I. Circuit Diagram Let's take a look at the figure. This figure is a circuit commonly used by lm4890 in MPN. 2. Introduction to lm48920 1. General description
Lm4890 is an audio power amplifier designed primarily for applications in mobile phones and other portable communication devices. In a 5 v dc power supply, it can output 1 W continuous average power to 8 Ω BTL (what is BTL ?) Load, and the total harmonic distortion is less than 1%. Boomer audio power amplifiers are specially designed to provide high-quality output power using as few external components as possible. Lm4890 is ideal for mobile phones and other low-voltage applications because it does not require external coupling capacitors or self-lifting capacitors. The main requirement of these applications is to minimize power consumption.
Lm4890 features low power consumption in shutdown mode. When the power-off pin is low, the system enters the power-off mode. In addition, lm4890
Another feature is the internal thermal shutdown protection mechanism.
The lm4890 contains a special circuit used to eliminate the noise generated when switching from on to off.
The unit gain of lm4890 is stable and can be configured by setting the external gain resistance. Note: BTL (Bridge-tied-load) indicates a bridge load. The two ends of the load are connected to the output ends of the two amplifiers respectively. The output of one amplifier is the image output of another amplifier. That is to say, the signal added at both ends of the load is only 180 ° different in phase. Load will generate 2 times the voltage of the original single-ended output. Theoretically, the output power of the circuit will increase by 4 times. The BTL Circuit can make full use of the system voltage. Therefore, the BTL structure is often used in low-voltage systems or battery power supply systems. In automobile audios, when the power of each sound channel exceeds 10 W, most of them adopt the BTL format. The BTL form is different from the push-pull form. Each amplified signal of BTL is a complete signal, but only the output signal of the two amplifiers is reversed. To construct a BTL amplifier using an integrated amplifier, a dual-channel or two-channel amplifier is required. However, not all the plug-ins are applicable to the BTL form, and several methods of the BTL form have their own advantages and disadvantages. Typical Power Amplifier integration blocks include tda2030a
Lm1875 lm4766 lm3886 tda1514. Iii. Application description 1. Example of a bridge structure As shown in 1, lm4890 has two operational amplifiers, which can have different amplifier structures. The gain of the first amplifier can be set externally, while the gain of the second amplifier is an internal fixed unit gain, reversely structured. The closed loop gain of the first amplifier is determined by the ratio of RF to RI, and the gain of the second amplifier is fixed by the internal resistance of two 10kb. Figure 1 shows that the output of the first amplifier serves as the input of the second amplifier, so that the output of the two amplifiers is equal in the amplitude, and the phase difference is 180 degrees. Therefore, the differential gain of the entire circuit is AVD = 2 * (RF/RI) The structure of Differential Drive load through vo1 and vo2. it is called a "bridge structure ". The bridge structure works different from the classical one-end output while the other end is mounted to the grounded amplifier structure. Compared with a single-ended amplifier, the design of the bridge structure has its unique advantages. It can differential drive the load, so when the operating voltage is certain, the output voltage swing can be doubled. Under the same conditions, the output power is 4 times the single-ended structure. To select the closed-loop gain of the amplifier and avoid extra distortion, see the Audio Power Amplifier Design Section. The bridge structure, such as the application in lm4890, has another advantage over the single-ended structure. Because it is a differential output, vo1
And VDD at 1/2 offset, so there is no DC voltage on the load. In this way, the output coupling capacitor is not required, and this capacitor is required in the single-Power Supply single-ended output amplifier. Without the output coupling capacitor, the bias of 1/2 VDD on the load can lead to the power consumption and possible loss of sound in the integrated circuit. (1) Power Consumption
Whether it is a bridge or a single-end power amplifier, power consumption is a major consideration when designing a successful power amplifier. The increase in the power output from the bridge amplifier to the load will directly increase the internal power consumption. Since lm4890 is integrated with two operational amplifiers internally, the maximum internal power consumption is 4 times that of the single-ended architecture. For an application, the maximum power consumption can be obtained from the power consumption curve or equation (1 ). Pdmax = 4 * (VDD) 2/(2 π 2rl) (1) The maximum temperature of tjmax cannot exceed 150 ℃. Tjmax uses pdmax and PCB Area from the power consumption curve. The additional copper platinum reduces the internal resistance of the application to 150 ℃/W, resulting in a high pdmax. The extra copper platinum can be introduced with any wires connected to the lm4890. This effect is particularly significant when the VDD, Gnd, and output pin are connected. If tjmax is greater than 150 ℃, it must be changed. These changes include reducing the power supply voltage, high load impedance, or reducing the surrounding temperature. Internal power consumption is the function of output power. Refer to the typical power consumption curve to obtain the power consumption information under different output power and output load. (2) Power Supply pass-through
For any amplifier, proper power supply passthrough is critical for low noise performance and high power suppression ratio. The capacitance on the power supply pin and bypass pin should be as close as possible to the device. Typical applications use 5 V voltage and 10 UF ta capacitor, electrical capacitor and ceramic capacitor to stabilize the power supply voltage. However, the power supply node of lm4890 still needs pass-through filtering. The selection of pass-through Capacitors, especially CB, is determined by PSRR requirements, burrs and pop performance, performance consumption, and size restrictions. (3) Shutdown
To reduce power consumption when not in use, lm4890 contains a shutdown pin used to offset current through the external shutdown amplifier. When the logic level of the shutdown pin is low, the amplifier is shut down. When the shutdown pin is connected to the geographic line, the leakage current of lm4890 can be minimized. When the voltage of the shutdown pin is less than VDC, the device will not work. The leakage current of the grounding pin may be larger than the typical shutdown current of 0.1ua.
In many applications, the output of a microcontroller or microprocessor is often used to control the shutdown circuit so that the circuit can be quickly and smoothly switched to the shutdown mode. Another method is to use a single-pole single-Throw Switch to connect an external pull-up resistor. When the switch is closed, the power-off pin is grounded so that the amplifier cannot work. When the switch is disconnected, the external pull-up resistor will make the lm4890 work. This solution ensures that the shutdown pin will not be left blank, so it will not lead to unexpected state changes. (4) proper selection of external components
In the application of integrated power amplifier, proper selection of external components is critical to optimizing the performance of devices and systems. Because lm4890 has loose requirements on external components, the value of the external components must be considered to optimize the overall performance of the system. The lm4890 unit gain is very stable, which gives the designer the maximum system flexibility. Lm4890 should use a low-gain structure, which can minimize thd + N and maximize the signal-to-noise ratio. The low-gain structure requires a large input signal to obtain a certain output power. Generally, the input signal obtained from the signal source, such as the audio decoder, must be equal to or slightly greater than 1 VRMS. Please refer to the design section of the Audio Amplifier for a more complete explanation of the appropriate gain selection.
In addition to gain, the bandwidth of the closed loop of the amplifier is another major consideration for the amplifier. In many cases, bandwidth is determined by the choice of external components, as shown in (1. The input coupled capacitor CI forms the first-level high-pass filter, which limits the response of low-frequency signals. The selection of these values should be based on the frequency response requirements under specific conditions. (5) Selection of input capacitor size
Large input capacitance is not only expensive, but also occupies space for the design of portable devices. Obviously, it is necessary to use a capacitor of a certain size to couple the low-frequency signal without weakening it. However, in many cases, the speakers in the portable system, whether external or built-in, cannot regenerate signals from less than 100Hz to 150Hz. Therefore, using a large input capacitor will not improve the actual performance of the system.
In addition, the cost and size of the system, Glitch and pop performance are also affected by the input coupling capacitor CI size. A large input capacitor requires more charge to reach the required static voltage DC (generally 1/2 VDD ). These charges come from the output via feedback and are prone to pop noise. Therefore, based on the required low-frequency response, making the input capacitor as small as possible can minimize the pop noise at startup. In addition to the input capacitor, the size of the pass-through capacitor must be carefully considered. The pass-through capacitor CB determines the lm4890 enabling speed. Therefore, it is the most critical component to minimize the pop noise. The slower the lm4890 output signal changes relative to the static voltage DC, the less noise it will start. Select 1.0 UF
The CB and small ci (0.1 UF to 0.39 UF) can enable no glitch and noise during shutdown. When the CB is 1.0 UF, the device is more susceptible to Glitch and pop noise when the device works normally (without high-frequency and low-frequency vibrations. Therefore, it is recommended that CB be 1.0 UF in addition to very sensitive designs. (6) Design of Audio Power Amplifier
A 1 W/8 Ω Audio Amplifier
Requirements:
Power 1 wrms
Load Impedance 8Ω
Input Level 1 VRMS
Input Impedance 20kb
Bandwidth: 100Hz to 20 kHz ± 0. 25db
A designer must first determine the minimum supply voltage to obtain the required output power. It is easy to find out the output power and power supply voltage. The second method is to use equation (2) and output voltage to calculate the required vopeak. In this way,
The smallest voltage is (vopeak + (vodtop + vodbot), where vodtop and vodbot are derived from the dropout voltage and power supply voltage curves. Vopeak = (2 rlpo) 1/2 (2) In many applications, 5 V is the standard voltage. It is often used as the power supply voltage. The extra power supply voltage will produce a large degree of voltage swing surplus,
This allows a peak value higher than 1 w without distortion. At this point, the designer must ensure that the selection of power supply voltage and output impedance do not violate the power consumption.
Some mentioned conditions. Once the power consumption equation is determined, the required difference gain can be determined by equation (3. AVD ≥( porl) 1/2/(VIN) = vorms/vinrms (3) RF/Ri = AVD/2 From equation (3), the minimum AVD is 2.83. We use AVD = 3. The expected input impedance is 20 KB, AVD = 2, RF/Ri = 1.5/1, rI = 20 KB, Rf = 30kb. The final design step is to determine the bandwidth, which is determined by two points whose frequency drops by 3 dB. At this point, the frequency is 5 times lower than 0.17db. This is better than 0.25db.
FL = 100 kHz/5 = 20Hz
FH = 20 kHz * 5 = 100 kHz
As described in the external component section, RI and CI form a Qualcomm filter.
CI ≥ 1/(2 π * 20 k Ω * 20Hz) = 0.397 UF;
We use 0.39 UF. The high-frequency cutoff point is determined by the required high-frequency cutoff frequency (FH) and differential gain (AVD.
When AVD = 3, FH = 100 kHz, gbwp = 150 kHz, which is much lower than the 4 MHz gbwp of lm4890. This shows that the designer wants to design a high-difference gain amplifier, and the lm4890 can still be used without putting it into the bandwidth limit. (7) Higher-gain Audio Amplifiers
The unit gain of lm4890 is very stable. In typical applications, apart from the gain setting resistance, input coupling capacitance, and appropriate power supply straight
Filter without other external components. However, when the closed loop gain is greater than 10, a feedback capacitor C4 is required,
2. This feedback capacitor forms a low-pass filter to eliminate possible high-frequency vibrations. When calculating the-frequency, you must note that improper combinations of R3 and C4 will lead to roloff before 20 kHz. The typical combination of high-frequency roloff Feedback resistance and capacitance within the audio range is R3 = 20kb and C4 = 25pf. These components make the-3 dB point about 320 kHz. Feedback resistance and
Capacitor is not recommended. |
