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IC DATA Manufacturer’s data on the LM3876 audio amplifier IC Used in the 50W power module described elsewhere in this issue, the LM3876 is a high performance audio power amplifier with very low noise and distortion. It features SPiKeTM pro­tection circuitry and 100 watts peak output capability. By LEO SIMPSON The LM3876 is described as having an 11-pin TO-220 package although it does not look very similar to the familiar 3-lead TO-220 package as used for 3-terminal regulators. The LM3876 package is 20mm wide and has 11 leads which are cranked to in­crease their spacing. The metal tab is not isolated and connects to the negative supply rail for the IC. Fig.1 shows the package details. Maximum power dissipation is 125 watts. The LM3876 is capable of delivering 100W peak power into an 8-ohm load. In normal use, it will deliver around 50W into 4Ω or 8Ω loads. Some of the main features of the LM3876 IC include: • S/N ratio: 114dB A-weighted, with respect to 40W • • • • • • THD <0.06%, 20Hz to 20kHz <at> 40W IMD (SMPTE) <0.004% 84V maximum supply rail Input mute function Supply under-voltage protection Short circuit and over-voltage protection • 30mA quiescent current • Open loop gain typically 120dB • 120dB power supply rejection ratio National Semiconductor rate the LM3876 to deliver 40W into 8Ω but it delivers quite a bit more in practice. Hence, we have rated the amplifier module featured elsewhere in this issue at 50 watts. The LM3876 is one of a family of monolithic power amplifi­ ers from National Semiconductor. Others in the range are the LM3875 which is virtually identical to the LM3876, except that it lacks the audio mute Fig.1: physical dimensions & package outline of the LM3876T audio power amplifier. 80  Silicon Chip facility, and the LM2876 which can be regard­ed as a de-rated version of the LM3876. Fig.2: a typical audio power amplifier application circuit (dual supply rails). Single or dual supply? Although it is possible to run the device on a single supply rail, it does require extra circuitry compared with the dual supply circuit. The single supply circuit also has the input and output AC-coupled and is likely to produce a solid turn-on thump as the output coupling capacitor is charged. The LM3876 was really designed to be a dual rail amplifier and that is how we recommend its use. Fig.2 shows National Semiconductor’s suggested dual voltage amplifier circuit and this is very similar to the 50W audio module published elsewhere in this issue. Fig.3 shows the equivalent schematic of the LM3876, exclud­ing the active protection circuitry. This shows a more or less conventional power op amp circuit with quasi complementary output stage (ie, all NPN transistors). Note that there is no facility for adjusting the quiescent current as this is taken care of during the IC manufacture. Mute operation As noted above, the LM3876 has an inbuilt mute feature and as can be seen from Fig.3 this entails an NPN transistor with its base grounded and its emitter connected to pin 8 (the Mute pin) via two diodes and a 1kΩ resistor. For normal operation, pin 8 must be pulled to the negative supply rail and a minimum of 0.5mA must flow for the transistor to be correctly biased. In turn, the transistor controls the operation of a PNP differential pair which mutes the output when no current flows through pin 8. Fig.4 shows the relationship between the mute input current and the output reduction. The important thing to note is that the current through pin 8 needs to be at least 0.5mA to ensure that there is no attenuation in the output signal. This only becomes critical if supply rails are reduced from the normal ±35V down to, say, ±20V pass filter at 400Hz, we measured the total harmonic distortion at 1kHz to be 0.002% at 40W RMS output. SPiKeTM protection “SPiKe” stands for “Self Peak Instantaneous Temperature” (in degrees Kelvin) and is National Semi- conductor’s name for the protection system in the LM3876. In effect, the chip continually monitors its internal temperature and sets its safe area of operation accordingly. It can be likened to the mechanism whereby a 3-terminal regulator will reduce its output current delivery if its internal THD vs. output power Fig.5 shows the THD + noise vs output power for the device operating at 1kHz into an 8-ohm load. As you can see, from 0.5W and up, the THD+N is about 0.01%. This is measured with a band­width of 80kHz. Using a high Fig.3: equivalent schematic of the LM3876 audio power amplifier, excluding the active protection circuitry. March 1994  81 connections to the output stage transistors and protects against shorting the output to ground (0V) or the supply lines. The output current is initially limited to about 6A peak until the thermal protec­tion cuts in. Thermal protection Fig.4: mute current (mA) vs. output muting (dB). Fig.5: total harmonic distortion plus noise (THD + N) vs. output power. The THD + N is generally around .01%. temperature becomes excessive. However, SPiKeTM is more comprehensive than that. Fig.6 shows a simplified schematic of the LM3876 with the SPiKeTM features de­picted. It incorporates current limiting and over voltage protec­tion. The current limiting works via second emitter Not depicted in the schematic of Fig.6, the LM3876’s ther­mal protection shuts down the device when the temperature on the die reaches 165°C. When the die temperature drops below 155°C, the device starts operating again but if the temperature again rises, shutdown will occur at 165°C. Therefore the device will heat up rapidly if a short circuit occurs and then will cycle on and off until the fault is removed. As far as we can determine from the literature supplied on these devices, the thermal protection limit of 165°C applies only when heavy currents are being delivered. SPiKeTM protection, on the other hand, works to a temperature limit of 250°C which is 100°C higher than the nominal maximum junction operating temperature for this device or for any plastic encapsulated semiconductor. Conventional monolithic power amplifier ICs provide their SOA (safe operating area) protection by monitoring the voltage and current conditions in the output stage and limiting the signal drive before the SOA conditions are exceeded. This pro­tects the device but it often severely limits the power which can be delivered and no account is taken of the device’s operating temperature. The SPiKeTM protection circuit, by contrast, senses the temperature of the output transistors and operates as the temper­ature reaches 250°C. Depending on the transistor tempera­ture, the safe operating area is reduced for all pulse widths as the case temperature rises. The graphs of Figs. 7, 8 & 9 show the progressive reduction of SOA for case temperatures of 25°C, 75°C and 125°C. Hence, by dynamically varying the SOA, the LM3876 is able to deliver a peak power output of as much as 100 watts – not bad for a device with a maximum power dissipation of 125 watts. Importantly, to get the maximum power out of the LM3876, you must not skimp on the heatsink. If you use a skimpy heatsink, you’ll get skimpy power output. Over voltage protection The over voltage protection circuitry protects the LM3876 against voltage spikes which can be developed at the output when driving inductive loads. These spikes can far exceed the voltage ratings unless they are clamped. In conventional amplifiers, this is done by clamping diodes to the supply rails from the output but in the LM3876 this function is Fig.6: equivalent schematic diagram of the LM3876 amplifier with simplified SPiKeTM protection circuitry. 82  Silicon Chip FIG.7 FIG.8 FIG.9 These graphs show the progressive reduction of SOA for case temperatures of 25°C, 75°C & 125°C. By dynamically varying the SOA, the LM3876 is able to deliver a peak power output of as much as 100 watts. performed by the output transistors themselves, these being turned on to limit the voltage. In this mode, they can sink 6A peak. Under voltage protection Also depicted on the diagram of Fig.6 is under voltage protection although we regard this as a misnomer. It should be called “under voltage shutdown”. The device is not actually protected against low voltages (nor could they damage it) but the output stages are biased off for supply voltages of less than ±9V. This prevents any turn on or turn off thumps for the speakers which are usually the result of a power amplifier losing control of the output stage when the supply voltage is very low. This under voltage protection feature should not be confused with the pin 8 muting feature described above. For best results, the external muting is operated with a capacitor at pin 8 and this adds to the internal muting effect. PC board layout is critical to achieve the very good per­formance available from the LM3876. Keeping the output and input ground returns separated is essential and the use of “star earth­ing” SC is strongly advised. March 1994  83