SM6451 Application Note Electronic Audio Volume Control IC OVERVIEW This document comprises usage precautions, external component descriptions, and frequently asked questions (FAQ) for using the SM6451 electronic audio volume control IC with high-performance audio characteristics. HIGH PERFORMANCE USAGE MDT 16 2 ADRS1 MCK 15 3 ADRS2 MLEN 14 4 DVDD + 10uF 5 LOUT 0.01uF 6 LIN 7 AVDD + 10uF 0.01uF 8 VRL + + 10uF 1uF SM6451 1 RSTN CPU DVSS 13 ROUT 12 RIN 11 AVSS 10 VRR 9 + 0.01uF 0.01uF 10uF + 1uF R ch. OUT L ch. OUT L ch. IN R ch. IN Figure 1. Typical application circuit 1 Digital and Analog Separation Digital noise mixing with the analog signals can cause a deterioration in audio characteristics. ■ ■ ■ Physically separate the digital supply (DVDD, DVSS) and the analog supply (AVDD, AVSS) PCB wiring as far as possible. Connect decoupling capacitors of approximately 10µF between the supply pins DVDD to DVSS and AVDD to AVSS. Connect bypass capacitors of approximately 0.01µF between the supply pins to prevent incorrect operation due to digital signal mixing. Reference Voltage (VRL, VRR) ■ ■ Connect decoupling capacitors between VRL to AVSS and VRR to AVSS, just as for the supply pins. Connect bypass capacitors of approximately 0.01µF between the supply pins to prevent noise caused by mixing with digital signals during data transfer. Input Coupling Capacitors The SM6451 uses an internal reference voltage of VDD/2 (analog ground). Connect coupling capacitors to LIN and RIN inputs, which are self biased, to block the analog input signal DC offset voltage. The capacitor results in a low-frequency cutoff given by: 1 fc = --------------- [Hz] 2πRC where R is the internal resistance of 50kΩ (typ). For example, the circuit in figure 1 has a cutoff given by: 1 1 fc = --------------- = -------------------------------------------- = 3.18[Hz] 2πRC 2π × 50kΩ × 1µF NIPPON PRECISION CIRCUITS INC.—1 SM6451 Input Signal Amplitude The SM6451 employs input reference amplitudes of 1.2Vrms (SM6451A) and 0.8Vrms (SM6451B). Accordingly, the signal amplitude at the input should match the input reference amplitude, as shown in figure 2. R 3.3R 3.3R R L ch Input LIN 4Vrms LOUT L ch Output 1.2Vrms R SM6451 3.3R 3.3R R ch Input R RIN ROUT R ch Output Figure 2. 4Vrms input example Input Protection Diodes All inputs have built-in protection circuits to protect the device against electrostatic breakdown. However, if there is a possibility that the input signal may exceed the supply voltage, connect external input protection diodes as shown in figure 3. Take care when selecting diodes as these can greatly affect device characteristics. AVDD L ch Input LIN LOUT L ch Output SM6451 R ch Input RIN ROUT R ch Output AVSS Figure 3. Input protection diodes NIPPON PRECISION CIRCUITS INC.—2 SM6451 SIMPLIFYING CIRCUIT STRUCTURE The circuit structure can have a direct affect on the analog characteristics, and this should be kept in mind when considering simplifying the circuit. MDT 16 2 ADRS1 MCK 15 3 ADRS2 MLEN 14 4 DVDD 5 LOUT 6 LIN 7 AVDD + 1uF 8 VRL 0.01uF SM6451 1 RSTN DVSS 13 ROUT 12 RIN 11 AVSS 10 VRR 9 + 0.68uF + CPU 1uF + 0.68uF + 1uF R ch. OUT L ch. OUT L ch. IN R ch. IN Figure 4. Typical application circuit 2 Common Digital and Analog Supplies There is no inherent operating problem with using a common supply for both the digital and analog circuits. Decoupling Capacitors Decoupling capacitors can have capacitance as low as 1µF. VRL and VRR Decoupling capacitors can be as low as 0.33 to 0.68µF. The bypass capacitors can be eliminated, although noise may be generated during digital signal data transfers between device and microcontroller. Input Signal Amplitude, Input Protection Diodes If the signal amplitude is less than the supply voltage, there is no need to match the amplitude to the input reference amplitude. Also, the LIN and RIN input protection diodes are not necessary. NIPPON PRECISION CIRCUITS INC.—3 SM6451 FREQUENTLY ASKED QUESTIONS (FAQ) Q: Are output op-amps built-in? A: The SM6451 has built-in op-amps for both inputs and outputs. LIN Input Buffer RIN Attenuation Control Q: What is the phase relationship between inputs and outputs? A: The output signals are in-phase with the input signals. LOUT Output Buffer ROUT Figure 5. SM6451 input/output block Q: Is the current consumption higher than devices by other manufacturers? A: The SM6451 has built-in input/output op-amps, so the rated current consumption is higher than devices without built-in op-amps from other manufacturers. Q: What is the total IC current consumption? A: During normal operation, the current consumption is the sum of the digital circuit IDDD1 and the analog circuit IDDA currents. When transferring data from the CPU, the digital circuit current (during data transfer) is IDDD2. Q: What is the device state after system reset? A: The attenuation level is set to mute. Q: Is a system reset required when power is applied? A: A reset is required to reset internal attenuation decoder data. After power is applied, RSTN should be held LOW for a minimum of 100ns and then be set HIGH. VDD RSTN MIN 100ns Figure 6. Reset timing Q: What is the affect of the state of the microcontroller interface and chip address select pins? A: RSTN, ADRS1, and ADRS2 inputs all have pull-up resistors built-in. If a single SM6451 device is being used, it is recommended that the chip address select pins be tied HIGH (connected to DVDD or left open circuit) to reduce current consumption. When set to LOW, the current consumed is only the input current per pin IIL which is 230µA (SM6451A) and 70µA (SM6451B). Q: What are the limits on microcontroller interface data transfer speed? A: There is no lower limit on transfer speed. At high speed, the MDT data setup time (tMDS = 50ns min) and hold time (tMDH = 50ns min) place an upper limit on the clock speed of approximately 10MHz. NIPPON PRECISION CIRCUITS INC.—4 SM6451 Q: Are the so-called A curve, B curve, and C curve characteristics not possible? A: The SM6451 can control the attenuation level in 1dB steps (min). The attenuation step width can be changed by data, allowing A, B, and C curves to be closely approximated. Q: Can input amplitudes other than the input reference amplitude be used? A: The analog characteristics change with input amplitude, but operation is still possible for small input amplitudes. However, when the input amplitude is large, the signal waveform is clipped close to the supply voltage, and this condition becomes worse with larger amplitudes. Note that if the signal amplitude exceeds the supply voltage, input protection diodes become necessary. 1 1 f=1kHz ATT=0dB 20kHz LPF 0.1 THD+N(%) THD + N(%) f = 1kHz ATT = 0dB 20kHz LPF 0.01 0.001 0.1 1 1.2 2 VIN(Vrms) Figure 7. THD+N vs. input amplitude (SM6451A) 0.1 VDD=3.3V VDD=3.0V VDD=2.7V 0.01 0.001 .1 .2 .5 1 1.2 VIN(Vrms) Figure 8. THD+N vs. input amplitude (SM6451B) NIPPON PRECISION CIRCUITS INC.—5 SM6451 Please pay your attention to the following points at time of using the products shown in this document. The products shown in this document (hereinafter “Products”) are not intended to be used for the apparatus that exerts harmful influence on human lives due to the defects, failure or malfunction of the Products. Customers are requested to obtain prior written agreement for such use from NIPPON PRECISION CIRCUITS INC. (hereinafter “NPC”). Customers shall be solely responsible for, and indemnify and hold NPC free and harmless from, any and all claims, damages, losses, expenses or lawsuits, due to such use without such agreement. NPC reserves the right to change the specifications of the Products in order to improve the characteristic or reliability thereof. NPC makes no claim or warranty that the contents described in this document dose not infringe any intellectual property right or other similar right owned by third parties. Therefore, NPC shall not be responsible for such problems, even if the use is in accordance with the descriptions provided in this document. Any descriptions including applications, circuits, and the parameters of the Products in this documents are for reference to use the Products, and shall not be guaranteed free from defect, inapplicability to the design for the mass-production products without further testing or modification. Customers are requested not to export or re-export, directly or indirectly, the Products to any country or any entity not in compliance with or in violation of the national export administration laws, treaties, orders and regulations. Customers are requested appropriately take steps to obtain required permissions or approvals from appropriate government agencies. NIPPON PRECISION CIRCUITS INC. 4-3, Fukuzumi 2-chome, Koto-ku, Tokyo 135-8430, Japan Telephone: +81-3-3642-6661 Facsimile: +81-3-3642-6698 http://www.npc.co.jp/ Email: [email protected] NK9903BE 2003.12 NIPPON PRECISION CIRCUITS INC.—6