LM4701T National Semiconductor, LM4701T Datasheet - Page 13

• Manufacturer Part Number: LM4701T
• Description: LM4701 Overture Audio Power Amplifier Series 30W Audio Power Amplifier with Mute and Standby Modes
• Manufacturer: National Semiconductor

Application Information
The LM4701 possesses a mute and standby function with in-
ternal 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 LM4701, a “level shifter”, like the one shown
in Figure 5 , must be employed. A level shifter is not needed
in a split-supply configuration since ground is also
half-supply.
When the voltage at the Logic Input node is 0V, the 2N3904
is “off” and thus resistor R
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
ates an RC time constant that reduces transients when the
mute or standby functions are enabled or disabled. Addition-
ally, R
gates of the LM4701 which insures device reliability. Refer to
the Mute Mode and Standby Mode sections in the Applica-
tion Information section for a more detailed description of
these functions.
CLICKS AND POPS
In the typical application of the LM4701 as a split-supply au-
dio power amplifier, the IC exhibits excellent “click” and “pop”
performance when utilizing the mute and standby functions.
In addition, the device employs Under-Voltage Protection,
which eliminates unwanted power-up and power-down tran-
sients. The basis for these functions are a stable and con-
stant 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
This makes the task of attaining a clickless and popless
turn-on more challenging. Any uneven charging of the ampli-
fier inputs will result in output clicks and pops due to the dif-
ferential input topology of the LM4701.
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 LM4701. In Figure 3 , the resistor R
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
SHIFT
limits the current supplied by the internal logic
FIGURE 5. Level Shift Circuit
CC
C
pulls up mute or standby input to
.
SHIFT
, along with C
(Continued)
SHIFT
DS100835-9
, cre-
INP
13
based upon a specific application loading and thus, the sys-
tem designer may need to adjust these values for optimum
performance.
As shown in Figure 3 , the resistors labeled R
the LM4701 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 pop-
less turn-on. Adjusting these resistors values slightly may re-
duce pops resulting from power supplies that ramp
extremely quick or exhibit overshoot during system turn-on.
AUDIO POWER AMPLlFIER DESIGN
Design a 25W/8
Given:
A designer must first determine the power supply require-
ments in terms of both voltage and current needed to obtain
the specified output power. V
equation (4) and I
To determine the maximum supply voltage, the following
conditions must be considered. Add the dropout voltage to
the peak output swing V
rent of I
unloaded voltage which is usually about 15% higher. The
supply voltage will also rise 10% during high line conditions.
Therefore the maximum supply voltage is obtained from the
following equation:
Max Supplies
For 25W of output power into an 8
PEAK
ing V
are
(5). At this point it is a good idea to check the Power Output
vs Supply Voltage to ensure that the required output power is
obtainable from the device while maintaining low THD+N. In
addition, the designer should verify that with the required
power supply voltage and load impedance, that the required
heatsink value
constraints. Once the heatsink issues have been addressed,
the required gain can be determined from equation (6).
From equation (6), the minimum A
By selecting a gain of 21, and with a feedback resistor, R
20 k , the value of R
Thus with R
Since the desired input impedance was 47 k , a value of 47
k
the bandwidth requirements which must be stated as a pair
of −3 dB frequency points. Five times away from a −3 dB
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
was selected for R
±
OPEAK
is 20V. A minimum supply rail of
31.7V and the required I
OPEAK
and V
J
. The regulation of the supply determines the
= 1 k
±
SA
OD
OPEAK
(V
Audio Amplifier
BI
. With regulation, the maximum supplies
is feasible given system cost and size
OPEAK
, namely 10 k and 200 k . These re-
I
IN
a non-inverting gain of 21 will result.
follows from equation (7).
OPEAK
. The final design step is to address
from equation (5).
R
I
+ V
= R
OPEAK
, to get the supply rail at a cur-
OPEAK
20 Hz to 20 kHz
OD
F
(A
) (1 + Regulation) (1.1)
V
V
can be determined from
is A
±
− 1)
load, the required V
is 2.5A from equation
25V results from add-
V
14.14.
BI
1 Vrms(max)
help bias up
±
www.national.com
25 Wrms
0.25 dB
47 k
8
F
(4)
(5)
(6)
(7)
O -
=