Datasheet

LM1876
www.ti.com SNAS097C –MAY 1999–REVISED APRIL 2013
A direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in internal
power dissipation. For each operational amplifier in a bridge configuration, the internal power dissipation will
increase by a factor of two over the single ended dissipation. Thus, for an audio power amplifier such as the
LM1876, which has two operational amplifiers in one package, the package dissipation will increase by a factor
of four. To calculate the LM1876's maximum power dissipation point for a bridged load, multiply Equation 1 by a
factor of four.
This value of P
DMAX
can be used to calculate the correct size heat sink for a bridged amplifier application. Since
the internal dissipation for a given power supply and load is increased by using bridged-mode, the heatsink's θ
SA
will have to decrease accordingly as shown by Equation 3. Refer to DETERMINING THE CORRECT HEAT SINK
for a more detailed discussion of proper heat sinking for a given application.
SINGLE-SUPPLY AMPLIFIER APPLICATION
The typical application of the LM1876 is a split supply amplifier. But as shown in Figure 4, the LM1876 can also
be used in a single power supply configuration. This involves using some external components to create a half-
supply bias which is used as the reference for the inputs and outputs. Thus, the signal will swing around half-
supply much like it swings around ground in a split-supply application. Along with proper circuit biasing, a few
other considerations must be accounted for to take advantage of all of the LM1876 functions.
The LM1876 possesses a mute and standby function with internal logic gates that are half-supply referenced.
Thus, to enable either the Mute or Standby function, the voltage at these pins must be a minimum of 2.5V above
half-supply. In single-supply systems, devices such as microprocessors and simple logic circuits used to control
the mute and standby functions, are usually referenced to ground, not half-supply. Thus, to use these devices to
control the logic circuitry of the LM1876, a “level shifter,” like the one shown in Figure 45, must be employed. A
level shifter is not needed in a split-supply configuration since ground is also half-supply.
Figure 45. Level Shift Circuit
When the voltage at the Logic Input node is 0V, the 2N3904 is “off” and thus resistor R
c
pulls up mute or standby
input to the supply. This enables the mute or standby function. When the Logic Input is 5V, the 2N3904 is “on”
and consequently, the voltage at the collector is essentially 0V. This will disable the mute or standby function,
and thus the amplifier will be in its normal mode of operation. R
shift
, along with C
shift
, creates an RC time constant
that reduces transients when the mute or standby functions are enabled or disabled. Additionally, R
shift
limits the
current supplied by the internal logic gates of the LM1876 which insures device reliability. Refer to MUTE MODE
and STANDBY MODE in Application Information for a more detailed description of these functions.
CLICKS AND POPS
In the typical application of the LM1876 as a split-supply audio power amplifier, the IC exhibits excellent “click”
and “pop” performance when utilizing the mute and standby modes. In addition, the device employs Under-
Voltage Protection, which eliminates unwanted power-up and power-down transients. The basis for these
functions are a stable and constant half-supply potential. In a split-supply application, ground is the stable half-
supply potential. But in a single-supply application, the half-supply needs to charge up just like the supply rail,
V
CC
. This makes the task of attaining a clickless and popless turn-on more challenging. Any uneven charging of
the amplifier inputs will result in output clicks and pops due to the differential input topology of the LM1876.
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