Manufacturer’s data on
the LM1875 20W audio
power amplifier IC
As used in the amplifier module elsewhere
in this issue, the LM1875 IC requires only a
few external components to deliver 25W into
8 ohms. It has quite impressive specifications
for its size, as well as in-built thermal & short
circuit protection.
By DARREN YATES
The LM1875 Audio Amplifier IC
from National Semiconductor is now
a few years old but it is still one of
the most cost-effective devices available when it comes to simplicity and
output power.
The LM1875 comes in a 5-lead
TO-220 package. The heatsink tab is
connected to the negative supply rail
of the amplifier (ie, to pin 3). However,
it must be isolated from the heatsink
via a TO-220 insulating kit otherwise
earth loops are likely to be a problem.
Fig.: this diagram shows the pinout
details for the LM1875. The device
must be isolated from its heatsink
using a TO-220 mounting kit.
Fig.3: THD vs power
output.
40 Silicon Chip
Incidentally, even if this IC is not
driving a load, it must be bolted to a
heatsink as the quiescent current of
70mA is enough to cause the thermal
protection circuitry to switch in (more
on that later).
Main features
The main features and specifications
of the LM1875 are as follows:
• Up to 30W power output into 8
ohms;
• Typical harmonic distortion of
0.015% <at> 1kHz, 20W output;
• Short circuit protection;
• Supply voltage range of 20-60V;
• 94dB supply rejection ratio;
• In-built thermal protection;
• Low noise (S/N ratio in excess of
100dB);
• Open loop gain typically 90dB;
Fig.4: THD vs frequency
for 4Ω & 8Ω loads at 10W.
•
70mA (typical) quiescent current.
The LM1875 can drive either 4Ω or
8Ω loads but it delivers slightly more
power into 8Ω loads. With 4Ω loads,
the maximum output power is 20W.
Although the data sheets indicate
that the device can deliver a maximum output power of 30W into 8Ω,
this is at its absolute maximum supply
voltage of 60V. With practical power
supplies, some allowance must be
made for variations in mains voltage
and therefore 25W is a more realistic
rating.
Single or dual rails
The LM1875 can be operated from
dual or single supply rails and the
amplifier module project featured in
this issue shows both supply arrangements. The pinout diagram can be
seen in Fig.1.
Fig.2 shows the internal circuit
diagram of the IC. Two NPN devices,
Q35 and Q39, are the output transistors. If you look closely, the emitter
resistor for Q35 is split in half and
this split feeds another NPN device,
Q36, which monitors the output
current on positive half cycles of the
output signal. In fact, Q36 and Q37
form part of a “load-line” protection
system which shuts down drive to the
Fig.5: power output vs
supply voltage (8Ω load).
Fig.6: PSRR vs frequency
(positive & negative rails).
Fig.2: the internal circuit diagram of the LM1875. Q35 & Q39 are the output transistors, while Q36 & Q37 form part of
a “load-line” protection system which shuts down drive to the output stage if the loading conditions are excessive.
output stage if the loading conditions
are excessive.
Fig.3 shows the distortion vs output
power for both 4Ω and 8Ω loads while
Fig.4 depicts distortion as a function
of frequency. As you might expect, the
device has increased distortion at both
ends of the audio spectrum.
Power output
Fig.5 shows the expected power output at 1% total harmonic distortion for
supply rails of between ±10V to ±30V
(RL = 8Ω). Power supply rejection
characteristics vs frequency are shown
in Fig.6. Note the difference between
the positive and negative rails, with
the negative rail being some 30dB or
Fig.7: power dissipation vs
power output (RL = 4Ω).
so worse at 20kHz. The maximum
figure of 94dB is relative to a 0Ω signal
source resistance, a 4Ω load and at a
frequency of 1kHz.
The in-built thermal protection
activates when the die temperature
reaches 175°C and shuts down the
device, which remains off until the
die cools down to 145°C. In the case
of a continuous load or over-drive
problem, if the die rises to 150°C the
device will again shut down. The beauty of this is that if the fault is a one-off
event, the thermal circuitry will allow
the die to heat up further than if it is
a continuous fault.
Figs.7 & 8 show details on the
power output vs power dissipation
Fig.8: power dissipation vs
power output (RL = 8Ω).
for 4Ω and 8Ω loads. Notice how the
device dissipation is much higher for
4Ω loads. In fact, even with a 1°C/W
heatsink, the LM1875’s internal thermal shutdown circuitry switches on
once the power output reaches 20W.
By this stage, the power dissipation
has reached about 32W and the die
temperature has surpassed the 175°C
mark.
With an 8Ω load, the LM1875 will
happily deliver 25W con
tinuously
without running the risk of thermal
shutdown.
Stability
Most power amplifiers don’t drive
capacitive loads all that well and the
LM1875 doesn’t either. Long speaker
leads can produce enough capacitance
to drive some amplifiers into VHF
oscillation. In this case, the manufacturer’s data sheets recommend that
you add a Zobel network consisting of
a 0.22µF capacitor and a 1Ω resistor to
the output. This has been included in
the amplifier module in this month’s
issue.
As with most designs, PC board
layout is important in minimising
the noise and distortion components.
Keeping the input signal away from
the supply rails will help keep the
SC
distortion low.
December 1993 41
