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LM4962 bảng dữ liệu(PDF) 10 Page - National Semiconductor (TI) |
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LM4962 bảng dữ liệu(HTML) 10 Page - National Semiconductor (TI) |
10 / 22 page Application Information BRIDGE CONFIGURATION EXPLANATION The Audio Amplifier portion of the LM4962 has two internal amplifiers allowing different amplifier configurations. The first amplifier’s gain is externally configurable, whereas the sec- ond amplifier is internally 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 internal 20k Ω resistors. Figure 1 shows that the output of amplifier one serves as the input to amplifier two. This results in both amplifiers producing sig- nals identical in magnitude, but out of phase by 180˚. Con- sequently, the differential gain for the Audio Amplifier 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 classic 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 differential drive to the load, thus doubling the output swing for a specified supply voltage. AMPLIFIER POWER DISSIPATION Power dissipation is a major concern when designing a successful amplifier, whether the amplifier is bridged or single-ended. A direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in internal power dissipation. Since the amplifier portion of the LM4962 has two operational amplifiers, the maximum inter- nal power dissipation is 4 times that of a single-ended am- plifier. The maximum power dissipation for a given BTL application can be derived from Equation 1. P DMAX(AMP) = 4(VDD) 2 /(2 π2Z L) (1) where Z L =Ro1+Ro2 +1/2 πfc BOOST CONVERTER POWER DISSIPATION At higher duty cycles, the increased ON-time of the switch FET means the maximum output current will be determined by power dissipation within the LM4962 FET switch. The switch power dissipation from ON-time conduction is calcu- lated by Equation 2. P DMAX(SWITCH) =DCxIIND(AVE) 2 xR DS(ON) (2) where DC is the duty cycle. There will be some switching losses as well, so some derat- ing needs to be applied when calculating IC power dissipa- tion. TOTAL POWER DISSIPATION The total power dissipation for the LM4962 can be calculated by adding Equation 1 and Equation 2 together to establish Equation 3: P DMAX(TOTAL) = [4*(VDD) 2/ 2 π2Z L]+[DCxIIND(AVE) 2xR DS(ON)] (3) The result from Equation 3 must not be greater than the power dissipation that results from Equation 4: P DMAX =(TJMAX -TA)/ θ JA (4) For the LQA28A, θ JA = 73˚C/W. TJMAX = 125˚C for the LM4962. Depending on the ambient temperature, T A,ofthe system surroundings, Equation 4 can be used to find the maximum internal power dissipation supported by the IC packaging. If the result of Equation 3 is greater than that of Equation 4, then either the supply voltage must be in- creased, the load impedance increased or T A reduced. For typical applications, power dissipation is not an issue. Power dissipation is a function of output power and thus, if typical operation is not around the maximum power dissipation point, the ambient temperature may be increased accord- ingly. START-UP SEQUENCE For the LM4962 correct start-up sequencing is important for optimal device performance. Using the correct start up se- quence will improve click/pop performance as well as avoid transients that could reduce battery life. For ringer/ loudspeaker mode, the supply voltage should be applied first and both the boost converter and the amplifier should be in shutdown. The boost converter can then be activated fol- lowed by the amplifier (see timing diagram Figure 3). If the boost converter shutdown is toggled while the amplifier is active a very audible pop will be heard. SHUTDOWN FUNCTION In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry to provide a quick, smooth transition into shutdown. Another solution is to use a single-pole, single-throw switch connected between V DD and Shutdown pins. BAND SWITCH FUNCTION The LM4962 features a Band Switch function which allows the user to use one amplifier for both receiver (earpiece) mode and ringer/loudspeaker mode. When the boost con- verter and the amplifier are both active the device is is in ringer mode. This enables the boost converter and sets the externally configurable closed loop gain selection to BW1. If the boost converter is in the shutdown and the amplifier is active the device is in receiver mode. In this mode the gain selection is switched to BW2. This allows the amplifier to be powered directly from the battery minus the voltage drop across the Schottky diode. SD Boost SD Amp Receiver Mode (BW2) Low High Boosted Ringer Mode (BW1) High High Shutdown Low Low BOOTSTRAP PIN The bootstrap pin, featured in the LM4962, provides a volt- age supply for the internal switch driver. Connecting the bootstrap pin to V1 (See Figure 1) allows for a higher voltage to drive the gate of the switch thereby reducing the Ron. This configuration is necessary in applications with heavier loads. The bootstrap pin can be connected to VDD when driving lighter loads to improve device performance (Iddq, THD+N, Noise, etc.). www.national.com 10 |
Số phần tương tự - LM4962 |
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Mô tả tương tự - LM4962 |
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