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Abstract: This paper introduces the main performance characteristics and basic working principle of On Semi-class A-B audio power amplifier NCP2890, and gives the typical application circuit and design method of designing audio power amplifier with NCP2890.Key words: NCP2890; class AB; audio amplifier
1 Overview
The NCP2890 is the first audio power amplifier in the cost-effective, full-featured audio family from On Semi. It is designed for battery-powered wireless devices such as cell phones and PDAs. A very good quality wireless application A-B audio power amplifier that provides customers with superior audio performance. The device combines external control gain features with adjustable power-on and shutdown delays with superior power-supply rejection ratio (PSRR) and total harmonic distortion plus noise (THD+N) characteristics, with "on and off" control The circuit eliminates the audible noise generated when turning on and off such an audio power amplifier and can be flexibly applied to the design of portable audio equipment. The NCP2890 allows direct power to the Li-Ion or Li-Polymer battery, eliminating the need for an additional low-dropout regulator (LDO) while reducing board space and overall cost.
2 chip structure and performance parameters
In order to meet the needs of specific markets, the NCP2890 is available in two different packages, Mi-crobump-9 (2.25mm2) and Micro-8 (14.7mm2). Figure 1 shows the pinout diagram for each pin. The functions are listed in Table 1.
| Microbump -9 | Micro8 | Name | Features |
| A1 | 4 | INM | Audio signal reverse input, external Rf and Rin |
| A2 | 5 | OUTA | Inverting output, external load |
| A3 | 3 | INP | Non-inverting input |
| B1 | NA | VM_P | Ground terminal |
| B2 | 7 | VM | Ground terminal |
| B3 | 6 | Vp | Power input, voltage range is 2.2V-5.5V |
| C1 | 2 | BYPASS | External bypass capacitor |
| C2 | 8 | OUTB | Non-inverting output, external load |
| C3 | 1 | SHUTDOWN | Shutdown control terminal, active low |
The NCP2890 includes 671 transistors and 1899 MOS gates for excellent audio performance. Table 2 shows the main performance parameters of the NCP2890.
Table 2 NC2890 main performance parameters
| Parameter | Minimum | Standard | Largest | Unit |
| Power supply voltage (Vp) | 2.6 | 5.5 | V | |
| Output work (Po) | 0.28 | 1.08 | W | |
| Efficiency (η) | 63 | % | ||
| Quiescent current (Idd) | 1.5 | 1.7 | 4.0 | mA |
| Total higher harmonics plus noise (THD+N) | 0.02 | % | ||
| Power supply rejection ratio (PSRR V+) | -74 | dB | ||
| Signal to noise ratio (SNR) | 84 | dB | ||
| Start voltage V (SDIH) | 1.2 | V | ||
| Shutdown voltage (VSDIL) | 0.4 | V | ||
| Start time (TWU) | 285 | Ms | ||
| Closed time (TSD) | 385 | Ms |
3 application circuit design
Figure 2 shows a typical application circuit of the NCP2890 audio power amplifier. As can be seen from Figure 2, the NCP2890 has only a few components, such as two resistors for adjusting the gain, one input coupling capacitor, and one bypass capacitor. Extremely simple.
3.1 Circuit working principle
The NCP2890 contains two identical power amplifiers. The input audio signal is amplified by the first power amplifier and output from OUTA. The voltage gain is determined by the ratio of the external resistors Rf to Rin. The amplified audio signal is then inverted and amplified by a second power amplifier with a gain of one and output from OUTB. Since the audio power signals outputted by the OUTA terminal and the OUTB terminal are equal in magnitude and opposite in phase, and the DC static potentials of the two output terminals (OUTA and OUTB) are the same (Vp/2), the speaker can be directly connected to the OUTA and OUTB terminals. Without increasing the output coupling circuit. The output stages of both power amplifiers are specifically designed with PMOS and NMOS transistors. When it is normally turned on, its channel resistance is less than 0.6Ω, so its output waveform distortion is very small.
In general, the power amplifier generates noise that can be heard by the human ear during the opening and closing process. To eliminate this audible noise, a noise cancellation circuit is specially designed inside the NCP2890. When turned on, a logic high level is applied to the switch control terminal, and the DC voltage value on the bypass capacitor Cby begins to increase exponentially. When the voltage value reaches the common mode voltage value (Vp/2), the output power starts. 50ms); When the power is off, the control terminal is connected to the low level, the load is connected to the ground, and the output power is zero. At this time, the DC quiescent current of the circuit is less than 100nA.
3.2 circuit component parameter settings
Rin and Rf are used to set the closed-loop gain of the amplifier. In order to optimize the performance of the NCP2890, the closed-loop gain of the amplifier should be set at a lower level. At this time, the THD is the smallest, the signal-to-noise ratio is the largest, and the frequency response range is the widest. Therefore, in most cases, the closed-loop gain of the amplifier is generally set between 2 and 5. Therefore, the input resistance (Rin) value D is suitable at 20kΩ, and Rf is used to adjust the closed-loop gain to control the output power.
The input coupling capacitor Cin is used to isolate the DC voltage at the input of the amplifier and can be combined with Rin to form a high-pass filter, but it affects the lower cut-off frequency of the filter. In order to prevent the low frequency signal from being excessively attenuated, in theory, Cin should take a larger value, and the larger capacity capacitor has a longer charge and discharge time. Therefore, it takes a long time for the static potential of the input terminal to reach Vp/2. And this easily causes the output to generate boot noise. Therefore, in most cases, the value of Cin is generally between 0.1 and 0.39 μF (Rin = 20 kΩ).
The bypass capacitor Cby is a common-mode voltage (Vp/2) filter. It is a key component that determines the length of the boot time and reduces the startup noise. In most cases, the bypass capacitor Cby is suitable for 1.0μF.
   The voltage dividing circuit composed of R1 and R2 is used to generate the starting voltage of the chip. Generally, the value of R1 is 100kΩ. The resistance value of R2 can be selected according to the voltage value, as long as the voltage dividing value is greater than 1.2V.
3.3 Precautions for use
When the circuit reaches the maximum output power? Porms=1.0W, Vp=5.0V, RL=8.0Ω?, the peak current on the load is 500mA. In order to prevent excessive output current when the output load is disconnected, an output current detection circuit is provided in the chip, which limits the maximum output circuit to 800mA. Thus, once the output current exceeds 800 mA, the four MOS transistors at the output will be turned off by the gate voltage and no longer output current.
When the chip temperature exceeds 160 ° C, the internal amplifier will be turned off and stopped working until the temperature is lower than 140 ° C, the internal amplifier will restart and start working.
Although the NCP2890 contains overcurrent and overheat protection circuits internally, it must be noted that the power supply voltage must not exceed its limit value to avoid chip damage.
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