LM48556TLX/NOPB National Semiconductor, LM48556TLX/NOPB Datasheet - Page 11

• Manufacturer Part Number: LM48556TLX/NOPB
• Description: IC AMP AUDIO PWR MONO AB 12USMD
• Manufacturer: National Semiconductor
• Series: Boomer®r
• Type: Class ABr

Specifications of LM48556TLX/NOPB

• Output Type: 1-Channel (Mono)
• Voltage - Supply: 2.7 V ~ 4.5 V
• Features: Depop, Differential Inputs, Shutdown
• Mounting Type: Surface Mount
• Package / Case: 12-MicroSMD
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Max Output Power X Channels @ Load: -
• Other names: LM48556TLX

Application Information
GENERAL AMPLIFIER FUNCTION
The LM48556 is a fully differential ceramic speaker driver that
utilizes National’s inverting charge pump technology to deliv-
er the high drive voltages required by ceramic speakers,
without the need for noisy, board-space consuming inductive
based regulators. The low-noise, inverting charge pump cre-
ates a negative supply (CPV
(PV
supplies, the maximum output voltage swing for each ampli-
fier is doubled compared to a traditional single supply device.
Additionally, the LM48556 is configured as a bridge-tied load
(BTL) device, quadrupling the maximum theoretical output
voltage range when compared to a single supply, single-end-
ed output amplifier, see Bridged Configuration Explained sec-
tion. The charge pump and BTL configuration allow the
LM48556 to deliver over 17V
speaker while operating from a single 4.5V supply .
DIFFERENTIAL AMPLIFIER EXPLANATION
The LM48556 features a differential input stage, which offers
improved noise rejection compared to a single-ended input
amplifier. Because a differential input amplifier amplifies the
difference between the two input signals, any component
common to both signals is cancelled. An additional benefit of
the differential input structure is the possible elimination of the
DC input blocking capacitors. Since the DC component is
common to both inputs, and thus cancelled by the amplifier,
the LM48556 can be used without input coupling capacitors
when configured with a differential input signal.
BRIDGE CONFIGURATION EXPLAINED
The LM48556 is designed to drive a load differentially, a con-
figuration commonly referred to as a bridge-tied load (BTL).
The BTL configuration differs from the single-ended configu-
ration, where one side of the load is connected to ground. A
BTL amplifier offers advantages over a single-ended device.
Driving the load differentially doubles the output voltage com-
pared to a single-ended amplifier under similar conditions.
Any component common to both outputs is cancelled, thus
there is no net DC voltage across the load, eliminating the DC
blocking capacitors required by single-ended, single-supply
amplifiers.
SHUTDOWN FUNCTION
The LM48556 features a low current shutdown mode. Set
SD = GND to disable the amplifier and reduce supply current
to 0.1µA. Switch SD between V
rent consumption in shutdown. The LM48556 may be dis-
abled with shutdown voltages less than 0.45V, however, the
idle current will be greater than the typical 0.1µA value.
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 device as possible. Place a 4.7µF tantalum
capacitor in parallel with a 0.1µF ceramic capacitor from
V
required.
Charge Pump Capacitor Selection
Use low ESR ceramic capacitors (less than 100mΩ) for opti-
mum performance.
DD
DD
to GND. Additional bulk capacitance may be added as
). Because the amplifiers operate from these bipolar
P-P
SS
DD
) from the positive supply
at 1kHz to a 1µF ceramic
and GND for minimum cur-
11
Charge Pump Flying Capacitor (C1)
The flying capacitor (C1) affects the load regulation and out-
put impedance of the charge pump. A C1 value that is too low
results in a loss of current drive, leading to a loss of amplifier
headroom. A higher valued C1 improves load regulation and
lowers charge pump output impedance to an extent. Above
4.7µF, the R
of C1 and C
capacitor can be used in systems with low maximum output
power requirements.
Charge Pump Hold Capacitor (C
The value and ESR of the hold capacitor (C
the ripple on CPV
put ripple. Decreasing the ESR of C
ripple and charge pump output impedance. A lower value ca-
pacitor can be used in systems with low maximum output
power requirements.
Gain Setting Resistor Selection
The amplifier gain of the LM48556 is set by four external re-
sistors, two per each input, R
amplifier gain is given by equation (1):
Careful matching of the resistor pairs, R
and R
between the resistors results in a differential gain error that
leads to an increase in THD+N, decrease in PSRR and CM-
RR, as well as an increase in output offset voltage. Resistors
with a tolerance of 1% or better are recommended.
The gain setting resistors should be placed as close to the
device as possible. Keeping the input traces close together
and of the same length increases noise rejection in noisy en-
vironments. Noise coupled onto the input traces which are
physically close to each other will be common mode and eas-
ily rejected.
Feedback Capacitor Selection
Due to their capacitive nature, ceramic speakers poorly re-
produce high frequency audio content. At high frequencies, a
ceramic speaker presents a low impedance load to the am-
plifier, increasing the required drive current. The higher output
current can drive the device into clipping, increasing THD+N.
Low-pass filtering the audio signal improves audio quality by
decreasing the signal amplitude at high frequencies, reducing
the speaker drive current. Adding a capacitor in parallel with
each feedback resistor creates a simple low-pass filter with
the -3dB point determined by equation (2):
Where R
equation (1) in the Gain Setting Resistors Selection section,
and C
capacitor is optional and not required for normal operation.
Input Capacitor Selection
Input capacitors block the DC component of the audio signal,
eliminating any conflict between the DC component of the
audio source and the bias voltage of the LM48556. The input
capacitors create a high-pass filter with the input resistors
R
tion (3) below.
IN
. The -3dB point of the high pass filter is found using Equa-
IN-
F
is the value of the feedback capacitor. The feedback
, is required for optimum performance. Any mismatch
F
is the value of the feedback resistor determined by
SS
DS(ON)
dominate the output impedance. A lower value
f
−3dB
A
SS
V
of the charge pump switches and the ESR
. Increasing the value of C
= 1 / 2
= R
F
/ R
π
R
IN
F
C
IN_
F
(V/V)
SS
and R
(Hz)
)
SS
reduces both output
F+
F_
SS
and R
(Figure 1). The
) directly affects
SS
reduces out-
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F-
, and R
(1)
(2)
IN+