LM4865MX National Semiconductor, LM4865MX Datasheet - Page 10

• Manufacturer Part Number: LM4865MX
• Manufacturer: National Semiconductor

Specifications of LM4865MX

• Operational Class: Class-AB
• Audio Amplifier Output Configuration: 1-Channel Mono
• Output Power (typ): 1x1@8Ohm/0.11x1@8OhmW
• Audio Amplifier Function: Headphone/Speaker
• Total Harmonic Distortion: 0.6@8Ohm@300mW%
• Single Supply Voltage (typ): 3/5V
• Dual Supply Voltage (typ): Not RequiredV
• Power Supply Requirement: Single
• Rail/rail I/o Type: No
• Power Supply Rejection Ratio: 50dB
• Single Supply Voltage (min): 2.7V
• Single Supply Voltage (max): 5.5V
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Operating Temp Range: -40C to 85C
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 8
• Package Type: SOIC N
• Lead Free Status / Rohs Status: Not Compliant

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Application Information
voltage applied to the HP-Sense pin (pin 3) at approximately
50mV. This 50mV enables the LM4865 and places it in
bridged mode operation.
While the LM4865 operates in bridged mode, the DC poten-
tial across the load is essentially 0V. Since the HP-Sense
threshold is set at 4V, even in an ideal situation, the output
swing cannot cause a false single-ended trigger. Connecting
headphones to the headphone jack disconnects the head-
phone jack contact pin from V
HP Sense pin up to V
tion, turns off Amp2, and mutes the bridged speaker. The
amplifier then drives the headphones, whose impedance is
in parallel with resistor R2. Resistor R2 has negligible effect
on output drive capability since the typical impedance of
headphones is 32Ω. The output coupling capacitor blocks
the amplifier’s half supply DC voltage, protecting the head-
phones.
A microprocessor or a switch can replace the headphone
jack contact pin. When a microprocessor or switch applies a
voltage greater than 4V to the HP Sense pin, a bridge-
connected speaker is muted and Amp1 drives the head-
phones.
PROPERLY SELECTING EXTERNAL COMPONENTS
Optimizing the LM4865’s performance requires properly se-
lecting external components. Though the LM4865 operates
well when using external components having wide toler-
ances, the best performance is achieved by optimizing com-
ponent values.
Input Capacitor Value Selection
Amplification of the lowest audio frequencies requires high
value input coupling capacitors. These high value capacitors
can be expensive and may compromise space efficiency in
portable designs. In many cases, however, the speakers
used in portable systems, whether internal or external, have
little ability to reproduce signals below 150Hz. In application
5 using speakers with this limited frequency response, a
large input capacitor will offer little improvement in system
performance.
Figure 1 shows that the nominal input impedance (R
10kΩ at maximum volume and 110kΩ at minimum volume.
Together, the input capacitor, C
high pass filter cutoff frequency that is found using Equation
(4).
FIGURE 3. Headphone Circuit
CC
. This enables the headphone func-
O
1 and allows R1 to pull the
i
, and R
IN
(Continued)
, produce a -3dB
10102534
IN
) is
10
As the volume changes from minimum to maximum, R
decrease from 110kΩ to 10kΩ. Equation (4) reveals that the
-3dB frequency will increase as the volume increases. The
nominal value of C
should be calculated with R
using a speaker with a low frequency limit of 150Hz, C
using Equation (4) is 0.1µF. The 0.22µF C
is optimized for a speaker whose response extends down to
75Hz.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to value of the bypass capacitor C
Since C
is the most critical when minimizing turn-on pops. The slower
the LM4865’s outputs ramp to their quiescent DC voltage
(nominally V
equal to 1.0µF, along with a small value of C
0.1µF to 0.39µF), produces a clickless and popless shut-
down function. Choosing C
mize clicks and pops.
CLICK AND POP CIRCUITRY
The LM4865 contains circuitry that minimizes turn-on and
shutdown transients or ’clicks and pops’. For this discussion,
turn-on refers to either applying the power supply voltage or
when the shutdown mode is deactivated. While the power
supply is ramping to its final value, the LM4865’s internal
amplifiers are configured as unity gain buffers. An internal
current source changes the voltage of the bypass pin in a
controlled, linear manner. Ideally, the input and outputs track
the voltage applied to the bypass pin. The gain of the internal
amplifiers remains unity until the voltage on the bypass pin
reaches 1/2 V
is stable, the device becomes fully operational and the gain
is set by the external voltage applied to the DC Vol/SD pin.
Although the bypass pin current cannot be modified, chang-
ing the size of C
magnitude of ’clicks and pops’. Increasing the value of C
reduces the magnitude of turn-on pops. However, this pre-
sents a tradeoff: as the size of C
increases. There is a linear relationship between the size of
CB and the turn-on time. Shown below are some typical
turn-on times for various values of C
In order eliminate ’clicks and pops’, all capacitors must be
discharged before turn-on. Rapidly switching V
allow the capacitors to fully discharge, which may cause
’clicks and pops’. In a single-ended configuration, the output
coupling capacitor, C
pacitor discharges through an internal 20kΩ resistor. De-
pending on the size of C
relatively large. To reduce transients in single-ended mode,
B
determines how fast the LM4865 turns on, its value
DD
DD
/2), the smaller the turn-on pop. Choosing C
. As soon as the voltage on the bypass pin
B
alters the device’s turn-on time and the
i
for lowest desired frequency response
C
OUT
0.01µF
0.22µF
0.47µF
B
0.1µF
1.0µF
, is of particular concern. This ca-
i
OUT
IN
as small as possible helps mini-
= 10kΩ . As an example when
, the time constant can be
B
increases, the turn-on time
T
ON
B
200ms
420ms
840ms
:
20ms
2sec
i
shown in Figure 1
i
(in the range of
DD
may not
(4)
IN
B
B
B
i
,
.