The Industry's Most Efficient Class D Speaker Amplifier LSI

ROHM has recently completed development of the BD5421EFS Class D stereo amplifier LSI for audio-equipped sets with strict thermal design and dimensional requirements, such as liquid crystal and plasma televisions, minicomponent stereos, power speakers, and gaming devices. The efficiency of conventional analog amplifiers is often poor, approximately 50%, with half of the power being converted to heat. This requires large heat sink or cooling fans, making it difficult to reduce set thickness. ROHM's Class D stereo amplifier LSI utilizes cutting-edge BiCDMOS processes for minimal line resistance and output transistor ON-resistance, resulting in an industry-leading 90% efficiency. 34W (17W+17W) output is enabled using a small back-surface metal plate with excellent thermal dissipation characteristics, eliminating the need for a heat sink. As a result, sets can be made smaller and thinner and power consumption reduced dramatically. Additional features include low input conversion noise (5.6μVrms) – well below that of conventional Class D power amplifiers – for clear audio, as well as proprietary soft switching technology for a smooth signal waveform envelope when turning the mute ON and OFF. And PWM modulation sampling clock Master/Slave functionality eliminates beat noise generated due to clock differences during synchronous multiple amplifier operation, such as in 5.1-channel systems. (Sample price: 4.2$)

· Main Features of the BD5421EFS:

1. Industry-leading 90% efficiency (10W + 10W output at 8Ω load), low heat generation.
2. HTSSOP-A44 package features incorporates a small back-surface metal package enabling 34W (17W + 17W) output without the need for a heat sink.
3. Can drive standard loads up to 4Ω.
4. Reduces popping noises during power interruption when turning ON and OFF.
5. Soft-switch technology provides high-quality sound muting.
6. Beat noise resulting from differences in sampling clock frequency is eliminated using a synchronous Master/Slave function when multiple amplifiers are used simultaneously.
7. Gain adjustable via external resistance (20dB to 37dB).
8. Internal PWM sampling clock frequency can be adjusted (200 to 400kHz), reducing unwanted radiation in the AM radio band.
9. Enables configuration of a high-power 34W mono amplifier by connecting stereo outputs in parallel.

· Glossary:

• Class D speaker amplifier, PWM sampling clock:
  Class D speaker amplifiers, often referred to as digital speaker amplifiers, integrate a comparator that first compares the input audio signal with a triangular wave (generated by a PWM reference clock), then converts the input audio signal into a rectangular wave with a pulse width that changes with time. The output FETs are switched based on the PWM signal, and the speakers are driven through an external low-pass filter. Because the output stage operates by switching, amplification is carried out efficiently without loss using minimal power. This results in low heat generation. Class D amps are normally compact, making them a popular choice for flat panel televisions, low-cost audio devices, and mobile phones.
• Input conversion noise:
  The noise generated in an amplifier is almost entirely generated at the amplifier inputs. If this noise, referred to as input conversion noise, is equal to or greater than the signal input level the signal will be masked by noise itself, resulting in poor quality audio.
• 5.1-channel systems:
  Stereo systems reproduce sound using two speakers. With 5.1-channel systems, six speakers are utilized: front, front-right, front-left, back-right, back-left and a subwoofer for bass output. Since the subwoofer speaker has a restricted output range it only counts as 0.1 channel. A 5.1-channel system can be configured using three Class D stereo speaker amplifiers.
• Master/Slave function:
  Functions that control other systems or devices are referred to as Master functions, while functions that are controlled by other systems or devices are referred to as Slave functions.
• Beat noise:
  Beat noise refers to frequency noise resulting from interference between two signals with similar frequencies. In extreme cases, noise will be heard at the speakers.

· For reference

BD5421EFS: Output Power vs. Efficiency (at VCC = 12V, RL = 8Ω, Gain = 29.6dB, f = 1kHz)

	Class D Speaker Amplifier LSI