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LM4830M Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM4830M Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 14 page Application Information (Continued) ground return lines should not be tied together with AC input reference ground return lines. In addition, the layout of these ground lines should be physically located as far as reason- ably possible from each other so that large signal coupling cannot occur. To further exemplify this point, the outputs and output load returns for the power amplifier, which have volts of signal on them, should be physically isolated from the sen- sitive inputs and AC input ground returns associated with the preamp. It is easy for large signals to couple into the sensi- tive low voltage microphone preamp inputs. TABLE 1. 4-Bit Attenuation Control LD Input Bits Attenuation Bridge Pin msb: lsb Level (dB) Amplifier D3–D0 Gain (dB) 1 0000 0 dB 6 dB 1 0001 −2 dB 4 dB 1 0010 −4 dB 2 dB 1 0011 −6 dB 0 dB 1 0100 −8 dB −2 dB 1 0101 −10 dB −4 dB 1 0110 −12 dB −6 dB 1 0111 −14 dB −8 dB 1 1000 −16 dB −10 dB 1 1001 −18 dB −12 dB 1 1010 −20 dB −14 dB 1 1011 −22 dB −16 dB 1 1100 −24 dB −18 dB 1 1101 −26 dB −20 dB 1 1110 −28 dB −22 dB 1 1111 −30 dB −24 dB 0 XXXX NC NC 0 — Logic Low (0V) 1 — Logic High (5V) X — Don’t Care NC — No Change DIGITAL ATTENUATION CONTROL The Load (LD) pin, pin 9, has two modes of operation. When this input pin is a logic high, 5V, the power amp’s attenuation control is in “transparent mode” where the voltages on bits D0–D3 will cause the appropriate attenuation level to be latched and decoded within the IC. For normal attenuation, pin 9 should be at 5V. When the LD input pin is a logic low, 0V, the power amp’s attenuation control is “locked-out” so that any change in the input bits will not cause a subsequent change in the amp’s attenuation level. The attenuation level is preset to −16 dB when the IC is first powered up, assuming that LD is a logic low until the IC is fully biased up. To provide the best click and pop performance when chang- ing attenuation levels, each step should be utilized. If a mute-type function is desired, it is recommended that each of the attenuation steps be “ramped through” quicker than the normal attenuation ramp. To ensure that attenuation steps are flawless when data is transitioning with load, refer to the timing diagram for proper setup and hold times. SELECTION OF EXTERNAL CAPACITORS The IC’s low frequency power supply rejection can be im- proved by using a larger bypass capacitor, C b. By increasing this capacitor value, the THD performance at low frequen- cies will also be improved. For cost sensitive designs, 0.1 µF is recommended, however, for best performance at least 1 µF should be used. The selection of the microphone input coupling capacitors should be based on desired low frequency coupling. Since the input resistance for those inputs is around 20 k Ω, the coupling cap should be 0.47 µF for 17 Hz coupling or 0.047 µF for 170 Hz coupling. Similarly, the selection of the power amplifier input coupling capacitors should be based on an input resistance of 40 k Ω, so for flatband 20 Hz reproduction, 0.47 µF caps or larger should be used. VOICE-BAND DESIGN The preamp on this IC is intended to be used for microphone amplification. Depending upon the frequency response of the microphone, the preamplifier’s response can be config- ured to fit the microphone. Simple capacitors can be used to bandwidth limit the frequency response of the preamplifier and improve the system’s performance. Once the gain of the preamp is chosen, the values for the resistors and capacitors can be selected based upon desired cutoff frequencies using the equations below. A VCL = 1+Rf/R i (4) flp = 1/(2 π R fCf ) (5) fhp = 1/(2 π R iCi ) (6) As an example, lets assume that the desired closed-loop gain is 40 dB and the desired voice-band is 300 Hz to 3 kHz. Using Equation 4, we choose R f = 100 kΩ and Ri = 1kΩ. The desired value in dB is equal to 20 log (A VCL). Then, solv- ing for C f and Ci using flp = 3 kHz, fhp = 300 Hz, Rf = 100 k Ω, and R i = 1kΩ we get the following: Cf = 530 pF and C i = 0.53 µF. COMPUTER APPLICATION CIRCUIT The LM4830 can also be used to drive both an internal sys- tem speaker and stereo headphones simultaneously, as shown in Figure 2. The internally configured unity-gain buffer requires the preamp to also be set up in an inverting unity-gain fashion to maintain proper signal phase between channels for the stereo headphone amplifier. The unity-gain configured circuit also requires that the AC input signal dy- namic range be properly conditioned for the 2.5 V PK signal swing. Please refer to the Typical Performance Characteristics curves for THD+N vs P O and frequency of the MIC preamp and buffer. SHUTDOWN FUNCTION In order to reduce current consumption while not in use, the LM4830 contains a shutdown pin to externally turn off the IC’s bias circuitry. This shutdown feature turns the IC off when a logic high is placed on the shutdown pin. The trigger point between a logic low and logic high is typically half-supply. Quiescent current consumption will depend upon the value of this voltage. It is best for this voltage to be forced to V DDto obtain the guaranteed shutdown current. The shutdown feature reduces quiescent supply current con- sumption from a typical 11 mA to under 2 µA for the whole IC. www.national.com 10 |
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