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LM4961 Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM4961 Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 16 page Application Information (Continued) and Y5F have such severe loss of capacitance due to effects of temperature variation and applied voltage, they may pro- vide as little as 20% of rated capacitance in many typical applications. Always consult capacitor manufacturer’s data curves before selecting a capacitor. High-quality ceramic capacitors can be obtained from Taiyo-Yuden. POWER SUPPLY BYPASSING As with any amplifier, proper supply bypassing is critical for low noise performance and high power supply rejection. The capacitor location on both V1 and V DD pins should be as close to the device as possible. SELECTING INPUT CAPACITOR FOR AUDIO AMPLIFIER One of the major considerations is the closedloop bandwidth of the amplifier. To a large extent, the bandwidth is dictated by the choice of external components shown in Figure 1. The input coupling capacitor, C i, forms a first order high pass filter which limits low frequency response. This value should be chosen based on needed frequency response for a few distinct reasons. High value input capacitors are both expensive and space hungry in portable designs. Clearly, a certain value capacitor is needed to couple in low frequencies without severe at- tenuation. But ceramic speakers used in portable systems, whether internal or external, have little ability to reproduce signals below 100Hz to 150Hz. Thus, using a high value input capacitor may not increase actual system perfor- mance. In addition to system cost and size, click and pop perfor- mance is affected by the value of the input coupling capaci- tor, C i. A high value input coupling capacitor requires more charge to reach its quiescent DC voltage (nominally 1/2 V DD). This charge comes from the output via the feedback and is apt to create pops upon device enable. Thus, by minimizing the capacitor value based on desired low fre- quency response, turn-on pops can be minimized. SELECTING BYPASS CAPACITOR FOR AUDIO AMPLIFIER Besides minimizing the input capacitor value, careful consid- eration should be paid to the bypass capacitor value. Bypass capacitor, C B, is the most critical component to minimize turn-on pops since it determines how fast the amplifer turns on. The slower the amplifier’s outputs ramp to their quies- cent DC voltage (nominally 1/2 V DD), the smaller the turn-on pop. Choosing C B equal to 1.0µF along with a small value of C i (in the range of 0.039µF to 0.39µF), should produce a virtually clickless and popless shutdown function. Although the device will function properly, (no oscillations or motor- boating), with C B equal to 0.1µF, the device will be much more susceptible to turn-on clicks and pops. Thus, a value of C B equal to 1.0µF is recommended in all but the most cost sensitive designs. SELECTING FEEDBACK CAPACITOR FOR AUDIO AMPLIFIER The LM4961 is unity-gain stable which gives the designer maximum system flexability. However, to drive ceramic speakers, a typical application requires a closed-loop differ- ential gain of 10. In this case a feedback capacitor (C f2) will be needed as shown in Figure 1 to bandwidth limit the amplifier. This feedback capacitor creates a low pass filter that elimi- nates possible high frequency noise. Care should be taken when calculating the -3dB frequency because an incorrect combination of R f and Cf2 will cause rolloff before the de- sired frequency SELECTING OUTPUT CAPACITOR (C O) FOR BOOST CONVERTER A single 4.7µF to 10µF ceramic capacitor will provide suffi- cient output capacitance for most applications. If larger amounts of capacitance are desired for improved line sup- port and transient response, tantalum capacitors can be used. Aluminum electrolytics with ultra low ESR such as Sanyo Oscon can be used, but are usually prohibitively expensive. Typical AI electrolytic capacitors are not suitable for switching frequencies above 500 kHz because of signifi- cant ringing and temperature rise due to self-heating from ripple current. An output capacitor with excessive ESR can also reduce phase margin and cause instability. In general, if electrolytics are used, we recommended that they be paralleled with ceramic capacitors to reduce ringing, switching losses, and output voltage ripple. SELECTING INPUT CAPACITOR (Cs1) FOR BOOST CONVERTER An input capacitor is required to serve as an energy reservoir for the current which must flow into the coil each time the switch turns ON. This capacitor must have extremely low ESR, so ceramic is the best choice. We recommend a nominal value of 4.7µF, but larger values can be used. Since this capacitor reduces the amount of voltage ripple seen at the input pin, it also reduces the amount of EMI passed back along that line to other circuitry. SETTING THE OUTPUT VOLTAGE (V 1) OF BOOST CONVERTER The output voltage is set using the external resistors R1 and R2 (see Figure 1). A value of approximately 13.3k Ω is rec- ommended for R2 to establish a divider current of approxi- mately 92µA. R1 is calculated using the formula: R1=R2X(V 2/1.23 − 1) (5) FEED-FORWARD COMPENSATION FOR BOOST CONVERTER Although the LM4961’s internal Boost converter is internally compensated, the external feed-forward capacitor C f is re- quired for stability (see Figure 1). Adding this capacitor puts a zero in the loop response of the converter. The recom- mended frequency for the zero fz should be approximately 6kHz. C f1 can be calculated using the formula: C f1=1/(2XR1Xfz) (6) SELECTING DIODES The external diode used in Figure 1 should be a Schottky diode. A 20V diode such as the MBR0520 from Fairchild Semiconductor is recommended. The MBR05XX series of diodes are designed to handle a maximum average current of 0.5A. For applications exceed- ing 0.5A average but less than 1A, a Microsemi UPS5817 can be used. www.national.com 10 |
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