介绍了安森美半导体(OnSemi

Abstract: ON Semiconductor (OnSemi) Company A-B-type main NCP2890 audio power amplifier

performance characteristics and the basic working principle, give the design of the NCP2890 audio

amplifier with a typical application circuit and design method.

Key words: NCP2890; AB type; audio amplifier

1 outlines

ON Semiconductor NCP2890 are (OnSemi) has introduced cost-effective, full-featured audio Series

Products of the first audio power amplifier, it is designed for mobile phone and PDA, such as

battery-powered wireless devices designed for production, are one kinds of very good quality

wireless applications A-B-type audio power amplifier can provide customers with superior audio

performance. The device has a superior power supply rejection ratio (PSRR) and total harmonic

distortion plus noise (THD + N) characteristics at the same time, a combination of external control

gain characteristics and adjustable boot and shutdown time delay function, and have a "startup

and shutdown "control circuit that can eliminate the opening and closing of such audio power

amplifier generated audible noise, can be flexibly used in portable audio equipment design.

NCP2890 allows lithium-ion or lithium polymer battery direct power supply, and thus obviates the

extra Low Dropout Regulators (LDO), while reducing board space and reduces the overall cost.

2-chip structure and performance parameters

To meet the specific needs of the market, NCP2890 currently have Mi-crobump-9 (2.25mm2) and Micro-8

(14.7mm2) two different kinds of packaging forms, shown in Figure 1 with its pin diagram, all

cited the function of the foot such as Table 1 NCP2890 include 671 internal transistor, MOS gate

1899, which has excellent audio performance, Table 2 give the NCP2890 the main performance

parameters.

Table 2NC2890 main performance parameters

3 Application Circuit Design

Figure 2 shows a typical NCP2890 audio amplifier application circuit, Figure 2 can be seen from,

NCP2890 external only used to adjust the gain of the two resistors, an input coupling capacitor,

a bypass capacitor, such as a small number of components, therefore the external requirements

devices extremely easy

3.1 Working Principle circuit

NCP2890 contains two identical power amplifiers, the input audio signal by the first power

amplifier amplified the output from the OUTA. Voltage gain by external resistor Rf and Rin the

ratio of the decision. The audio signal amplified by the gain of one and then the second power

amplifier RP enlarge, and OUTB output. Because of OUTA and OUTB client-side audio output signal

power equal to the size, phase contrast, and two outputs (OUTA and OUTB) DC the same static

potential (Vp / 2), so, speakers can be connected directly to the client OUTA and OUTB , rather

than increase the output coupling circuit. Two power amplifier output stage using PMOS and NMOS

transistors by the special design. Normal conduction, the channel resistance of less than 0.6Ω,

and thus its output waveform distortion is very l power amplifier in the open and

turn off the process will produce the human ear can hear the noise, in order to eliminate audible

noise, in-house designed NCP2890 noise elimination circuit. Boot time, add the switch control

logic high end of bypass capacitor Cby on the DC voltage values begin to increase exponentially

when the voltage value reaches the value of common-mode voltage (Vp / 2) when the beginning of

output power (this process around 50ms); and shutdown, the control low-level termination, load

to be connected to the grounding terminal, the output power to zero, when the DC circuit quiescent

current less than 100nA

3.2 circuit element parameters

Rin and Rf amplifier used to set the closed-loop gain, NCP2890 in order to optimize the performance

of the closed-loop gain amplifier should be set at a relatively low level, THD at this time the

smallest, the largest signal to noise ratio, wide frequency response range. So in most cases,

the closed-loop gain amplifier installed in general between 2 ~ 5, therefore, input resistance

(Rin) values of D in the 20kΩ more appropriate, and Rf is used to adjust the closed-loop gain

to control the output power. Input coupling capacitor Cin input amplifier used to isolate the

DC voltage, Rin simultaneously with the formation of a high-pass filter, but it will affect the

filter cutoff frequency threshold. In order to enable the low-frequency signals do not decay

too great, in theory, Cin should take a larger value, while the larger capacitor charge and

discharge time is long, therefore, required a longer period of time to make input to reach the

static potential Vp / 2, which is easy to make the output noise generated boot. Therefore, in

most cases, Cin values generally between 0.1 ~ 0.39μF more appropriate (Rin = 20kΩ pm). Bypass

capacitor Cby are common-mode voltage (Vp / 2) of the filter, is to determine the length of the

boot, boot noise reduction as a key component, in most cases, 1.0μF bypass capacitor Cby check

more appropriate. R1 and R2 components of the partial pressure circuit chips used to produce the

start voltage, general R1 value 100kΩ, designed according to voltage value to select the

resistance R2, as long as the pressure so that the value is greater than 1.2V can be about

3.3 Caution

When the circuit reaches the maximum output power  Porms = 1.0W, Vp = 5.0V, RL = 8.0Ω  when

the load on the peak current of 500mA. In order to prevent the output load circuit when the output

current is too large, set up chip output current detection circuit, it can be limited to a maximum

output circuit 800mA. In this way, once the output current of over 800mA, the output side of

the four MOS transistors will be gated off and no longer voltage output current. When the chip

temperature exceeds 160 ℃, the internal amplifier will be shut down and stop working until the

temperature is below 140 ℃, the internal amplifier to begin work to restart. Despite the NCP2890

contains an internal over-current and over-temperature protection circuit, but when in use, we

must pay attention to power supply voltage should not exceed their limits, so as to avoid chip

damage.

摘要：介绍了安森美半导体(OnSemi)公司Ａ－Ｂ类音频功率放大器NCP2890的主要性能特点和基本工作原理，给出了用NCP2890设计音频功放的典型应用电路和设计方法。 关键词：NCP2890；A-B类；音频放大器

1 概述

ＮＣＰ２８９０是安森美半导体(ＯｎＳｅｍｉ)公司推出的经济高效、功能齐备的音频系列产品中的第一款音频功率放大器，它是专为手机和ＰＤＡ等电池供电的无线设备而设计生产的，是一种质量非常优秀的无线应用Ａ－Ｂ类音频功率放大器，可为客户提供卓越的音频性能。该器件在具有出众的电源抑制比(ＰＳＲＲ)和总谐波失真加噪声(ＴＨＤ＋Ｎ)特性的同时，综合了外部控制增益特征和可调式开机与关机延时功能，并具有“开机和关机”控制电路，能消除开启和关闭此类音频功率放大器时产生的可听噪声，可灵活应用于便携式音频设备的设计中。ＮＣＰ２８９０允许锂离子或锂聚合物电池直接供电，因而省却了额外的低压降稳压器(ＬＤＯ)，同时减少了电路板的占用空间并降低了整体成本.

2 芯片结构与性能参数

为满足特定市场的需求，ＮＣＰ２８９０目前有Ｍｉ-ｃｒｏｂｕｍｐ－９(２．２５ｍｍ２)和Ｍｉｃｒｏ－８(１４．７ｍｍ２)两种不同的封装形式，图１所示为其引脚排列图，各引脚的功能如表１列. ＮＣＰ２８９０内部包括６７１个晶体管、１８９９个ＭＯＳ门电路，因而具有极佳的音频性能，表２给出了ＮＣＰ２８９０的主要性能参数.

３ 应用电路设计

图２所示为ＮＣＰ２８９０音频功放的典型应用电路，由图２可见，ＮＣＰ２８９０外围只有用来调节增益的两个电阻、一个输入耦合电容、一个旁路电容等少数几个元件，因此所需外围器件极其简单

３．１电路工作原理

ＮＣＰ２８９０内含两个完全一样的功率放大器，输入的音频信号经第一个功率放大器放大后从ＯＵＴＡ输出。电压增益由外接电阻Ｒｆ与Ｒｉｎ的比值决定。放大后的音频信号再经增益为１的第二个功率放大器进行反相放大，并从ＯＵＴＢ输出。由于ＯＵＴＡ端与ＯＵＴＢ端输出的音频功率信号大小相等、相位相反，且两个输出端(ＯＵＴＡ和ＯＵＴＢ)的直流静态电位相同（Ｖｐ／２），所以，扬声器可以直接连接到ＯＵＴＡ与ＯＵＴＢ端，而不用增加输出耦合电路。两个功率放大器的输出级均采用ＰＭＯＳ和ＮＭＯＳ晶体管特殊设计而成。正常导通时，其沟道电阻小于０．６Ω，因而其输出波形失真非常小. 一般的功率放大器在开启和关闭过程中会产生人耳可听到的噪声，为了消除这种可听噪声，在ＮＣＰ２８９０内部专门设计了消噪声电路。开机时，逻辑高电平加到开关控制端，旁路电容Ｃｂｙ上的直流电压值开始按指数规律增加，当电压值达到共模电压值（Ｖｐ／２）时，开始输出功率（此过程大约５０ｍｓ）；而关机时，控制端接低电平，负载被连接到接地端，输出功率为零，此时电路的直流静态电流小于１００ｎＡ.

３．２电路元件参数设置

Ｒｉｎ与Ｒｆ用来设置放大器的闭环增益，为了优化ＮＣＰ２８９０的性能，放大器的闭环增益应该设置在较低的水平，此时ＴＨＤ最小，信噪比最大，频率响应范围最宽。所以在多数情况下，放大器的闭环增益一般设置在２～５之间，因此，输入电阻(Ｒｉｎ)的取值Ｄ在２０ｋΩ比较合适，而Ｒｆ则用来调节闭环增益以控制输出功率. 输入耦合电容Ｃｉｎ用来隔离放大器输入端的直流电压，同时可与Ｒｉｎ组成一个高通滤波器，但它会影响滤波器的下限截止频率。为了使低频信号不至于衰减过大，理论上，Ｃｉｎ应该取较大的值，而较大容量电容的充放电时间较长，因此，需要较长的时间才能使输入端的静态电位达到Ｖｐ／２，而这易使输出端产生开机噪声。所以，在多数情况下，Ｃｉｎ的取值一般在０．１～０．３９μＦ之间比较合适（Ｒｉｎ＝２０ｋΩ时）. 旁路电容Ｃｂｙ是共模电压（Ｖｐ／２）

的滤波器，是决定开机时间的长短、减少开机噪声的一个关键元件，在多数情况下，旁路电容Ｃｂｙ取１．０μＦ比较合适。Ｒ１和Ｒ２组成的分压电路用于产生芯片的启动电压，一般Ｒ１取值１００ｋΩ，设计时可根据电压值来选择Ｒ２的阻值，只要使分压值约大于１．２Ｖ即可。

３．３使用注意事项

当电路达到最大输出功率Ｐｏｒｍｓ＝１．０Ｗ，Ｖｐ＝５．０Ｖ，ＲＬ＝８．０Ω时，负载上的峰值电流为５００ｍＡ。为了防止输出负载断路时产生过大的输出电流，芯片内设置了输出电流检测电路，它可将最大输出电路限定为８００ｍＡ。这样，一旦输出电流超过８００ｍＡ时，输出端的四个ＭＯＳ晶体管将被门控电压关断而不再输出电流。当芯片温度超过１６０℃时，内部放大器将被关闭而停止工作，直到温度低于１４０℃时，内部放大器才重新启动开始工作

尽管ＮＣＰ２８９０内部含有过流和过热保护电路，但是在使用时，一定要注意供电电源电压不能超过其极限值，以免造成芯片损坏