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LM4668 bảng dữ liệu(PDF) 8 Page - National Semiconductor (TI) |
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LM4668 bảng dữ liệu(HTML) 8 Page - National Semiconductor (TI) |
8 / 14 page General Features SYSTEM FUNCTIONAL INFORMATION Modulation Technique Unlike typical Class D amplifiers that use single-ended com- parators to generate a pulse-width modulated switching waveform and RC timing circuits to set the switching fre- quency, the LM4668 uses a balanced differential floating modulator. Oscillation is a result of injecting complimentary currents onto the respective plates of a floating, on-die ca- pacitor. The value of the floating capacitor and value of the components in the modulator’s feedback network and sets the nominal switching frequency at 450kHz. Modulation re- sults from imbalances in the injected currents. The amount of current imbalance is directly proportional to the applied input signal’s magnitude and frequency. Using a balanced, floating modulator produces a Class D amplifier that is immune to common mode noise sources such as substrate noise. This noise occurs because of the high frequency, high current switching in the amplifier’s out- put stage. The LM4668 is immune to this type of noise because the modulator, the components that set its switch- ing frequency, and even the load all float with respect to ground. The balanced modulator’s pulse width modulated output drives the gates of the LM4668’s H-bridge configured output power MOSFETs. The pulse-train present at the power MOSFETs’ output is applied to an LC low pass filter that removes the 450kHz energy component. The filter’s output signal, which is applied to the driven load, is an amplified replica of the audio input signal. Shutdown Function The LM4668’s active-low shutdown function allows the user to place the amplifier in a shutdown mode while the system power supply remains active. Activating shutdown deacti- vates the output switching waveform and minimizes the quiescent current. Applying logic 0 (GND) to pin 8 enables the shutdown function. Applying logic 1 (4V ≤ V LOGIC ≤ V DD) to pin 8 disables the shutdown function and restores full amplifier operation. Under Voltage Protection The under voltage protection disables the output driver sec- tion of the LM4668 while the supply voltage is below 8V. This condition may occur as power is first applied or during low line conditions, changes in load resistance, or when power supply sag occurs. The under voltage protection ensures that all of the LM4668’s power MOSFETs are off. This action eliminates shoot-through current and minimizes output tran- sients during turn-on and turn-off. The under voltage protec- tion gives the digital logic time to stabilize into known states, further minimizing turn output transients. Turn-On Time The LM4668 has an internal timer that determines the am- plifier’s turn-on time. After power is first applied or the part returns from shutdown, the nominal turn-on time is 600ms. This delay allows all externally applied capacitors to charge to a final value of V DD/2. Further, during turn-on, the outputs are muted. This minimizes output transients that may occur while the part settles into is quiescent operating mode. Output Stage Fault Detection And Protection The output stage MOSFETs are protected against output conditions that could otherwise compromise their operational status. An onboard fault detection circuit continuously moni- tors the signal on each output MOSFET’s gate and com- pares it against the respective drain voltage. When a condi- tion is detected that violates a MOSFET’s Safe Operating Area (SOA), the drive signal is disconnected from the output MOSFETs’ gates. The fault detect circuit maintains this pro- tective condition for approximately 600ms, at which time the drive signal is reconnected. If the fault condition is no longer present, normal operation resumes. If the fault condition remains, however, the drive signal is again disconnected. Thermal Protection The LM4668 has thermal shutdown circuitry that monitors the die temperature. Once the LM4668 die temperature reaches 170˚C, the LM4668 disables the output switching waveform and remains disabled until the die temperature falls below 140˚C (typ). Over-Modulation Protection The LM4668’s over-modulation protection is a result of the preamplifier’s (AMP1 and AMP2, Figure 1) inability to pro- duce signal magnitudes that equal the power supply volt- ages. Since the preamplifier’s output magnitude will always be less than the supply voltage, the duty cycle of the ampli- fier’s switching output will never reach zero. Peak modula- tion is limited to a nominal 95%. Application Hints SUPPLY BYPASSING Correct power supply bypassing has two important goals. The first is to reduce noise on the power supply lines and minimize deleterious effects that the noise may cause to the amplifier’s operation. The second is to help stabilize an unregulated power supply and to improve the supply’s tran- sient response under heavy current demands. These two goals require different capacitor value ranges. Therefore, various types and values are recommended for supply by- passing. For noise de-coupling, generally small ceramic ca- pacitors (0.01µF to 0.1µF) are recommended. Larger value (1µF to 10µF) tantalum capacitors are needed for the tran- sient current demands. These two capacitors in parallel will do an adequate job of removing most noise from the supply rails and providing the necessary transient current. These capacitors should be placed as close as possible to each IC’s supply pin(s) using leads as short as possible. The LM4668 has two V DD pins: a power VDD (PVDD) and a signal V DD (SVDD). The parallel combination of the low value ceramic (0.1µF) and high value tantalum (10µF) should be used to bypass the PV DD pin. A small value (0.1µF) ceramic or tantalum can be used to bypass the SV DD pin. AMPLIFIER OUTPUT FILTERING The LM4668 requires a lowpass filter connected between the amplifier’s bridge output and the load. The second-order LC output filter shown in Figure 1 creates the lowpass re- sponse that is necessary to attenuate signal energy at the amplifier’s switching frequency. It also serves to suppress EMI. Together, the output filter’s 0.27µF capacitors and the recommended minimum inductor value of 27µH produce a www.national.com 8 |
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