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LM48556 Datasheet(PDF) 11 Page - Texas Instruments |
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LM48556 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 23 page LM48556 www.ti.com SNAS452A – JUNE 2008 – REVISED DECEMBER 2008 Charge Pump Hold Capacitor (CSS) The value and ESR of the hold capacitor (CSS) directly affects the ripple on CPVSS. Increasing the value of CSS reduces output ripple. Decreasing the ESR of CSS reduces both output ripple and charge pump output impedance. A lower value capacitor can be used in systems with low maximum output power requirements. Gain Setting Resistor Selection The amplifier gain of the LM48556 is set by four external resistors, two per each input, RIN_ and RF_ Figure 1. The amplifier gain is given by Equation 1: AV = RF / RIN (V/V) (1) Careful matching of the resistor pairs, RF+ and RF-, and RIN+ and RIN-, is required for optimum performance. Any mismatch between the resistors results in a differential gain error that leads to an increase in THD+N, decrease in PSRR and CMRR, as well as an increase in output offset voltage. Resistors with a tolerance of 1% or better are recommended. The gain setting resistors should be placed as close to the device as possible. Keeping the input traces close together and of the same length increases noise rejection in noisy environments. Noise coupled onto the input traces which are physically close to each other will be common mode and easily rejected. Feedback Capacitor Selection Due to their capacitive nature, ceramic speakers poorly reproduce high frequency audio content. At high frequencies, a ceramic speaker presents a low impedance load to the amplifier, increasing the required drive current. The higher output current can drive the device into clipping, increasing THD+N. Low-pass filtering the audio signal improves audio quality by decreasing the signal amplitude at high frequencies, reducing the speaker drive current. Adding a capacitor in parallel with each feedback resistor creates a simple low-pass filter with the - 3dB point determined by Equation 2: f−3dB = 1 / 2πRFCF (Hz) Where • RF is the value of the feedback resistor determined by Equation 1 in the Gain Setting Resistor Selection section • CF is the value of the feedback capacitor (2) The feedback capacitor is optional and not required for normal operation. Input Capacitor Selection Input capacitors block the DC component of the audio signal, eliminating any conflict between the DC component of the audio source and the bias voltage of the LM48556. The input capacitors create a high-pass filter with the input resistors RIN. The -3dB point of the high pass filter is found using Equation 3 below. f = 1 / 2 πRINCIN (Hz) Where • the value of RIN is determined by Equation 1 in the Gain Setting Resistor Selection section (3) When the LM48556 is using a single-ended source, power supply noise on the ground is seen as an input signal. Setting the high-pass filter point above the power supply noise frequencies, 217Hz in a GSM phone, for example, filters out the noise such that it is not amplified and heard on the output. Capacitors with a tolerance of 1% or better are recommended for impedance matching and improved CMRR and PSRR. SINGLE-ENDED AUDIO AMPLIFIER CONFIGURATION The LM48556 is compatible with single-ended sources. Figure 4 shows the typical single-ended applications circuit. Copyright © 2008, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM48556 |
Similar Part No. - LM48556_14 |
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Similar Description - LM48556_14 |
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