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
