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LM4731TA Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM4731TA Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 16 page Application Information (Continued) These instabilities can be eliminated through multiple by- passing utilizing a large tantalum or electrolytic capacitor (10 µF or larger) which is used to absorb low frequency varia- tions and a small ceramic capacitor (0.1 µF) to prevent any high frequency feedback through the power supply lines. If adequate bypassing is not provided, the current in the supply leads which is a rectified component of the load current may be fed back into internal circuitry. This signal causes distortion at high frequencies requiring that the sup- plies be bypassed at the package terminals with an electro- lytic capacitor of 470 µF or more. BRIDGED AMPLIFIER APPLICATION The LM4731 has two operational amplifiers internally, allow- ing for a few different amplifier configurations. One of these configurations is referred to as “bridged mode” and involves driving the load differentially through the LM4731’s outputs. This configuration is shown in Figure 2. Bridged mode op- eration is different from the classical single-ended amplifier configuration where one side of its load is connected to ground. A bridge amplifier design has a distinct advantage over the single-ended configuration, as it provides differential drive to the load, thus doubling output swing for a specified supply voltage. Consequently, theoretically four times the output power is possible as compared to a single-ended amplifier under the same conditions. This increase in attainable output power assumes that the amplifier is not current limited or clipped. A direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in internal power dissipation. For each operational amplifier in a bridge con- figuration, the internal power dissipation will increase by a factor of two over the single ended dissipation. Thus, for an audio power amplifier such as the LM4731, which has two operational amplifiers in one package, the package dissipa- tion will increase by a factor of four. To calculate the LM4731’s maximum power dissipation point for a bridged load, multiply equation (1) by a factor of four. This value of P DMAX can be used to calculate the correct size heat sink for a bridged amplifier application. Since the inter- nal dissipation for a given power supply and load is in- creased by using bridged-mode, the heatsink’s θ SA will have to decrease accordingly as shown by equation (3). Refer to the section, Determining the Correct Heat Sink, for a more detailed discussion of proper heat sinking for a given appli- cation. SINGLE-SUPPLY AMPLIFIER APPLICATION The typical application of the LM4731 is a split supply am- plifier. But as shown in Figure 3, the LM4731 can also be used in a single power supply configuration. This involves using some external components to create a half-supply bias which is used as the reference for the inputs and outputs. Thus, the signal will swing around half-supply much like it swings around ground in a split-supply application. Along with proper circuit biasing, a few other considerations must be accounted for to take advantage of all of the LM4731 functions. The LM4731 possesses a mute and standby function with internal logic gates that are half-supply referenced. Thus, to enable either the Mute or Standby function, the voltage at these pins must be a minimum of 2.5V above half-supply. In single-supply systems, devices such as microprocessors and simple logic circuits used to control the mute and standby functions, are usually referenced to ground, not half-supply. Thus, to use these devices to control the logic circuitry of the LM4731, a “level shifter,” like the one shown in Figure 5, must be employed. A level shifter is not needed in a split-supply configuration since ground is also half-supply. When the voltage at the Logic Input node is 0V, the 2N3904 is “off” and thus resistor R c pulls up mute or standby input to the supply. This enables the mute or standby function. When the Logic Input is 5V, the 2N3904 is “on” and consequently, the voltage at the collector is essentially 0V. This will disable the mute or standby function, and thus the amplifier will be in its normal mode of operation. R shift, along with Cshift, creates an RC time constant that reduces transients when the mute or standby functions are enabled or disabled. Additionally, R shift limits the current supplied by the internal logic gates of the LM4731 which insures device reliability. Refer to the Mute Mode and Standby Mode sections in the Application Information section for a more detailed description of these functions. CLICKS AND POPS In the typical application of the LM4731 as a split-supply audio power amplifier, the IC exhibits excellent “click” and “pop” performance when utilizing the mute and standby modes. In addition, the device employs Under-Voltage Pro- tection, which eliminates unwanted power-up and power- down transients. The basis for these functions are a stable and constant half-supply potential. In a split-supply applica- tion, ground is the stable half-supply potential. But in a single-supply application, the half-supply needs to charge up just like the supply rail, V CC. This makes the task of attaining a clickless and popless turn-on more challenging. Any un- even charging of the amplifier inputs will result in output clicks and pops due to the differential input topology of the LM4731. 20060354 FIGURE 5. Level Shift Circuit www.national.com 11 |
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