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LM4940 Scheda tecnica(PDF) 11 Page - Texas Instruments |
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LM4940 Scheda tecnica(HTML) 11 Page - Texas Instruments |
11 / 23 page LM4940 www.ti.com SNAS219B – MAY 2004 – REVISED MARCH 2011 TA = TJMAX - PDMAX-SEθJA (3) For a typical application with a 12V power supply and two 4 Ω SE loads, the maximum ambient temperature that allows maximum stereo power dissipation without exceeding the maximum junction temperature is approximately 113°C for the KTW package. TJMAX = PDMAX-SEθJA + TA (4) Equation (6) gives the maximum junction temperature TJMAX. If the result violates the LM4940's 150°C, reduce the maximum junction temperature by reducing the power supply voltage or increasing the load resistance. Further allowance should be made for increased ambient temperatures. The above examples assume that a device is operating around the maximum power dissipation point. Since internal power dissipation is a function of output power, higher ambient temperatures are allowed as output power or duty cycle decreases. If the result of Equation (3) is greater than that of Equation (4), then decrease the supply voltage, increase the load impedance, or reduce the ambient temperature. Further, ensure that speakers rated at a nominal 4 Ω do not fall below 3 Ω. If these measures are insufficient, a heat sink can be added to reduce θJA. The heat sink can be created using additional copper area around the package, with connections to the ground pins, supply pin and amplifier output pins. Refer to the TYPICAL PERFORMANCE CHARACTERISTICS curves for power dissipation information at lower output power levels. POWER SUPPLY VOLTAGE LIMITS Continuous proper operation is ensured by never exceeding the voltage applied to any pin, with respect to ground, as listed in the Absolute Maximum Ratings section. POWER SUPPLY BYPASSING As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply rejection. Applications that employ a voltage regulator typically use a 10µF in parallel with a 0.1µF filter capacitors to stabilize the regulator's output, reduce noise on the supply line, and improve the supply's transient response. However, their presence does not eliminate the need for a local 1.0µF tantalum bypass capacitance connected between the LM4940's supply pins and ground. Do not substitute a ceramic capacitor for the tantalum. Doing so may cause oscillation. Keep the length of leads and traces that connect capacitors between the LM4940's power supply pin and ground as short as possible. Connecting a 10µF capacitor, CBYPASS, between the BYPASS pin and ground improves the internal bias voltage's stability and improves the amplifier's PSRR. The PSRR improvements increase as the bypass pin capacitor value increases. Too large, however, increases turn-on time and can compromise the amplifier's click and pop performance. The selection of bypass capacitor values, especially CBYPASS, depends on desired PSRR requirements, click and pop performance (as explained in the section, SELECTING EXTERNAL COMPONENTS), system cost, and size constraints. MICRO-POWER SHUTDOWN The LM4940 features an active-low shutdown mode that disables the amplifier's bias circuitry, reducing the supply current to 40 μA (typ). Connect SHUTDOWN to a voltage between 2V to VDD/2 for normal operation. Connect SHUTDOWN to GND to disable the device. A voltage that is greater than GND can increase shutdown current. Copyright © 2004–2011, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM4940 |
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