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LM4961 Folha de dados(PDF) 8 Page - Texas Instruments |
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LM4961 Folha de dados(HTML) 8 Page - Texas Instruments |
8 / 24 page LM4961, LM4961LQBD SNAS242K – AUGUST 2004 – REVISED MAY 2013 www.ti.com APPLICATION INFORMATION BRIDGE CONFIGURATION EXPLANATION The Audio Amplifier portion of the LM4961 has two internal amplifiers allowing different amplifier configurations. The first amplifier’s gain is externally configurable, whereas the second 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 2 shows that the output of amplifier one serves as the input to amplifier two. This results in both amplifiers producing signals identical in magnitude, but out of phase by 180°. Consequently, the differential gain for the Audio Amplifier is AVD = 2 *(Rf/Ri) (1) 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 configuration 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. Four times the output power is possible as compared to a single-ended amplifier under the same conditions. The bridge configuration also creates a second advantage over single-ended amplifiers. 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 configuration. Without an output coupling capacitor, the half-supply bias across the load would result in both increased internal IC power dissipation and also possible loudspeaker damage. 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 LM4961 FET switch. The switch power dissipation from ON-time conduction is calculated by Equation 3. PD(SWITCH) = DC x IIND(AVE) 2 x R DS(ON) (2) where DC is the duty cycle. There will be some switching losses as well, so some derating needs to be applied when calculating IC power dissipation. MAXIMUM 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 LM4961 has two operational amplifiers, the maximum internal power dissipation is 4 times that of a single-ended amplifier. The maximum power dissipation for a given BTL application can be derived from Equation 2. PDMAX(AMP) = (2VDD 2) / (π2R L) (3) where RL = Ro1 + Ro2 (4) MAXIMUM TOTAL POWER DISSIPATION The total power dissipation for the LM4961 can be calculated by adding Equation 2 and Equation 3 together to establish Equation 5: PDMAX(TOTAL) = (2VDD 2) / (π2EFF2R L) (5) where EFF = Efficiency of boost converter RL = Ro1 + Ro2 The result from Equation 5 must not be greater than the power dissipation that results from Equation 6: PDMAX = (TJMAX - TA) / θJA (6) 8 Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated Product Folder Links: LM4961 LM4961LQBD |
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