LM48820
Ground-Referenced, Ultra Low Noise, Fixed Gain, 95mW
Stereo Headphone Amplifier
General Description
Key Specifications
The LM48820 is a ground referenced, fixed-gain audio power
amplifier capable of delivering 95mW of continuous average
power into a 16Ω single-ended load, with less than 1% THD
+N from a 3V power supply.
The LM48820 features a new circuit technology that utilizes
a charge pump to generate a negative reference voltage. This
allows the outputs to be biased about ground, thereby eliminating output-coupling capacitors typically used with normal
single-ended loads.
Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal number of
external components. The LM48820 does not require output
coupling capacitors or bootstrap capacitors, and therefore is
ideally suited for mobile phone and other portable applications.
The LM48820 features a low-power consumption shutdown
mode selectable for each channel and a soft start function that
reduces start-up current transients. Additionally, the
LM48820 features an internal thermal shutdown protection
mechanism.
The LM48820 contains advanced pop & click circuitry that
eliminates noises which would otherwise occur during turn-on
and turn-off transitions.
The LM48820 has an internal fixed gain of 1.5V/V.
■ Improved PSRR at 217Hz
80dB (typ)
■ Power Output at VDD = 3V,
RL = 16Ω, THD+N = 1%
95mW (typ)
■ Shutdown Current
0.05µA (typ)
■ Internal Fixed Gain
1.5V/V (typ)
■ Wide Operating Voltage Range
1.6V to 4.5V
Features
■ Available in space saving 0.4mm pitch micro SMD
package
Fixed Logic Levels
Ground referenced outputs
High PSRR
Ultra low current shutdown mode
Improved pop & click circuitry eliminates noises during
turn-on and turn-off transitions
■ No output coupling capacitors, snubber networks,
bootstrap capacitors, or gain-setting resistors required
■ Shutdown either channel independently
■ Soft start feature reduces start up transient current
■
■
■
■
■
Applications
■
■
■
■
■
Mobile Phones
MP3 Players
PDAs
Portable electronic devices
Notebook PCs
Boomer® is a registered trademark of National Semiconductor Corporation.
© 2007 National Semiconductor Corporation
202023
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LM48820 Ground-Referenced, Ultra Low Noise, Fixed Gain, 95mW Stereo Headphone Amplifier
June 2007
LM48820
Typical Application
202023b8
FIGURE 1. Typical Audio Amplifier Application Circuit
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2
LM48820
Connection Diagrams
micro SMD Package
14 – Bump TM Marking
20202378
Top View
XY – Date Code
TT – Lot Traceability
G – Boomer Family
I7 – LM48820TM
20202309
Top View
Order Number LM48820TM
See NS Package Number TME14AAA
TME14 Package View
20202397
3
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LM48820
Pin Descriptions
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Pin
Name
A1
RIN
Function
Right Channel Input
A2
SGND
Signal Ground
A3
CPVDD
Charge Pump Power Supply
A4
CCP+
B1
SD_RC
Positive Terminal - Charge Pump Flying Capacitor
Active-Low Shutdown, Right Channel
B2
SD_LC
Active-Low Shutdown, Left Channel
Power Ground
B4
PGND
C1
LIN
C2
ROUT
Right Channel Output
C4
CCP-
Negative Terminal - Charge Pump Flying Capacitor
D1
AVDD
Positive Power Supply - Amplifier
D2
LOUT
Left Channel Output
D3
-AVDD
D4
VCP_OUT
Left Channel Input
Negative Power Supply - Amplifier
Charge Pump Power Output
4
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage
Storage Temperature
Input Voltage
Power Dissipation (Note 3)
ESD Susceptibility (Note 4)
ESD Susceptibility (Note 5)
θJA (Note 9)
86°C/W (typ)
Operating Ratings
4.75V
−65°C to +150°C
-0.3V to VDD + 0.3V
Internally Limited
2000V
200V
Electrical Characteristics VDD = 3V
150°C
Temperature Range
TMIN ≤ TA ≤ TMAX
−40°C ≤ TA ≤ 85°C
1.6V ≤ VDD ≤ 4.5V
Supply Voltage (VDD)
(Notes 1, 2)
The following specifications apply for VDD = 3V, 16Ω load, and the conditions shown in “Typical Audio Amplifier Application Circuit” (see Figure 1) unless otherwise specified. Limits apply to TA = 25°C.
LM48820
Symbol
IDD
ISD
Parameter
Quiescent Power Supply Current
Full Power Mode
Conditions
Typical
(Note 6)
Limit
(Notes 7, 8)
VIN = 0V, inputs terminated
both channels enabled
4.7
5.5
VIN = 0V, inputs terminated
one channel enabled
3
Shutdown Current
SD_LC = SD_RC = GND
VOS
Output Offset Voltage
RL = 32Ω, VIN = 0V
AV
Voltage Gain
ΔAV
Gain Match
RIN
PO
THD+N
Total Harmonic Distortion + Noise
1
2
µA (max)
5
mV (max)
V/V
1
20
mA (max)
mA (max)
–1.5
Input Resistance
Output Power
0.05
Units
(Limits)
%
15
25
kΩ (min)
kΩ (max)
THD+N = 1% (max); f = 1kHz,
one channel
95
mW
THD+N = 1% (max); f = 1kHz,
RL = 32Ω, one channel
80
mW
THD+N = 1% (max); f = 1kHz,
two channels in phase
50
40
mW (min)
THD+N = 1% (max); f = 1kHz,
RL = 32Ω, two channels in phase
55
45
mW (min)
PO = 60mW, f = 1kHz,
single channel
0.01
%
PO = 50mW, f = 1kHz, RL = 32Ω
single channel
0.007
%
80
75
58
dB
dB
dB
100
dB
PSRR
Power Supply Rejection Ratio
Full Power Mode
VRIPPLE = 200mVP-P, Input Referred
f = 217Hz
f = 1kHz
f = 20kHz
SNR
Signal-to-Noise Ratio
RL = 32Ω, PO = 20mW,
(A-weighted)
f = 1kHz, BW = 20Hz to 22kHz
VIH
Shutdown Input Voltage High
VDD = 1.8V to 4.2V
VIL
Shutdown Input Voltage Low
VDD = 1.8V to 4.2V
XTALK
Crosstalk
PO = 1.6mW, f = 1kHz
70
ZOUT
Output Impedance
SD_LC = SD_RC = GND
Input Terminated
Input not terminated
30
30
5
1.2
V (min)
0.45
V (max)
dB
25
kΩ (min)
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LM48820
Junction Temperature
Thermal Resistance
Absolute Maximum Ratings (Notes 1, 2)
LM48820
LM48820
Symbol
Parameter
Conditions
Typical
(Note 6)
Limit
(Notes 7, 8)
8
2
Units
(Limits)
SD_LC = SD_RC = GND
ZOUT
Output Impedance
IL
Input Leakage
–500mV ≤ VOUT ≤ VDD +500mV
(Note 10)
±0.1
kΩ (min)
nA
Note 1: All voltages are measured with respect to the GND pin unless otherwise specified.
Note 2: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
functional but do not guarantee specific performance limits. Electrical Characteristics state DC and AC electrical specifications under particular test conditions
that guarantee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where
no limit is given; however, the typical value is a good indication of device performance.
Note 3: The maximum power dissipation must be derated at elevated temperatures and is dictated by TJMAX, θJA, and the ambient temperature, TA. The maximum
allowable power dissipation is PDMAX = (TJMAX - TA) / θJA or the number given in Absolute Maximum Ratings, whichever is lower. See power dissipation curves
for more information
Note 4: Human body model, 100pF discharged through a 1.5kΩ resistor.
Note 5: Machine Model, 220pF - 240pF discharged through all pins.
Note 6: Typicals are measured at 25°C and represent the parametric norm.
Note 7: Limits are guaranteed to National's AOQL (Average Outgoing Quality Level).
Note 8: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.
Note 9: θJA value is measured with the device mounted on a PCB with a 3” x 1.5”, 1oz copper heatsink.
Note 10: VOUT refers to signal applied to the LM48820 outputs.
External Components Description
(Figure 1)
Components
1.
CINR/INL
Functional Description
Input coupling capacitor which blocks the DC voltage at the amplifier's input terminals. Also creates a high-pass
filter with Ri at fC = 1/(2πRiCIN). Refer to the section Proper Selection of External Components, for an explanation
of how to determine the value of Ci.
2
CC
Flying capacitor. Low ESR ceramic capacitor (≤100mΩ)
3.
CSS
Output capacitor. Low ESR ceramic capacitor (≤100mΩ)
4.
CS1
Tantalum capacitor. Supply bypass capacitor which provides power supply filtering. Refer to the Power Supply
Bypassing section for information concerning proper placement and selection of the supply bypass capacitor.
5.
CS2
Ceramic capacitor. Supply bypass capacitor which provides power supply filtering. Refer to the Power Supply
Bypassing section for information concerning proper placement and selection of the supply bypass capacitor.
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THD+N vs Frequency
VDD = 1.6V, RL = 16Ω, Stereo, PO = 3mW
THD+N vs Frequency
VDD = 1.6V, RL = 32Ω, Stereo, PO = 3mW
202023a1
202023a0
THD+N vs Frequency
VDD = 3V, RL = 16Ω, Stereo, PO = 25mW
THD+N vs Frequency
VDD = 3V, RL = 32Ω, Stereo, PO = 25mW
202023a3
202023a5
THD+N vs Frequency
VDD = 3V, RL = 16Ω, One channel, PO = 60mW
THD+N vs Frequency
VDD = 3V, RL = 32Ω, One channel, PO = 50mW
202023a4
202023a2
7
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LM48820
Typical Performance Characteristics
LM48820
THD+N vs Output Power
VDD = 1.6V, RL = 16Ω, One channel
THD+N vs Output Power
VDD = 1.6V, RL = 32Ω, One channel
20202321
20202322
THD+N vs Output Power
VDD = 1.6V, RL = 32Ω, Stereo
THD+N vs Output Power
VDD = 1.6V, RL = 16Ω, Stereo
20202323
20202324
THD+N vs Output Power
VDD = 3V, RL = 16Ω, One channel
THD+N vs Output Power
VDD = 3V, RL = 32Ω, One channel
20202326
20202398
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LM48820
THD+N vs Output Power
VDD = 3V, RL = 16Ω, Stereo
THD+N vs Output Power
VDD = 3V, RL = 32Ω, Stereo
20202327
20202350
Output Power vs Power Supply Voltage
RL = 16Ω, f = 1kHz, Stereo
Output Power vs Power Supply Voltage
RL = 16Ω, f = 1kHz, Mono
20202372
20202371
Output Power vs Power Supply Voltage
RL = 32Ω, f = 1kHz, Mono
Output Power vs Power Supply Voltage
RL = 32Ω, f = 1kHz, Stereo
20202374
20202375
9
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LM48820
Power Dissipation vs Output Power
VDD = 1.6V, RL = 16Ω, f = 1kHz
Power Dissipation vs Output Power
VDD = 1.6V, RL = 32Ω, f = 1kHz
20202389
20202390
Power Dissipation vs Output Power
VDD = 3V, RL = 32Ω, f = 1kHz
Power Dissipation vs Output Power
VDD = 3V, RL = 16Ω, f = 1kHz
20202392
20202391
PSRR vs Frequency
VDD = 1.6V, RL = 16Ω
PSRR vs Frequency
VDD = 1.6V, RL = 32Ω
20202366
20202364
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LM48820
PSRR vs Frequency
VDD = 3V, RL = 16Ω
PSRR vs Frequency
VDD = 3V, RL = 32Ω
20202367
20202368
Power Supply Current vs Power Supply Voltage
VIN = 0V, Mono
Power Supply Current vs Power Supply Voltage
VIN = 0V, Stereo
20202376
20202377
11
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LM48820
amplifier. Thus the maximum package dissipation point is
56mW. The maximum power dissipation point obtained must
not be greater than the power dissipation that results from
Equation 3:
Application Information
SUPPLY VOLTAGE SEQUENCING
Before applying any signal to the inputs or shutdown pins of
the LM48820, it is important to apply a supply voltage to the
VDD pins. After the device has been powered, signals may be
applied to the shutdown pins (see MICRO POWER SHUTDOWN) and input pins.
PDMAX = (TJMAX - TA) / (θJA) (W)
For this micro SMD package, θJA = 86°C/W and TJMAX = 150°
C. Depending on the ambient temperature, TA, of the system
surroundings, Equation 3 can be used to find the maximum
internal power dissipation supported by the IC packaging. If
the result of Equation 1 is greater than that of Equation 1, then
either the supply voltage must be decreased, the load
impedance increased or TA reduced. For the typical application of a 3V power supply, with a 16Ω load, the maximum
ambient temperature possible without violating the maximum
junction temperature is approximately 127°C provided that
device operation is around the maximum power dissipation
point. Power dissipation is a function of output power and
thus, if typical operation is not around the maximum power
dissipation point, the ambient temperature may be increased
accordingly.
ELIMINATING THE OUTPUT COUPLING CAPACITOR
The LM48820 features a low noise inverting charge pump that
generates an internal negative supply voltage. This allows the
outputs of the LM48820 to be biased about GND instead of a
nominal DC voltage, like traditional headphone amplifiers.
Because there is no DC component, the large DC blocking
capacitors (typically 220µF) are not necessary. The coupling
capacitors are replaced by two, small ceramic charge pump
capacitors, saving board space and cost.
Eliminating the output coupling capacitors also improves low
frequency response. In traditional headphone amplifiers, the
headphone impedance and the output capacitor form a high
pass filter that not only blocks the DC component of the output, but also attenuates low frequencies, impacting the bass
response. Because the LM48820 does not require the output
coupling capacitors, the low frequency response of the device
is not degraded by external components.
In addition to eliminating the output coupling capacitors, the
ground referenced output nearly doubles the available dynamic range of the LM48820 when compared to a traditional
headphone amplifier operating from the same supply voltage.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply rejection. Applications that employ a 3V power supply typically use
a 4.7µF capacitor in parallel with a 0.1µF ceramic filter capacitor to stabilize the power supply output, reduce noise on
the supply line, and improve the supply's transient response.
Keep the length of leads and traces that connect capacitors
between the LM48820's power supply pin and ground as short
as possible.
OUTPUT TRANSIENT ('CLICK AND POPS') ELIMINATED
The LM48820 contains advanced circuitry that virtually eliminates output transients ('clicks and pops'). This circuitry prevents all traces of transients when the supply voltage is first
applied or when the part resumes operation after coming out
of shutdown mode.
MICRO POWER SHUTDOWN
The voltage applied to the SD_LC (shutdown left channel) pin
and the SD_RC (shutdown right channel) pin controls the
LM48820’s shutdown function. When active, the LM48820’s
micropower shutdown feature turns off the amplifiers’ bias
circuitry, reducing the supply current. The trigger point is
0.45V (max) for a logic-low level, and 1.2V (min) for logic-high
level. The low 0.05µA (typ) shutdown current is achieved by
applying a voltage that is as near as ground a possible to the
SD_LC/SD_RC pins. 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 microprocessor, or a microcontroller. When using a switch,
connect an external 100kΩ pull-up resistor between the
SD_LC/SD_RC pins and VDD. Connect the switch between
the SD_LC/SD_RC pins and ground. Select normal amplifier
operation by opening the switch. Closing the switch connects
the SD_LC/SD_RC pins to ground, activating micro-power
shutdown. The switch and resistor guarantee that the
SD_LC/SD_RC pins will not float. This prevents unwanted
state changes. In a system with a microprocessor or microcontroller, use a digital output to apply the control voltage to
the SD_LC/SD_RC pins. Driving the SD_LC/SD_RC pins
with active circuitry eliminates the pull-up resistor.
AMPLIFIER CONFIGURATION EXPLANATION
As shown in Figure 2, the LM48820 has two internal operational amplifiers. The two amplifiers have internally configured
gain, the closed loop gain is set by selecting the ratio of Rf to
Ri. Consequently, the gain for each channel of the IC is
AV = -(Rf / Ri) = 1.5 (V/V)
(1)
where RF = 30kΩ and Ri = 20kΩ.
POWER DISSIPATION
Power dissipation is a major concern when using any power
amplifier and must be thoroughly understood to ensure a successful design. Equation 1 states the maximum power dissipation point for a single-ended amplifier operating at a given
supply voltage and driving a specified output load.
PDMAX = (VDD) 2 / (2π2RL) (W)
(2)
Since the LM48820 has two operational amplifiers in one
package, the maximum internal power dissipation point is
twice that of the number which results from Equation 2. Even
with large internal power dissipation, the LM48820 does not
require heat sinking over a large range of ambient temperatures. From Equation 2, assuming a 3V power supply and a
16Ω load, the maximum power dissipation point is 28mW per
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(3)
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM48820's performance requires properly selecting external components. Though the LM48820 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component values.
12
f–3dB = 1 / 2πRiCIN (Hz)
(4)
Also, careful consideration must be taken in selecting a certain type of capacitor to be used in the system. Different types
of capacitors (tantalum, electrolytic, ceramic) have unique
performance characteristics and may affect overall system
performance.
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitors (CINL and CINR in Figure 1). A high
value capacitor can be expensive and may compromise
13
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LM48820
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.
Applications using speakers with this limited frequency response reap little improvement by using high value input and
output capacitors.
Besides affecting system cost and size, the input coupling
capacitor has an effect on the LM48820's click and pop performance. The magnitude of the pop is directly proportional
to the input capacitor's size. Thus, pops can be minimized by
selecting an input capacitor value that is no higher than necessary to meet the desired −3dB frequency.
As shown in Figure 1, the internal input resistor, Ri and the
input capacitor, CINL and CINR, produce a -3dB high pass filter
cutoff frequency that is found using Equation (4).
Charge Pump Capacitor Selection
Use low (<100mΩ) ESR (equivalent series resistance) ceramic capacitors with an X7R dielectric for best performance.
Low ESR capacitors keep the charge pump output
impedance to a minimum, extending the headroom on the
negative supply. Higher ESR capacitors result in reduced
output power from the audio amplifiers.
Charge pump load regulation and output impedance are affected by the value of the flying capacitor (CC). A larger valued
CC (up to 3.3μF) improves load regulation and minimizes
charge pump output resistance. The switch-on resistance
dominates the output impedance for capacitor values above
2.2μF.
The output ripple is affected by the value and ESR of the output capacitor (CSS). Larger capacitors reduce output ripple on
the negative power supply. Lower ESR capacitors minimize
the output ripple and reduce the output impedance of the
charge pump.
The LM48820 charge pump design is optimized for 2.2μF, low
ESR, ceramic, flying, and output capacitors.
LM48820
Revision History
Rev
Date
1.0
05/09/07
Initial release.
1.1
05/15/07
Added the BOM table.
1.2
06/25/07
Deleted and replaced some curves. Input text edits also.
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Description
14
LM48820
Physical Dimensions inches (millimeters) unless otherwise noted
14 – Bump micro SMD
Order Number LM48820TM
NS Package Number TME14AAA
X1 = X2 = 1.615±0.03mm, X3 = 0.600±0.075mm,
15
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LM48820 Ground-Referenced, Ultra Low Noise, Fixed Gain, 95mW Stereo Headphone Amplifier
Notes
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