Along with large scale integrated circuit’s development, the tradition the power amplifier which is composed of the separative element is being replaced highly by the integrated power module. Especially in the output is between the 20~50W power power source and middle the standard source design, many Manufacturers have used the advanced large scale integration power amplifier in the power output stage, this both saved the cost, and reduced the electric circuit debugging cycle, simultaneously strengthened instrument’s reliability, brings the very big profit to the product formalization and the standardized production, therefore the large scale integration power amplifier has become the designer’s first choice in the power amplifier design, the application prospect is getting more and more broad.
1 LM3886 pin function
LM3886 is a section of high fidelity power amplifier which the American Country Semiconductor Company (NSC) produces, uses 11 foot TO-220 seal pin arrangement as shown in Figure 1, between the chip and the radiator does not need to insulate, its main electrical parameter like table 1 arranges in order. Various pins’ function is as follows:
1st, 5 feet: Positive electricity source input;
3 feet: Output;
4 feet: Negative supply input;
7 feet: ;
8 feet: Static sound control;
9 feet: Inverted input;
10 feet: Synchronism input;
2nd, 6, 11 feet: Spatial foot.
LM3886 has the following characteristic:
* has the output voltage short circuit protection and the electric current opening protection function;
* has pressed with the overheated protection function;
* has the undervoltage protection function, namely the power line voltage is lower than 10V the automatic stopping work.
Table 1 LM3886 main parameter
| Parameter | Minimum value | Typical value | Maximum value | Unit |
| Supply voltage | ±10 | ±15 | ±42 | V |
| Quiescent current | 30 | 50 | 85 | mA |
| Output | 38 | 68 | 135/Peak | W |
| Open-loop voltage gain | 90 | 115 | dB | |
| Transition voltage gain | 8 | 19 | V/μs | |
| Signal-to-noise ratio | 92.5 | 110 | dB | |
| Output current | 7 | 11.5 | A |
2 LM3886 in power power source’s application
LM3886 mainly aims at a model of chip which the acoustic amplifier designs, but the acoustic amplifier and the power power source amplifier active status has the difference, the former prime task in instantaneous power condition, the latter, then works in the constant power condition, therefore in the radiation aspect, the former does not need the ventilator, but the latter must add the ventilator to help the radiation. Especially when serves as the current amplifier, because the power power source outputs the alternating current needs the constant flow output, no matter has the load is big or is small, its consumption’s total output is certain, when the load is zero or is very small, electric power supply’s complete voltage falls on LM3886, but outputs the electric current must maintain invariable, like the power which consumes on LM3886 will be big, even if increases radiator’s area and the volume, the LM3886 temperature rise speed is also quick, in several seconds will cause it to be unable the normal work. For example, when the LM3886 output alternating current is 5A, the output is 30W, needs the power line voltage for ±25V above, this time works as the load is zero (short circuit condition), ±25V the pressure drop the entire tribe on LM3886, the power which LM3886 itself will consume will achieve 125W, the rapid rise’s temperature will cause LM3886 to be at overheated protection condition being unable to work.
In order to solve the above problem, may design two piece of LM3886 in the power power source amplifier to take the electric current power amplifier, may have the magnitude of current which through two piece of LM3886 output current’s superimposition must output. Its concrete line as shown in Figure 2.
LM3886-1 is a transconductance power amplifier, its output current’s change by the input voltage signal control, may know by Figure 2, the LM3886-1 output voltage is V1, its size will change along with the load size; LM3886-2 is a voltage proportional amplifier (1:1), its output voltage follows V1 throughout the change, namely V2=V1; V1=IaRm Io (RL Rt), V2=IbRm Io (RL Rt) because of V1=V2, therefore Ia=Ib; But the output current is Io=Ia Ib=2Ia=ViRf/(R1Rt). May know through the above computation and the analysis, by two piece of LM3886 through the different working’s cascade, causes the output current and the power obtains the expansion.
