Motor de Búsqueda de Datasheet de Componentes Electrónicos |
|
LM4817 Datasheet(PDF) 14 Page - National Semiconductor (TI) |
|
|
LM4817 Datasheet(HTML) 14 Page - National Semiconductor (TI) |
14 / 21 page Application Information (Continued) SELECTING PROPER EXTERNAL COMPONENTS Optimizing the LM4817’s performance requires properly se- lecting external components. Though the LM4817 operates well when using external components with wide tolerances, best performance is achieved by optimizing component val- ues. The LM4817 is unity-gain stable, giving a designer maximum design flexibility. The gain should be set to no more than a given application requires. This allows the amplifier to achieve minimum THD+N and maximum signal-to-noise ra- tio. These parameters are compromised as the closed-loop gain increases. However, low gain demands input signals with greater voltage swings to achieve maximum output power. Fortunately, many signal sources such as audio CO- DECs have outputs of 1V RMS (2.83VP-P). Please refer to the Audio Power Amplifier Design section for more informa- tion on selecting the proper gain. Input Capacitor Value Selection Amplifying the lowest audio frequencies requires high value input coupling capacitor (C i in Figure 1). A high value capaci- tor can be expensive and may compromise space efficiency in portable designs. In many cases, however, the speakers used in portable systems, whether internal or external, have little ability to reproduce signals below 150Hz. Applications using speakers with this limited frequency response reap little improvement by using large input capacitor. Besides effecting system cost and size, C i has an affect on the LM4817’s click and pop performance. When the supply voltage is first applied, a transient (pop) is created as the charge on the input capacitor changes from zero to a quies- cent state. The magnitude of the pop is directly proportional to the input capacitor’s size. Higher value capacitors need more time to reach a quiescent DC voltage (usually V DD/2) when charged with a fixed current. The amplifier’s output charges the input capacitor through the feedback resistor, R f. Thus, pops can be minimized by selecting an input capacitor value that is no higher than necessary to meet the desired -3dB frequency. A shown in Figure 1, the input resistor (R I) and the input capacitor, C I produce a −3dB high pass filter cutoff frequency that is found using Equation (7). (7) As an example when using a speaker with a low frequency limit of 150Hz, C I, using Equation (4), is 0.063µF. The 1.0µF C I shown in Figure 1 allows the LM4817 to drive high effi- ciency, full range speaker whose response extends below 30Hz. Bypass Capacitor Value Selection Besides minimizing the input capacitor size, careful consid- eration should be paid to value of C B, the capacitor con- nected to the BYPASS pin. Since C B determines how fast the LM4817 settles to quiescent operation, its value is critical when minimizing turn−on pops. The slower the LM4817’s outputs ramp to their quiescent DC voltage (nominally 1/2 V DD), the smaller the turn−on pop. Choosing CB equal to 1.0µF along with a small value of C i (in the range of 0.1µF to 0.39µF), produces a click-less and pop-less shutdown func- tion. As discussed above, choosing C i no larger than neces- sary for the desired bandwidth helps minimize clicks and pops. OPTIMIZING CLICK AND POP REDUCTION PERFORMANCE The LM4817 contains circuitry to minimize turn-on and shut- down transients or "clicks and pop". For this discussion, turn-on refers to either applying the power supply voltage or when the shutdown mode is deactivated. While the power supply is ramping to its final value, the LM4817’s internal amplifiers are configured as unity gain buffers. An internal current source changes the voltage of the BYPASS pin in a controlled, linear manner. Ideally, the input and outputs track the voltage applied to the BYPASS pin. The gain of the internal amplifiers remains unity until the voltage on the bypass pin reaches 1/2 V DD. As soon as the voltage on the BYPASS pin is stable, the device becomes fully operational. Although the bypass pin current cannot be modified, chang- ing the size of C B alters the device’s turn-on time and the magnitude of "clicks and pops". Increasing the value of C B reduces the magnitude of turn-on pops. However, this pre- sents a tradeoff: as the size of C B increases, the turn-on time increases. There is a linear relationship between the size of C B and the turn-on time. Here are some typical turn-on times for various values of C B: C B T ON 0.01µF 20 ms 0.1µF 200 ms 0.22µF 440 ms 0.47µF 940 ms 1.0µF 2 Sec In order eliminate "clicks and pops", all capacitors must be discharged before turn-on. Rapidly switching V DD may not allow the capacitors to fully discharge, which may cause "clicks and pops". NO LOAD STABILITY The LM4817 may exhibit low level oscillation when the load resistance is greater than 10k Ω. This oscillation only occurs as the output signal swings near the supply voltages. Pre- vent this oscillation by connecting a 5k Ω between the output pins and ground. www.national.com 14 |
Número de pieza similar - LM4817 |
|
Descripción similar - LM4817 |
|
|
Enlace URL |
Política de Privacidad |
ALLDATASHEET.ES |
¿ALLDATASHEET es útil para Ud.? [ DONATE ] |
Todo acerca de Alldatasheet | Publicidad | Contáctenos | Política de Privacidad | Intercambio de Enlaces | Lista de Fabricantes All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |