lm48861tm National Semiconductor Corporation, lm48861tm Datasheet - Page 9

• Manufacturer Part Number: lm48861tm
• Description: Powerwise Ground-referenced, Ultra Low Noise, Stereo Headphone Amplifier
• Manufacturer: National Semiconductor Corporation

Application Information
GENERAL AMPLIFIER FUNCTION
The LM48861 headphone amplifier features National’s
ground referenced architecture that eliminates the large DC-
blocking capacitors required at the outputs of traditional head-
phone amplifiers. A low-noise inverting charge pump creates
a negative supply (CPV
(V
supplies, with the amplifier outputs biased about GND, in-
stead of a nominal DC voltage (typically V
amplifiers. Because there is no DC component to the head-
phone output signals, the large DC-blocking capacitors (typ-
ically 220μF) are not necessary, conserving board space and
system cost, while improving frequency response.
COMMON MODE SENSE
The LM48861 features a ground (common mode) sensing
feature. In noisy applications, or where the headphone jack is
used as a line out to other devices, noise pick up and ground
imbalance can degrade audio quality. The LM48861 COM in-
put senses and corrects any noise at the headphone return,
or any ground imbalance between the headphone return and
device ground, improving audio reproduction. Connect COM
directly to the headphone return terminal of the headphone
jack (Figure 2). No additional external components are re-
quired. Connect COM to GND if the common-mode sense
feature is not in use.
MICRO POWER SHUTDOWN
The voltage applied to the shutdown (SHDN) pin controls the
LM48861’s shutdown function. Activate micro-power shut-
down by applying a logic-low voltage to the SHDN pin. When
active, the LM48861’s micro-power shutdown feature turns
off the amplifier’s bias circuitry, reducing the supply current.
The trigger point is 0.4V (max) for a logic-low level, and 1.4V
(min) for a logic-high level. The low 0.1μA (typ) shutdown cur-
rent is achieved by applying a voltage that is as near as
ground as possible to the SHDN pin. A voltage that is higher
than ground may increase the shutdown current.
There are a few ways to control the micro-power shutdown.
These include using a single-pole, single-throw switch, a mi-
croprocessor, or a microcontroller. When using a switch,
connect an external 100kΩ pull-up resistor between the
SHDN pin and GND. Connect the switch between the
SHDN pin and V
ing the switch. Opening the switch connects the SHDN pin to
ground, activating micro-power shutdown. The switch and re-
sistor guarantee that the SHDN pin will not float. This prevents
unwanted state changes. In a system with a microprocessor
or microcontroller, use a digital output to apply the control
DD
). The headphone amplifiers operate from these bipolar
DD
. Select normal amplifier operation by clos-
SS
FIGURE 2.
) from the positive supply voltage
DD
/2), like traditional
30054101
9
voltage to the SHDN pin. Driving the SHDN pin with active
circuitry eliminates the pull-up resistor.
POWER DISSIPATION
Power dissipation is a major concern when using any power
amplifier, especially one in mobile devices. In the LM48861,
the power dissipation comes from the charge pump and two
operational amplifiers. Refer to the Power Dissipation vs Out-
put Power curve in the Typical Performance Characteristics
section of the datasheet to find the power dissipation associ-
ated the output power level of the LM48861. The power
dissipation should not exceed the maximum power dissipa-
tion point of the micro SMD package given in equation 1.
For the LM48861TM micro SMD package, θ
T
tem surroundings.
PROPER SELECTION OF EXTERNAL COMPONENTS
Power Supply Bypassing/Filtering
Proper power supply bypassing is critical for low noise per-
formance and high PSRR. Place the supply bypass capaci-
tors as close to the supply pins as possible. Place a 1μF
ceramic capacitor from V
tance may be added as required.
Charge Pump Capacitor Selection
Use low ESR ceramic capacitors (less than 100mΩ) for opti-
mum performance.
Charge Pump Flying Capacitor (C5)
The flying capacitor (C5) affects the load regulation and out-
put impedance of the charge pump. A C5 value that is too low
results in a loss of current drive, leading to a loss of amplifier
headroom. A higher valued C5 improves load regulation and
lowers charge pump output impedance to an extent. Above
2.2μF, the R
of C5 and C6 dominate the output impedance. A lower value
capacitor can be used in systems with low maximum output
power requirements.
Charge Pump Hold Capacitor (C6)
The value and ESR of the hold capacitor (C6) directly affects
the ripple on CPV
put ripple. Decreasing the ESR of C6 reduces both output
ripple and charge pump output impedance. A lower value ca-
pacitor can be used in systems with low maximum output
power requirements.
Power Supply Bypassing /Filtering
Proper power supply bypassing is critical for low noise per-
formance and high PSRR. Place the supply bypass capaci-
tors as close to the device as possible. Typical applications
employ a voltage regulator with 10µF and 0.1µF bypass ca-
pacitors that increase supply stability. These capacitors do
not eliminate the need for bypassing of the LM48861 supply
pins. A 1µF capacitor is recommended.
Input Capacitor Selection
The LM48861 requires input coupling capacitors. Input ca-
pacitors block the DC component of the audio signal, elimi-
nating any conflict between the DC component of the audio
source and the bias voltage of the LM48861. The input ca-
pacitors create a high-pass filter with the input resistors R
JMAX
= 150°C, and T
DS(ON)
P
DMAX
SS
of the charge pump switches and the ESR
. Increasing the value of C6 reduces out-
= (T
A
is the ambient temperature of the sys-
JMAX
DD
to GND. Additional bulk capaci-
- T
A
) / (θ
JA
)
JA
www.national.com
= 70°C/W.
(1)
IN
.