Class D power amplifier control IC chip LX1710/LX1711

    Abstract: LX1710/LX1711 is a digital power amplifier controller produced by Microsemi. It can be used with an external power switch to form a complete digital power amplifier. It is a control chip with Hi-Fi characteristics in medium-power digital power amplifiers. The article introduces its main electrical characteristics and external circuits, and gives the application circuit and actual indicators of the digital power amplifier evaluation board composed of it and Fairchild's MOSFET FDS4953 and FDS6612A.

    Keywords: medium power class D power amplifier digital power amplifier control IC LX1710 LX1711 FDS4953 FDS6612A

1 Overview

Class D digital power amplifier is a new chip that has appeared in the computer multimedia field in recent years. As this type of technology matures, it has begun to expand into the traditional audio field. Simply put, a digital power amplifier is just like a switching power supply, using switching signals to control the output of audio amplification. In digital power, the analog audio signal or digital audio signal is first converted into a PWM pulse width modulation encoded audio signal, and the PWM code controls the driver to control the switching of the high-power switching transistor. Then the output high-power PWM signal is filtered by the LPF low-energy filter and the analog audio signal obtained is used to drive the speaker load.

The All-IN-ONE chip, which integrates the PWM modulation and control part with the power components on one chip, can only be suitable for smaller power amplifiers. And if the control part is integrated separately, it is equivalent to using medium and high power amplification, so there is no need to install a heat sink, which makes the design of the whole machine more convenient. For high-power digital power amplifiers above 200W, this structure has a very obvious effect of reducing the size. Therefore, more and more digital power amplifiers with a few watts to tens of watts are beginning to use independent control part solutions. Another major advantage of this solution is that it has a wide selection of high-power MOSFETs, which is beneficial to improving audio quality and controlling costs, and is therefore suitable for different levels of Hi-Fi equipment. LX1710/LX1711 is one of the few digital power amplifier control chips with Hi-Fi characteristics.

2 Main electrical characteristics of LX1710/LX1711

LX1710/LX1711 adopts a 28-pin flat package, and its main parameters and characteristics are listed in Table 1. The difference between them is that the former has a maximum withstand voltage of 25V, while the latter is 30V. In addition, the maximum power and other corresponding current parameters are also different. However, their usage is exactly the same and can be directly replaced.

Table 1 Main electrical characteristics of LX1710/LX1711

3 Structural principles and application circuit

Figure 1 is the internal main structure and peripheral circuit wiring diagram of LX1710/LX1711. In this circuit, LX1710 is used as a control chip. When carrying a 2Ω load, it can obtain a maximum output power of 38W, and its efficiency can reach 84%. In addition, the heat dissipation of MOSFET tubes can only rely on the printed board, without the need for additional heat sinks.

The chip's PWM output pulses are generated in the ordinary way by three dewaveders and comparators. Because the external power component is composed of an H-shaped switching circuit, the feedback signal can be obtained in a differential manner. Because its analog inputs are also differential, common-mode noise is suppressed.

The chip contains a mute function that cuts off input signals and a sleep function that consumes 25μA. It also has an over-current protection function, which can protect the external power MOSFET from being burned when the load is short-circuited.

The peripheral components of the controlled class D power amplifier chip are much more than those of the monolithic integrated D power amplifier IC. Among them, the most important components are two pairs of switching power MOSFET tubes, and these two tubes are required to be driven with a small amount to obtain larger energy, and at the same time, they should maintain a small on-resistance when the energy is small. The ones selected in the picture are the low turn-on voltage power MOSFETs FDS4853 (N-channel, paired tube) and FDS6612 (P-channel, single tube) produced by Fairchild Company. These MOSFETs all have low turn-on voltage values.

Another special aspect of this circuit is the feedback circuit formed by R3 and R4. This feedback circuit is connected before the filter and can effectively suppress the deterioration of frequency characteristics and increase in distortion caused by the LC filter circuit composed of L1, L2, C20, and C21. If feedback is directly derived from the speaker load like a traditional analog power amplifier, the amount of feedback of high-frequency components will be reduced, which will have little effect on improving the analog waveform.

The high-end cutoff frequency of the LC second-order low-pass filter is fh=1/(π(LC)1/2), and the quality factor Q=RLC20/L1. Therefore, the size of the speaker impedance will have a direct impact on the frequency response of the high-frequency end. To make the high-end response flat, the phase-frequency changes are not drastic, and the Q value cannot be greater than 1. Theoretically, the low frequency of a Class D power amplifier can reach zero frequency, but due to the presence of input capacitors C3 and C14 in the circuit, the direct wave is blocked.

The power supply decoupling of Class D power amplifier is very important. Improper wiring may cause self-oscillation. Therefore, in addition to large-capacity electrolytic capacitors, other decoupling capacitors in the circuit should use low-inductance capacitors, and tantalum capacitors are suitable for capacitors of several μF. Among them, several 0.1μF capacitors should be as close as possible to the pins of the IC circuit to enhance the high-frequency decoupling effect. In addition, ordinary ground wires and power ground wires should also be routed separately.