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SI8241 데이터시트(PDF) 2 Page - Silicon Laboratories |
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SI8241 데이터시트(HTML) 2 Page - Silicon Laboratories |
2 / 12 page AN542 2 Rev. 0.1 2. The Si8241 Audio Gate Driver Every so often, a new IC is introduced that challenges the current technological hegemony. With features that make these products the perfect drivers for Class D amplification, the Silicon Labs Si8241/44 Audio Gate Drivers represent a new standard for the Class D amplifier industry. Key features are outlined in the following sections. 2.1. Programmable Dead Time It is well documented that a precise dead time setting is critical in Class D amplifiers. During dead time, both the high-side and low-side MOSFETs are off. However, the low-side MOSFET body diode continues to conduct current, which manifests itself as output distortion. Too short a dead time causes shoot-through current that reduces system efficiency; too long a dead time increases THD, negatively impacting audio quality. While competing audio drivers typically have coarse, digital dead-time settings (i.e. 1 of n delay values), the Si8241/44 Audio Gate Drivers have a precise linear dead time setting that programs with a single external resistor. This feature provides the resolution necessary to precisely set dead time for optimal system performance. The Audio Gate Driver dead time equation is shown in Equation 1. Equation 1. Audio Gate Driver Dead Time Per Equation 1, the Silicon Labs Class D amplifier uses a 2 k resistor to generate 20 ns of dead time. Changing this dead time value to 18 ns only requires changing the RDT to 1.8 k (connected from the dead-time pin (DT) input to ground). This setting mechanism allows dead time to be incrementally increased or decreased in nanosecond increments, instead of tens of nanoseconds like competitive products. 2.2. Input/Output Isolation Implementing a two-state, Class D amplifier can be difficult due to input level shifting requirements, and most available Class D drivers lack the capability to eliminate level shifting. Drivers that do eliminate level shifting have other peculiarities making them less-than-ideal for Class D operation (example: driver output ground terminal referenced to the –VBUS rail, requiring the input drive signal to be level-shifted). This is not the case with the Si8241 Audio Gate Driver where the isolation (i.e. level shift function) is implemented internally and is transparent to the user. The Si8241 Audio Gate Driver controlled by TTL input signal levels drives the outputs to ±VBUS, and only a single TTL PWM input signal is required to drive a two-state Class D amplifier. 2.3. High-Voltage Outputs The Si8241 is capable of switching very high voltages (up to a 1,500 Vdc peak driver-to-driver differential voltage is possible) allowing a ±750 VBUS. For practical Class D amplifier designs, a voltage of ±100 Vdc can deliver an astounding 600 W of audio power into 8 . 2.4. Output Current Drive Class D amplifier switching MOSFETs should not be "slammed" on and off by excessively high current gate drivers. With its 0.5 A peak current outputs, the Si8241 Audio Gate Driver hits the sweet spot for Class D operation up to 400 W. Power levels beyond 400 W typically require larger MOSFETs and, consequently, more gate drive. For applications of this type, the Si8244 (4A) Audio Gate Driver provides the required added gate drive, where rise and fall times can be adjusted with a series gate resistor. 2.5. High-Frequency Operation One of the best attributes of the Si8241 Audio Gate Driver is its 8 MHz maximum switching frequency, making it the fastest driver on the market for Class D operation. The Silicon Labs Class D reference design operates at approximately 500 kHz, and operating the amplifier between 500 kHz and 1 MHz dramatically reduces the high- frequency artifacts, resulting in a remarkably clean audio waveform. DT 10R DT where Dead Time (DT) is in ns and R DT is in k = |
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