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LM4925 Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM4925 Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 15 page Application Information BRIDGE (BTL) CONFIGURATION EXPLANATION The LM4925 is a stereo audio power amplifier capable of operating in bridged (BTL) mode. As shown in Figure 3, the LM4925 has two internal operational amplifiers. The first amplifier’s gain is externally configurable, 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 Rf to Ri while the second amplifier’s gain is 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 sig- nals identical in magnitude, but out of phase by 180˚. Con- sequently, the differential gain for the IC is A VD =2*(Rf /Ri) By driving the load differentially through outputs VoA and VoB, 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, as it provides differential drive to the load, thus doubling output swing for a specified supply voltage. Four times the output power is possible as compared to a single-ended amplifier under the same con- ditions. 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 ex- cessive clipping, please refer to the Audio Power Amplifier Design section. A bridge configuration, such as the one used in LM4925, also creates a second advantage over single-ended amplifi- ers. Since the differential outputs, VoA and VoB, 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 LM4925 can be configured for either single ended (see Figure2)orBTL mode (see Figure 3). When the SE/BTL pin has a logic high (V DD) applied to it, the LM4925 is in BTL mode. If a logic low (GND) is applied to SE/BTL, the LM4925 operates in single-ended mode. The slew rate of V DD must be greater than 2.5V/ms to ensure reliable Power on reset (POR). The circuit shown in Figure 4 presents an applica- tions solution to the problem of using different supply volt- ages with different turn-on times in a system with the LM4925. This circuit shows the LM4925 with a 25-50k Ω. Pull-up resistor connected from the shutdown pin to V DD. The shutdown pin of the LM4925 is also being driven by an open drain output of an external microcontroller on a sepa- rate supply. This circuit ensures that shutdown is disabled when powering up the LM4925 by either allowing shutdown to be high before the LM4925 powers on (the microcontroller powers up first) or allows shutdown to ramp up with V DD (the LM4925 powers up first). This will ensure the LM4925 pow- ers up properly and enters the correct mode of operation (BTL or SE). Please note that the SE/BTL pin should be tied to GND for single-ended (SE) mode, and to Vdd for BTL mode. 20121161 FIGURE 4. Recommended Circuit for Different Supply Turn-On Timing www.national.com 10 |
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