Audio Power Amplifier Principle and Application NCP2890

1 Overview

NCP2890 is a Semiconductor (On Semi) has introduced the cost-effective, full-featured audio products in the first audio amplifier, it is designed for mobile phones and PDA and other battery-powered wireless equipment design and production, is a kind of very good quality class AB audio power amplifier wireless applications, 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 combines the characteristics of external control and adjustable gain boot and shutdown delay function, and has a "startup and shutdown," the control circuit, can eliminate the opening and closing of such audio power amplifier, audible noise generated, can be flexibly applied to a portable audio equipment design. NCP2890 allows lithium-ion or lithium polymer battery direct power supply, which saves an additional LDO (LDO), while reducing board space and lower overall costs.

2-chip structure and performance parameters

To meet the specific needs of the market, NCP2890 present Mi-crobump-9 (2.25mm2) and Micro-8 (14.7mm2) in two different packages, as shown in Figure 1 for pin permutation, the functions of each pin Table 1.

Microbump -9 Micro8 function name
A1 4 INM audio signal anti-input, external Rf and Rin
A2 5 OUTA inverting output, an external load
A3 3 INP-phase input
B1 NA VM_P ground terminal
B2 7 VM ground terminal
B3 6 Vp power input, voltage range of 2.2V-5.5V
C1 2 BYPASS external bypass capacitor side
C2 8 OUTB-phase output, external load
C3 1 SHUTDOWN shutdown control side, low effective

NCP2890-house, including 671 transistors, MOS gate 1899, which has excellent audio performance, Table 2 shows the main performance parameters NCP2890.

Main performance parameters of Table 2 NC2890

Parameters of the largest units of the minimum standards
Supply voltage (Vp) 2.6 5.5 V
Output power (Po) 0.28 1.08 W
Efficiency (η) 63%
Quiescent Current (Idd) 1.5 1.7 4.0 mA
The total high harmonic 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
Close voltage (VSDIL) 0.4 V
Start time (TWU) 285 ms
Closing time (TSD) 385 ms

3 Application Circuit Design

Figure 2 shows a typical application NCP2890 audio amplifier circuit, can be seen from Figure 2, NCP2890 is only used to adjust the external gain two resistors, an input coupling capacitor, a bypass capacitor and a few number of components, peripheral devices required extremely simple.

3.1 The principle of the circuit

NCP2890 contains two identical power amplifiers, the input audio signal amplified by the first power amplifier output from the OUTA. Voltage gain by the external resistor Rf and Rin ratio of the decision. Amplified audio signal via a gain of 1, the first two power amplifier inverting amplifier, and output from OUTB. As OUTA and OUTB terminal end of audio output power signal equal and opposite phase, and two outputs (OUTA and OUTB) DC the same static potential (Vp / 2 ), so the speaker can be directly connected to OUTA and OUTB side, instead of increasing the output coupling circuit. two power amplifier output stage using PMOS and NMOS transistors are designed with special. normal conduction, its channel resistance of less than 0.6Ω, so the output waveform distortion is very small.

Audio Power Amplifier Principle and Application NCP2890

General power amplifier in the process of opening and closing will have the human ear can hear the noise, in order to eliminate audible noise, the NCP2890 is designed within the noise cancellation circuit. Boot, add the switch control logic high end , bypass capacitor Cby the DC voltage value began to increase exponentially when the voltage to common mode voltage (Vp / 2), the starting output power (this process is about 50ms); the shutdown, control, low-power termination level, the load is connected to ground terminal, the output power is zero, then the DC circuit quiescent current less than 100nA.

3.2 circuit element parameter

Rin and Rf used to set the amplifier's closed-loop gain, in order to optimize the performance of NCP2890, the amplifier closed-loop gain should be set at a lower level, then the minimum THD, SNR largest widest frequency response range. So in most cases the amplifier's closed-loop gain generally set at between 2 to 5, therefore, input resistance (Rin) of the 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 to isolation amplifier input DC voltage, at the same time with Rin to form a high pass filter, but it will affect the filter's lower cutoff frequency. Buzhi Yu to make low-frequency signal attenuation is too large, in theory, Cin should whichever is greater value, while the larger capacitor charge and discharge time is longer, therefore, require a longer time to make input static potential is Vp / 2, which makes this easy to turn the noise generated output. Therefore, in most cases, Cin's value is generally between 0.1 ~ 0.39μF more appropriate (Rin = 20kΩ pm).

Bypass capacitor Cby is a common mode voltage (Vp / 2) of the filter is to determine the length of the boot, the boot noise reduction is a key component, in most cases, 1.0μF bypass capacitor Cby take more appropriate.

Audio Power Amplifier Principle and Application NCP2890

R1 and R2 form a divider circuit for generating chip start-voltage, general R1 value 100kΩ, the design can be selected according to voltage resistance R2, as long as the partial pressure value to about greater than 1.2V can be.

3.3 Caution

When the circuit reaches the maximum output power Porms = 1.0W, Vp = 5.0V, RL = 8.0Ω, the load on the peak current of 500mA. In order to prevent the output load circuit when excessive output current, output current chip set detection circuit, it can limit the maximum output circuit 800mA. Thus, once the output current exceeds 800mA, the output of the four MOS transistors will be gated off and no longer relevant 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 ℃, only to restart within the amplifier to work.

Although the NCP2890 includes over-current and overheat the internal protection circuits, but when used, must pay attention to the power supply voltage can not exceed its limit, so as to avoid chip damage.