Datasheet

LM4780, LM4780TABD
www.ti.com SNAS193B –JULY 2003–REVISED APRIL 2013
Once the impedance seen by each amplifier in the parallel configuration is known then Equation (2) can be used
with this calculated impedance to find the amount of power dissipation for each amplifier. Total power dissipation
(P
DMAX
) within an IC package is found by adding up the power dissipation for each amplifier in the IC package.
Using the calculated P
DMAX
the correct heat sink size can be determined. Refer to the section, DETERMINING
THE CORRECT HEAT SINK, for more information and detailed discussion of proper heat sinking.
SINGLE-SUPPLY AMPLIFIER APPLICATION
The typical application of the LM4780 is a split supply amplifier. But as shown in Figure 5, the LM4780 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 LM4780 functions, like the mute
function.
CLICKS AND POPS
In the typical application of the LM4780 as a split-supply audio power amplifier, the IC exhibits excellent “click”
and “pop” performance when utilizing the mute mode. 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 at the same rate as 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 LM4780.
To achieve a transient free power-up and power-down, the voltage seen at the input terminals should be ideally
the same. Such a signal will be common-mode in nature, and will be rejected by the LM4780. In Figure 5, the
resistor R
INP
serves to keep the inputs at the same potential by limiting the voltage difference possible between
the two nodes. This should significantly reduce any type of turn-on pop, due to an uneven charging of the
amplifier inputs. This charging is based on a specific application loading and thus, the system designer may need
to adjust these values for optimal performance.
As shown in Figure 5, the resistors labeled R
BI
help bias up the LM4780 off the half-supply node at the emitter of
the 2N3904. But due to the input and output coupling capacitors in the circuit, along with the negative feedback,
there are two different values of R
BI
, namely 10kΩ and 200kΩ. These resistors bring up the inputs at the same
rate resulting in a popless turn-on. Adjusting these resistors values slightly may reduce pops resulting from
power supplies that ramp extremely quick or exhibit overshoot during system turn-on.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components is required to meet the design targets of an application. The choice of
external component values that will affect gain and low frequency response are discussed below.
The gain of each amplifier is set by resistors R
f
and R
i
for the non-inverting configuration shown in Figure 1. The
gain is found by Equation 7 below:
A
V
= 1 + R
f
/ R
i
(V/V) (7)
For best noise performance, lower values of resistors are used. A value of 1kΩ is commonly used for R
i
and then
setting the value of R
f
for the desired gain. For the LM4780 the gain should be set no lower than 10V/V and no
higher than 50V/V. Gain settings below 10V/V may experience instability and using the LM4780 for gains higher
than 50V/V will see an increase in noise and THD.
The combination of R
i
with C
i
(see Figure 1) creates a high pass filter. The low frequency response is determined
by these two components. The -3dB point can be found from Equation 8 shown below:
f
i
= 1 / (2πR
i
C
i
) (Hz) (8)
If an input coupling capacitor is used to block DC from the inputs as shown in Figure 6, there will be another high
pass filter created with the combination of C
IN
and R
IN
. When using a input coupling capacitor R
IN
is needed to
set the DC bias point on the amplifier's input terminal. The resulting -3dB frequency response due to the
combination of C
IN
and R
IN
can be found from Equation 9 shown below:
f
IN
= 1 / (2πR
IN
C
IN
) (Hz) (9)
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