LM386 circuit

LM1876 is a two-channel medium-power amplifier circuit, which is equivalent to packaging two LM1875s together. Its performance
is similar to that of LM1875, except that it has two more independent mute control terminals for two channels than LM1875. LM2876 is
a monolithic power amplifier circuit with a rated output power of 50w. When connected to form a BTL circuit, the rated output power can reach 12.OWoI_M3875 and
LM3876 are single-chip power amplifier circuits with a rated output power of 70W. When two pieces are connected to form a BTL circuit, the output power can reach
180Wo. The performances of LM3875 and LM3876 are basically similar, except that There are some differences in the protection circuit, and
there are slight differences in the minimum supply voltage and input offset voltage limit parameters.
    LM3886 is a monolithic power amplifier circuit with a rated output power of 78W. When connected to a BTL circuit, the output power can reach
200Wo. The main difference between LM3886 and LM3875/LM3876 is that the load capacity of LM3886 is better. When the output
load is 4n, the output power can be increased. As large as 87W, when the load is 4fl, the output power of LM3875/1_M38 76
is reduced to 56Wo. Therefore, if you need to connect a 4fl speaker, you can choose LM38860. Generally, the power amplifier is equipped with an 8fl
load , so you can choose LM3875/LM3876. fulfil requirements.
    The performance of the LM series audio power amplifier integrated circuits is very good, and the structural differences of the internal circuits are also very small. Now take
LM3875 as an example to introduce its performance characteristics. The power supply voltage range of LM3875 is 20--84V (single power supply, dual power supply is
10~42V). When the power supply voltage is 35V and the load is m, the distortion is <lo%, the output power is 70W, and the instantaneous peak
output power is 100W. ; In the frequency range 20H ~ 20kHz, when the output power is 40W, the total distortion plus noise
is <0.06%; the conversion rate is 11V4LSo.
    The LM series power amplifier integrated circuits all adopt the company's original SPIKE protection circuit, which is "self-peak instantaneous
temperature" protection. circuit. This kind of SPIKE protection circuit can dynamically detect and protect the safe operating area (meaning) A) of the output stage transistor
to fully realize overvoltage, undervoltage, overload, output short circuit (including short circuit to ground and short circuit to power supply), The protection function against thermal
runaway and instantaneous temperature shock. Because of its complete protection functions and high reliability, it
is completely unnecessary to add other protection circuits when used in the complete machine.
    Overvoltage protection. Usually when there is no input signal to the amplifier or when the power supply voltage rises unexpectedly and instantaneously, the power amplifier
transistor may be broken down due to exceeding the maximum rated voltage. For this reason, LM3875 and other models of IC have a unique overvoltage protection
circuit inside. When the sensitive device of the overvoltage protection circuit detects that the output terminal voltage exceeds the allowable value, it immediately turns off the output driver
tube to prevent the output tube from being broken down by overvoltage. When the overvoltage ends, the output driver tube resumes conduction. In this way,
the output tube and connected speakers can be protected from damage due to overvoltage.
    Model ICs such as LM3875 also have power supply voltage limiting protection circuits inside. When overvoltage occurs in the first half cycle of the output signal
, the voltage follow-up circuit limits Vc to about 8v. When an overvoltage occurs in the negative half cycle, the equivalent diode in the IC
limits Ve to about 0,8V for
    temperature limit protection. When the output terminal is short-circuited to ground or to the power supply, or when the output power
reaches the limit value due to instantaneous pulses within the input signal spectrum range , the internal protection circuit of the IC can continuously monitor the dynamics of the safe working area of ​​the output tube. When the junction temperature of the output tube reaches 250°C, the protection circuit can correspondingly reduce the base current of the driver tube so that the output tube always remains within the safe operating area.     Speaker impact protection when turning the power on and off o At the moment when the power is turned on or off, the normal working state of the amplifier has not yet established, and each stage is still in a transition state from unstable to stable. The output stage of the amplifier may output a relatively large amount of noise. A large impact voltage goes to the speaker, causing the speaker to make a loud "pop" sound, and sometimes even burning out the speaker. Since this transition stage of the amplifier is in an under-voltage state, power amplifiers such as the LM3875 use an under-voltage protection circuit to protect the speaker when the power is turned on and off. For LM3875, when the voltage between the positive and negative power lines has not exceeded 14V, for LM2876, LM3876 and LM3886, before the voltage on the negative power line has not been lower than -9V, the Vc control signal causes all current sources to be turned off, and the amplifier The output stage is in a high-impedance output state, and the DC voltage at the output of the amplifier remains ov. If the positive and negative power supply voltages rise at the same time after the power is turned on, each current source will be in the cut-off state of undervoltage protection before the negative power supply voltage reaches -9V or the difference between the positive and negative power supply voltages reaches 18v. Since the 9V threshold voltage of the undervoltage protection is referenced to the ground potential, it can prevent possible spike voltages in the power supply from causing the corresponding 14V undervoltage protection circuit to turn on in advance.     current limit. In order to protect the output tube from being damaged due to a short circuit at the output terminal to ground, power amplifier LC models such as the LM3875 are equipped with a current limiting circuit inside. The current limiting circuit can detect the emitter current of the output driver tube. When the emitter current is detected to increase to a certain extent, the current limiting circuit begins to reduce the base current of the output driver tube and the emitter current of the output driver tube. The larger the value, the greater the reduction in the base current of the driver tube, making the base current of the driver tube smaller. In this way, the maximum output current of the output tube can be limited to 6A (for LM3886, the maximum output current of the output tube is limited to 7A).     When the amplifier output terminal is short-circuited to the positive and negative power lines, the current limiting circuit can detect the driver tube respectively. emitter current and limits the output current. It should be noted that this protection must not last longer than a few seconds, otherwise the long-term reliability of the IC will not be guaranteed.     Overheating protection. Amplifiers generally have temperature compensation circuits that can compensate within a certain range to maintain the stability of the amplifier circuit. However, due to the characteristic error of the complementary transistor or insufficient temperature compensation due to excessive ambient temperature , the power consumption of the output tube increases excessively, causing the IC to overheat. Sometimes due to improper use of the radiator, the heat generated by the amplifier during long-term continuous operation cannot be dissipated well, and the temperature of the IC chip will rise rapidly. When the chip temperature rises to the upper limit of 165°C, the overheat protection circuit inside the power amplifier LC turns off the output tube and connects the output terminal to ground. at this time


























The amplifier stops working, and the output tube does not turn on and resume working until the chip temperature drops to 155°C.
    From the above introduction, it can be seen that the LM series power amplifier LC not only ensures excellent product performance and reliable quality in the production process
, but also In the design of the circuit (especially the design of the protection circuit), convenience and reliability of use are also fully considered.
The power amplifier designed and produced using this kind of power amplifier rc will inevitably have the advantages of simple circuit , excellent performance, basically no need for debugging
and high reliability.