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LM48556TL Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM48556TL Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 20 page Application Information GENERAL AMPLIFIER FUNCTION The LM48556 is a fully differential ceramic speaker driver that utilizes National’s inverting charge pump technology to deliv- er the high drive voltages required by ceramic speakers, without the need for noisy, board-space consuming inductive based regulators. The low-noise, inverting charge pump cre- ates a negative supply (CPV SS) from the positive supply (PV DD). Because the amplifiers operate from these bipolar supplies, the maximum output voltage swing for each ampli- fier is doubled compared to a traditional single supply device. Additionally, the LM48556 is configured as a bridge-tied load (BTL) device, quadrupling the maximum theoretical output voltage range when compared to a single supply, single-end- ed output amplifier, see Bridged Configuration Explained sec- tion. The charge pump and BTL configuration allow the LM48556 to deliver over 17V P-P at 1kHz to a 1µF ceramic speaker while operating from a single 4.5V supply . DIFFERENTIAL AMPLIFIER EXPLANATION The LM48556 features a differential input stage, which offers improved noise rejection compared to a single-ended input amplifier. Because a differential input amplifier amplifies the difference between the two input signals, any component common to both signals is cancelled. An additional benefit of the differential input structure is the possible elimination of the DC input blocking capacitors. Since the DC component is common to both inputs, and thus cancelled by the amplifier, the LM48556 can be used without input coupling capacitors when configured with a differential input signal. BRIDGE CONFIGURATION EXPLAINED The LM48556 is designed to drive a load differentially, a con- figuration commonly referred to as a bridge-tied load (BTL). The BTL configuration differs from the single-ended configu- ration, where one side of the load is connected to ground. A BTL amplifier offers advantages over a single-ended device. Driving the load differentially doubles the output voltage com- pared to a single-ended amplifier under similar conditions. Any component common to both outputs is cancelled, thus there is no net DC voltage across the load, eliminating the DC blocking capacitors required by single-ended, single-supply amplifiers. SHUTDOWN FUNCTION The LM48556 features a low current shutdown mode. Set SD = GND to disable the amplifier and reduce supply current to 0.1µA. Switch SD between V DD and GND for minimum cur- rent consumption in shutdown. The LM48556 may be dis- abled with shutdown voltages less than 0.45V, however, the idle current will be greater than the typical 0.1µA value. PROPER SELECTION OF EXTERNAL COMPONENTS Power Supply Bypassing/Filtering Proper power supply bypassing is critical for low noise per- formance and high PSRR. Place the supply bypass capaci- tors as close to the device as possible. Place a 4.7µF tantalum capacitor in parallel with a 0.1µF ceramic capacitor from V DD to GND. Additional bulk capacitance may be added as required. Charge Pump Capacitor Selection Use low ESR ceramic capacitors (less than 100m Ω) for opti- mum performance. Charge Pump Flying Capacitor (C1) The flying capacitor (C1) affects the load regulation and out- put impedance of the charge pump. A C1 value that is too low results in a loss of current drive, leading to a loss of amplifier headroom. A higher valued C1 improves load regulation and lowers charge pump output impedance to an extent. Above 4.7µF, the R DS(ON) of the charge pump switches and the ESR of C1 and C SS dominate the output impedance. A lower value capacitor can be used in systems with low maximum output power requirements. Charge Pump Hold Capacitor (C SS) The value and ESR of the hold capacitor (C SS) directly affects the ripple on CPV SS. Increasing the value of CSS reduces out- put ripple. Decreasing the ESR of C SS reduces both output ripple and charge pump output impedance. A lower value ca- pacitor 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 re- sistors, two per each input, R IN_ and RF_ (Figure 1). The amplifier gain is given by equation (1): A V = RF / RIN (V/V) (1) Careful matching of the resistor pairs, R F+ and RF-, and RIN+ and R IN-, 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 CM- RR, 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 en- vironments. Noise coupled onto the input traces which are physically close to each other will be common mode and eas- ily rejected. Feedback Capacitor Selection Due to their capacitive nature, ceramic speakers poorly re- produce high frequency audio content. At high frequencies, a ceramic speaker presents a low impedance load to the am- plifier, 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) (2) Where R F is the value of the feedback resistor determined by equation (1) in the Gain Setting Resistors Selection section, and C F is the value of the feedback capacitor. 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 R IN. The -3dB point of the high pass filter is found using Equa- tion (3) below. 11 www.national.com |
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