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LM4670 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM4670 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 20 page  Application Information (Continued) current usage while in the shutdown state. While the LM4670 may be disabled with shutdown voltages in between ground and supply, the idle current will be greater than the typical 0.01µA value. Increased THD may also be observed with voltages less than V DD on the Shutdown pin when in PLAY mode. The LM4670 has an internal resistor connected between GND and Shutdown pins. The purpose of this resistor is to eliminate any unwanted state changes when the Shutdown pin is floating. The LM4670 will enter the shutdown state when the Shutdown pin is left floating or if not floating, when the shutdown voltage has crossed the threshold. To mini- mize the supply current while in the shutdown state, the Shutdown pin should be driven to GND or left floating. If the Shutdown pin is not driven to GND, the amount of additional resistor current due to the internal shutdown resistor can be found by Equation (1) below. (V SD - GND) / 300k Ω (1) With only a 0.5V difference, an additional 1.7µA of current will be drawn while in the shutdown state. PROPER SELECTION OF EXTERNAL COMPONENTS The gain of the LM4670 is set by the external resistors, Ri in Figure 1, The Gain is given by Equation (2) below. Best THD+N performance is achieved with a gain of 2V/V (6dB). A V = 2 * 150 k Ω /R i (V/V) (2) It is recommended that resistors with 1% tolerance or better be used to set the gain of the LM4670. The Ri resistors should be placed close to the input pins of the LM4670. Keeping the input traces close to each other and of the same length in a high noise environment will aid in noise rejection due to the good CMRR of the LM4670. Noise coupled onto input traces which are physically close to each other will be common mode and easily rejected by the LM4670. Input capacitors may be needed for some applications or when the source is single-ended (see Figures 3, 5). Input capacitors are needed to block any DC voltage at the source so that the DC voltage seen between the input terminals of the LM4670 is 0V. Input capacitors create a high-pass filter with the input resistors, R i. The –3dB point of the high-pass filter is found using Equation (3) below. f C =1/(2 πR i Ci ) (Hz) (3) The input capacitors may also be used to remove low audio frequencies. Small speakers cannot reproduce low bass frequencies so filtering may be desired . When the LM4670 is using a single-ended source, power supply noise on the ground is seen as an input signal by the +IN input pin that is capacitor coupled to ground (See Figures 5 – 7). Setting the high-pass filter point above the power supply noise frequen- cies, 217Hz in a GSM phone, for example, will filter out this noise so it is not amplified and heard on the output. Capaci- tors with a tolerance of 10% or better are recommended for impedance matching. DIFFERENTIAL CIRCUIT CONFIGURATIONS The LM4670 can be used in many different circuit configu- rations. The simplest and best performing is the DC coupled, differential input configuration shown in Figure 2. Equation (2) above is used to determine the value of the R i resistors for a desired gain. Input capacitors can be used in a differential configuration as shown in Figure 3. Equation (3) above is used to determine the value of the C i capacitors for a desired frequency re- sponse due to the high-pass filter created by C i and Ri. Equation (2) above is used to determine the value of the R i resistors for a desired gain The LM4670 can be used to amplify more than one audio source. Figure 4 shows a dual differential input configuration. The gain for each input can be independently set for maxi- mum design flexibility using the R i resistors for each input and Equation (2). Input capacitors can be used with one or more sources as well to have different frequency responses depending on the source or if a DC voltage needs to be blocked from a source. SINGLE-ENDED CIRCUIT CONFIGURATIONS The LM4670 can also be used with single-ended sources but input capacitors will be needed to block any DC at the input terminals. Figure 5 shows the typical single-ended applica- tion configuration. The equations for Gain, Equation (2), and frequency response, Equation (3), hold for the single-ended configuration as shown in Figure 5. When using more than one single-ended source as shown in Figure 6, the impedance seen from each input terminal should be equal. To find the correct values for C i3 and Ri3 connected to the +IN input pin the equivalent impedance of all the single-ended sources are calculated. The single- ended sources are in parallel to each other. The equivalent capacitor and resistor, C i3 and Ri3, are found by calculating the parallel combination of all C ivalues and then all Ri val- ues. Equations (4) and (5) below are for any number of single-ended sources. C i3 =Ci1 +Ci2 +Cin ... (F) (4) R i3 = 1 / (1/Ri1 + 1/Ri2 + 1/Rin ...) ( Ω) (5) The LM4670 may also use a combination of single-ended and differential sources. A typical application with one single- ended source and one differential source is shown in Figure 7. Using the principle of superposition, the external compo- nent values can be determined with the above equations corresponding to the configuration. www.national.com 11 |
Similar Part No. - LM4670_06 |
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Similar Description - LM4670_06 |
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