either max or min volume, but worsens at other settings. 1µF LEFT-CHANNEL DIFFERENTIAL AUDIO INPUT VDD PVDD IN+ MAX9746 OUT+ U2 IN- OUTSYNC Figure 6. Master-Slave Stereo Configuration TOTAL HARMONIC DISTORTION PLUS NOISE vs. OUTPUT POWER CROSSTALK vs. FREQUENCY 100 0 VDD = 3.3V f = 1kHz RL = 8Ω SLAVE DEVICE VDD = 3.3V RL = 8Ω f = 1kHz VIN = 500mVP-P -20 CROSSTALK (dB) THD+N (%) 10 1 0.1 0.01 -40 MASTER TO SLAVE -60 -80 -100 0.001 -120 0 0.1 0.2 0.3 OUTPUT POWER (W) Figure 7. Master-Slave THD+N 0.4 0.5 10 100 1k 10k 100k FREQUENCY (Hz) Figure 8. Master-Slave Crosstalk ______________________________________________________________________________________ 13 MAX9746 Stereo Configuration VDD MAX9746 1.2W, Low-EMI, Filterless, Class D Audio Amplifier One solution is the configuration shown in Figure 9b. The potentiometer is connected between the differential inputs, and these “see” identical RC paths when the device powers up. The variable resistive element appears between the two inputs, meaning the setting affects both inputs the same way. The potentiometer is audio taper, as in Figure 9a. This significantly improves transient performance on power-up or release from shutdown. A similar approach can be applied when the MAX9746 is driven differentially and a volume control is required. UCSP Applications Information For the latest application details on UCSP construction, dimensions, tape carrier information, PC board techniques, bump-pad layout, and recommended reflow temperature profile, as well as the latest information on reliability testing results, refer to the Application Note: UCSP—A Wafer-Level Chip-Scale Package available on Maxim’s website at www.maxim-ic.com/ucsp. 1µF CW 22kΩ 1µF IN- 50kΩ CW 50kΩ MAX9746 IN+ IN- 1µF 22kΩ MAX9746 IN+ 1µF Figure 9a. Single-Ended Drive of MAX9746 Plus Volume 14 Figure 9b. Improved Single-Ended Drive of MAX9746 Plus Volume ______________________________________________________________________________________ 1.2W, Low-EMI, Filterless, Class D Audio Amplifier VDD 1µF VDD 0.1µF AUX_IN OUT 2.2kΩ OUT BIAS VDD PVDD IN+ OUT+ MAX9746 IN- OUT- SHDN SYNC CODEC/ BASEBAND PROCESSOR 2.2kΩ MAX4063 0.1µF IN+ VDD IN0.1µF 1µF VDD SHDN 1µF INL OUTL 1µF MAX9722 INR OUTR MICROCONTROLLER PVSS SVSS C1P CIN 1µF 1µF Chip Information TRANSISTOR COUNT: 3595 PROCESS: BiCMOS ______________________________________________________________________________________ 15 MAX9746 System Diagram Package Information (The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information go to www.maxim-ic.com/packages.) 12L, UCSP 4x3.EPS MAX9746 1.2W, Low-EMI, Filterless, Class D Audio Amplifier PACKAGE OUTLINE, 4x3 UCSP 21-0104 F 1 1 Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. 16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2006 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc.