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LM4916 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM4916 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 18 page  Typical Performance Characteristics (Continued) Mute Attenuation vs Load Resistance Shutdown Current Distribution 200487F2 200487F7 Application Information SINGLE ENDED (SE) CONFIGURATION EXPLANATION As shown in Figure 2, the LM4916 has two operational amplifiers internally, which have externally configurable gain. The closed loop gain of the two configurable amplifiers is set by selecting the ratio of Rf to Ri. Consequently, the gain for each channel of the IC is A VD = -(Rf /Ri) When the LM4916 operates in Single Ended mode, coupling capacitors are used on each output (VoA and VoB) and the SE/BTL pin (Pin 8) is connected to ground. These output coupling capacitors blocks the half supply voltage to which the output amplifiers are typically biased and couples the audio signal to the headphones or other single-ended (SE) loads. The signal return to circuit ground is through the headphone jack’s sleeve. BRIDGED (BTL) CONFIGURATION EXPLANATION As shown in Figure 3, the LM4916 has two internal opera- tional amplifiers. The first amplifier’s gain is externally con- figurable, while the second amplifier should be externally fixed in a unity-gain, inverting configuration. The closed-loop gain of the first amplifier is set by selecting the ratio of R f to R iwhile the second amplifier’s gain should be fixed by the two external 20k Ω resistors. Figure 3 shows that the output of amplifier one serves as the input to amplifier two which results in both amplifiers producing signals identical in mag- nitude, but out of phase by 180˚. Consequently, the differen- tial gain for the IC is A VD = 2 *(Rf /Ri). By driving the load differentially through outputs Vo1 and Vo2, an amplifier configuration commonly referred to as "bridged mode" is established. Bridged mode operation is different from the classical single-ended amplifier configura- tion where one side of the load is connected to ground. A bridge amplifier design has a few distinct advantages over the single-ended configuration. It provides a differential drive to the load, thus doubling output swing for a specified supply voltage. Four times the output power is possible as com- pared 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. In order to choose an amplifier’s closed-loop gain without causing excessive clipping, please refer to the Audio Power Amplifier Design section. A bridge configuration, such as the one used in LM4916, also creates a second advantage over single-ended amplifi- ers. Since the differential outputs, Vo1 and Vo2, are biased at half-supply, no net DC voltage exists across the load. This eliminates the need for an output coupling capacitor which is required in a single supply, single-ended amplifier configura- tion. MODE SELECT DETAIL The LM4916 can be configured in either Single Ended or BTL mode (see Figure 2 and Figure 3). The default state of the LM4916 at power up is single ended. During initial power up or return from shutdown, the LM4916 must detect the correct mode of operation by sensing the status of the SE/BTL pin. When the bias voltage of the part ramps up to 60mV (as seen on the Bypass pin), an internal comparator detects the status of SE/BTL; and at 10mV, latches that value in place. Ramp up of the bias voltage will proceed at a different rate from this point on depending upon operating mode. BTL mode will ramp up about 11 times faster than Single Ended mode. Shutdown is not a valid command during this time period (T WU) and should not enabled to ensure a proper power on reset (POR) signal. In addition, the slew rate of V DD must be greater than 2.5V/ms to ensure reliable POR. Recommended power up timing is shown in Figure 5 along with proper usage of Shutdown and Mute. The mode-select circuit is suspended during C B discharge time. The circuit shown in Figure 4 presents an applications solution to the problem of using different supply voltages with different turn-on times in a system with the LM4916. This circuit shows the LM4916 with a 25-50k Ω. Pull-up re- sistor connected from the shutdown pin to V DD. The shut- www.national.com 11 |
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