LM4780TA/NOPB National Semiconductor, LM4780TA/NOPB Datasheet - Page 18

• Manufacturer Part Number: LM4780TA/NOPB
• Description: IC AMP AUDIO PWR 60W AB TO220-27
• Manufacturer: National Semiconductor
• Series: Overture™r
• Type: Class ABr

Specifications of LM4780TA/NOPB

• Output Type: 2-Channel (Stereo)
• Max Output Power X Channels @ Load: 60W x 2 @ 8 Ohm
• Voltage - Supply: 20 V ~ 84 V, ±10 V ~ 42 V
• Features: Depop, Mute, Short-Circuit and Thermal Protection
• Mounting Type: Through Hole
• Package / Case: TO-220-27 (Bent and Staggered Leads)
• Amplifier Class: AB
• No. Of Channels: 2
• Output Power: 60W
• Supply Voltage Range: 20V To 84V
• Load Impedance: 8ohm
• Operating Temperature Range: -20°C To +85°C
• Amplifier Case Style: TO-220
• Rohs Compliant: Yes
• For Use With: LM4780TABD - BOARD EVALUATION LM4780TA
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM4780TA; *LM4780TA/NOPB; LM4780TA

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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 ampli-
fier. But as shown in
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 po-
tential. 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
turn-on more challenging. Any uneven charging of the ampli-
fier 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
to keep the inputs at the same potential by limiting the voltage
difference possible between the two nodes. This should sig-
nificantly 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
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
sistors 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 ex-
tremely 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 re-
sponse are discussed below.
The gain of each amplifier is set by resistors R
non-inverting configuration shown in
found by
For best noise performance, lower values of resistors are
used. A value of 1kΩ is commonly used for R
the value of R
should be set no lower than 10V/V and no higher than 50V/
DMAX
CC
. This makes the task of attaining a clickless and popless
the correct heat sink size can be determined. Refer to
Equation 6
Figure
f
for the desired gain. For the LM4780 the gain
A
V
= 1 + R
4, the resistors labeled R
BI
below:
Figure
, namely 10kΩ and 200kΩ. These re-
Figure
f
4, the LM4780 can also be used
/ R
i
(V/V)
4, the resistor R
Figure
i
and then setting
BI
1. The gain is
f
and R
help bias up
INP
i
serves
for the
(6)
18
V. Gain settings below 10V/V may experience instability and
using the LM4780 for gains higher than 50V/V will see an in-
crease in noise and THD.
The combination of R
pass filter. The low frequency response is determined by
these two components. The -3dB point can be found from
Equation 7
If an input coupling capacitor is used to block DC from the
inputs as shown in
filter created with the combination of C
a input coupling capacitor R
point on the amplifier's input terminal. The resulting -3dB fre-
quency response due to the combination of C
be found from
With large values of R
outputs when the inputs are left floating. Decreasing the value
of R
If the value of R
to increase in order to maintain the same -3dB frequency re-
sponse.
HIGH PERFORMANCE CONSIDERATIONS
Using low cost electrolytic capacitors in the signal path such
as C
formance. However, electrolytic capacitors are less linear
than other premium capacitors. Higher THD+N performance
may be obtained by using high quality polypropylene capac-
itors in the signal path. A more cost effective solution may be
the use of smaller value premium capacitors in parallel with
the larger electrolytic capacitors. This will maintain signal
quality in the upper audio band where any degradation is most
noticeable while also coupling in the signals in the lower audio
band for good bass response.
Distortion is introduced as the audio signal approaches the
lower -3dB point, determined as discussed in the section
above. By using larger values of capacitors such that the -3dB
point is well outside of the audio band will reduce this distor-
tion and improve THD+N performance.
Increasing the value of the large supply bypass capacitors will
improve burst power output. The larger the supply bypass
capacitors the higher the output pulse current without supply
droop increasing the peak output power. This will also in-
crease the headroom of the amplifier and reduce THD.
SIGNAL-TO-NOISE RATIO
In the measurement of the signal-to-noise ratio, misinterpre-
tations of the numbers actually measured are common. One
amplifier may sound much quieter than another, but due to
improper testing techniques, they appear equal in measure-
ments. This is often the case when comparing integrated
circuit designs to discrete amplifier designs. Discrete transis-
tor amps often “run out of gain” at high frequencies and
therefore have small bandwidths to noise as indicated below.
IN
IN
or not letting the inputs float will remove the oscillations.
and C
shown below:
i
(see Figures 1 - 5) will result in very good per-
Equation 8
f
IN
IN
f
i
is decreased then the value of C
= 1 / (2
= 1 / (2
Figure
i
IN
with C
oscillations may be observed on the
π
π
shown below:
5, there will be another high pass
R
R
IN
IN
i
i
C
C
(see
is needed to set the DC bias
i
) (Hz)
IN
) (Hz)
Figure
IN
and R
1) creates a high
IN
IN
. When using
and R
IN
will need
IN
can
(7)
(8)