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LM4730TA Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM4730TA Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 17 page  Application Information MUTE MODE By placing a logic-high voltage on the mute pins, the signal going into the amplifiers will be muted. If the mute pins are left floating or connected to a logic-low voltage, the amplifi- ers will be in a non-muted state. There are two mute pins, one for each amplifier, so that one channel can be muted without muting the other if the application requires such a configuration. Refer to the Typical Performance Character- istics section for curves concerning Mute Attenuation vs Mute Pin Voltage. STANDBY MODE The standby mode of the LM4730 allows the user to drasti- cally reduce power consumption when the amplifiers are idle. By placing a logic-high voltage on the standby pins, the amplifiers will go into Standby Mode. In this mode, the current drawn from the V CC supply is typically less than 10 µA total for both amplifiers. The current drawn from the V EE supply is typically 3.5mA. Clearly, there is a significant re- duction in idle power consumption when using the standby mode. There are two Standby pins, so that one channel can be put in standby mode without putting the other amplifier in standby if the application requires such flexibility. Refer to the Typical Performance Characteristics section for curves showing Supply Current vs. Standby Pin Voltage for both supplies. UNDER-VOLTAGE PROTECTION Upon system power-up, the under-voltage protection cir- cuitry allows the power supplies and their corresponding capacitors to come up close to their full values before turning on the LM4730 such that no DC output spikes occur. Upon turn-off, the output of the LM4730 is brought to ground before the power supplies such that no transients occur at power-down. OVER-VOLTAGE PROTECTION The LM4730 contains over-voltage protection circuitry that limits the output current while also providing voltage clamp- ing, though not through internal clamping diodes. The clamp- ing effect is quite the same, however, the output transistors are designed to work alternately by sinking large current spikes. THERMAL PROTECTION The LM4730 has a sophisticated thermal protection scheme to prevent long-term thermal stress of the device. When the temperature on the die exceeds 150˚C, the LM4730 shuts down. It starts operating again when the die temperature drops to about 145˚C, but if the temperature again begins to rise, shutdown will occur again above 150˚C. Therefore, the device is allowed to heat up to a relatively high temperature if the fault condition is temporary, but a sustained fault will cause the device to cycle in a Schmitt Trigger fashion be- tween the thermal shutdown temperature limits of 150˚C and 145˚C. This greatly reduces the stress imposed on the IC by thermal cycling, which in turn improves its reliability under sustained fault conditions. Since the die temperature is directly dependent upon the heat sink used, the heat sink should be chosen such that thermal shutdown will not be reached during normal opera- tion. Using the best heat sink possible within the cost and space constraints of the system will improve the long-term reliability of any power semiconductor device, as discussed in the Determining the Correct Heat Sink Section. DETERMlNlNG MAXIMUM POWER DISSIPATION Power dissipation within the integrated circuit package is a very important parameter requiring a thorough understand- ing if optimum power output is to be obtained. An incorrect maximum power dissipation calculation may result in inad- equate heat sinking causing thermal shutdown and thus limiting the output power. Equation (1) exemplifies the theoretical maximum power dissipation point of each amplifier where V CC is the total supply voltage. P DMAX =VCC2/2 π2R L (1) Thus by knowing the total supply voltage and rated output load, the maximum power dissipation point can be calcu- lated. The package dissipation is twice the number which results from equation (1) since there are two amplifiers in each LM4730. Refer to the graphs of Power Dissipation versus Output Power in the Typical Performance Charac- teristics section which show the actual full range of power dissipation not just the maximum theoretical point that re- sults from equation (1). DETERMINING THE CORRECT HEAT SINK The choice of a heat sink for a high-power audio amplifier is made entirely to keep the die temperature at a level such that the thermal protection circuitry does not operate under normal circumstances. The thermal resistance from the die (junction) to the outside air (ambient) is a combination of three thermal resistances, θ JC, θ CS, and θ SA. In addition, the thermal resistance, θ JC (junction to case), of the LM4730TA is 1.5˚C/W. Using Ther- malloy Thermacote thermal compound, the thermal resis- tance, θ CS (case to sink), is about 0.2˚C/W. Since convection heat flow (power dissipation) is analogous to current flow, thermal resistance is analogous to electrical resistance, and temperature drops are analogous to voltage drops, the power dissipation out of the LM4730 is equal to the following: P DMAX =(TJMAX−TAMB)/ θ JA (2) where T JMAX = 150˚C, TAMB is the system ambient tempera- ture and θ JA = θ JC + θ CS + θ SA. Once the maximum package power dissipation has been calculated using equation (1), the maximum thermal resis- tance, θ SA, (heat sink to ambient) in ˚C/W for a heat sink can be calculated. This calculation is made using equation (3) which is derived by solving for θ SA in equation (2). θ SA = [(TJMAX−TAMB)−PDMAX( θ JC + θ CS)]/PDMAX (3) Again it must be noted that the value of θ SA is dependent upon the system designer’s amplifier requirements. If the ambient temperature that the audio amplifier is to be working under is higher than 25˚C, then the thermal resistance for the heat sink, given all other things are equal, will need to be smaller. SUPPLY BYPASSING The LM4730 has excellent power supply rejection and does not require a regulated supply. However, to improve system performance as well as eliminate possible oscillations, the LM4730 should have its supply leads bypassed with low- inductance capacitors having short leads that are located close to the package terminals. Inadequate power supply bypassing will manifest itself by a low frequency oscillation known as “motorboating” or by high frequency instabilities. www.national.com 11 |
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