LM4875
750 mW Audio Power Amplifier with DC Volume Control
and Headphone Switch
General Description
The LM4875 is a mono bridged audio power amplifier with
DC voltage volume control. The LM4875 is capable of deliv-
ering 750mW of continuous average power into an 8
load
with less than 1% THD when powered by a 5V power supply.
Switching between bridged speaker mode and headphone
(single ended) mode is accomplished using the headphone
sense pin. To conserve power in portable applications, the
LM4875's micropower shutdown mode (I
Q
= 0.7A, typ) is
activated when less than 300mV is applied to the DC Vol/SD
pin.
Boomer audio power amplifiers are designed specifically to
provide high power audio output while maintaining high fidel-
ity. They require few external components and operate on
low supply voltages.
Applications
n
GSM phones and accessories, DECT, office phones
n
Hand held radio
n
Other portable audio devices
Key Specifications
j
P
O
at 1.0% THD+N into 8
750mW (typ)
j
P
O
at 10% THD+N into 8
1W (typ)
j
Shutdown current
0.7A(typ)
j
Supply voltage range
2.7V to 5.5V
Features
n
Precision DC voltage volume control
n
Headphone amplifier mode
n
"Click and pop" suppression
n
Shutdown control when volume control pin is low
n
Thermal shutdown protection
Typical Application
Connection Diagram
BOOMER
TM
is a trademark of National Semiconductor Corporation.
DS101042-1
FIGURE 1. Typical Audio Amplifier
Application Circuit
Small Outline Package (SO)
Mini Small Outline Package (MSOP)
DS101042-2
Top View
Order Number LM4875M, LM4875MM
See NS Package Number M08A, MUA08A
January 2002
LM4875
750
mW
Audio
Power
Amplifier
with
DC
V
olume
Control
and
Headphone
Switch
2002 National Semiconductor Corporation
DS101042
www.national.com
Absolute Maximum Ratings
(Note 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage
6.0V
Storage Temperature
-65C to +150C
Input Voltage
-0.3V to V
DD
+0.3V
Power Dissipation (Note 3)
Internally Limited
ESD Susceptibility (Note 4)
2000V
ESD Susceptibility (Note 5)
200V
Junction Temperature
150C
Soldering Information
Vapor Phase (60 sec.)
215C
Infrared (15 sec.)
220C
Thermal Resistanc
JC
(SOP)
35C/W
JA
(SOP)
150C/W
JC
(MSOP)
56C/W
JA
(MSOP)
190C/W
Operating Ratings
Temperature Range
T
MIN
T
A
T
MAX
-40C
T
A
+85C
Supply Voltage
2.7V
V
DD
5.5V
See AN-450 "Surface Mounting and their Effects on Product
Reliability" for other methods of soldering surface mount
devices.
Electrical Characteristics
(Notes 1, 2)
The following specifications apply for V
DD
= 5V, unless otherwise specified. Limits apply for T
A
= 25C.
Symbol
Parameter
Conditions
LM4875
Min
(Note 7)
Typical
(Note 6)
Max
(Note 7)
Units
V
DD
Supply Voltage
2.7
5.5
V
I
DD
Quiescent Power Supply
Current
V
IN
= 0V, I
O
= 0A, HP Sense = 0V
4
7
mA
V
IN
= 0V, I
O
- 0A, HP Sense = 5V
3.5
6
mA
I
SD
Shutdown Current
V
PIN4
0.3V
0.7
A
V
OS
Output Offset Voltage
V
IN
= 0V
5
50
mV
P
O
Output Power
THD = 1% (max), HP Sense
<
0.8V,
f = 1kHz, R
L
= 8
500
750
mW
THD = 10% (max), HP Sense
<
0.8V,
f = 1kHz, R
L
= 8
1.0
W
THD + N = 1%, HP Sense
>
4V,
f = 1kHz, R
L
= 32
80
mW
THD = 10%, HP Sense
>
4V,
f = 1kHz, R
L
= 32
110
mW
THD+N
Total Harmonic Distortion +
Noise
P
O
= 300 mWrms, f = 20Hz20kHz,
R
L
= 8
0.6
%
PSRR
Power Supply Rejection Ratio
V
RIPPLE
= 200mVrms, R
L
= 8
, C
B
=
1.0 F, f = 1kHz
50
dB
Gain
RANGE
Single-Ended Gain Range
Gain with V
PIN4
4.0V, (80% of V
DD
)
18.8
20
dB
Gain with V
PIN4
0.9V, (20% of V
DD
)
-70
-72
dB
V
IH
HP Sense High Input Voltage
4
V
V
IL
HP Sense Low Input Voltage
0.8
V
Note 1: All voltages are measured with respect to the ground 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 operates within the Operating Ratings. Specifications are not guaranteed for parameters
where no limit is given. The typical value, however, is a good indication of device performance.
Note 3: The maximum power dissipation must be derated at elevated temperatures and is dictated by T
JMAX
,
JA
, and the ambient temperature T
A
. The maximum
allowable power dissipation is P
DMAX
= (T
JMAX
- T
A
)/
JA
or the number given in the Absolute Maximum Ratings, whichever is lower. For the LM4875M, T
JMAX
=
150C.
Note 4: Human body model, 100pF discharged through a 1.5k
resistor.
Note 5: Machine Model, 220pF240pF discharged through all pins.
Note 6: Typicals are measured at 25C and represent the parametric norm.
Note 7: Limits are guaranteed to National's AOQL (Average Outgoing Quality Level).
Note 8: The quiescent power supply current depends on the offset voltage when a practical load is connected to the amplifier.
LM4875
www.national.com
2
External Components Description
(
Figure 1)
Components
Functional Description
1.
C
i
Input coupling capacitor blocks DC voltage at the amplifier's input terminals. It also creates a highpass
filter with the internal R
i
that produces an f
c
= 1/(2
R
i
C
i
) (10k
R
i
100k
). Refer to the Application
Information section, Selecting External Components, for an explanation of determining the value of C
i
.
2.
C
S
The supply bypass capacitor. Refer to the Power Supply Bypassing section for information about
properly placing, and selecting the value of, this capacitor.
3.
C
B
The capacitor, C
B
, filters the half-supply voltage present on the BYPASS pin. Refer to the Application
Information section, Selecting External Components, for information concerning proper placement and
selecting C
B
's value.
Typical Performance Characteristics
THD+N vs Frequency
DS101042-5
THD+N vs Frequency
DS101042-6
THD+N vs Output Power
DS101042-7
THD+N vs Output Power
DS101042-8
THD+N vs Output Power
DS101042-10
THD+N vs Output Power
DS101042-11
Power Dissipation vs Load
Resistance
DS101042-12
Power Dissipation vs Output Power
DS101042-13
Power Derating Curve
DS101042-14
LM4875
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3
Typical Performance Characteristics
(Continued)
Clipping Voltage vs RL
DS101042-15
Noise Floor
DS101042-16
Frequency Response vs
Input Capacitor Size
DS101042-17
Power Supply
Rejection Ratio
DS101042-18
Attenuation Level vs
DC-Vol Amplitude
DS101042-19
THD+N vs Frequency
DS101042-20
THD+N vs Frequency
DS101042-21
THD+N vs Frequency
DS101042-22
THD+N vs Output Power
DS101042-23
LM4875
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4
Typical Performance Characteristics
(Continued)
Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in
Figure 1, the LM4875 consists of two opera-
tional amplifiers internally. An external DC voltage sets the
closed-loop gain of the first amplifier, whereas two internal
20k
resistors set the second amplifier's gain at -1. The
LM4875 can be used to drive a speaker connected between
the two amplifier outputs or a monaural headphone con-
nected between V
O
1 and GND.
Figure 1 shows that the output of Amp1 serves as the input
to Amp2. This results in both amplifiers producing signals
that are identical in magnitude, but 180 out of phase.
Taking advantage of this phase difference, a load placed
between V
O
1 and V
O
2 is driven differentially (commonly
referred to as "bridge mode" ). This mode is different from
single-ended driven loads that are connected between a
single amplifier's output and ground.
Bridge mode has a distinct advantage over the single-ended
configuration: its differential drive to the load doubles the
output swing for a specified supply voltage. This results in
four times the output power when compared to a
single-ended amplifier under the same conditions. This in-
crease in attainable output assumes that the amplifier is not
current limited or the output signal is not clipped. To ensure
minimum output signal clipping when choosing an amplifier's
closed-loop gain, refer to the Audio Power Amplifier De-
sign section.
THD+N vs Output Power
DS101042-24
THD+N vs Output Power
DS101042-28
Output Power vs Load Resistance
DS101042-29
Clipping Voltage vs Supply Voltage
DS101042-30
Output Power vs Supply Voltage
DS101042-31
Output Power vs Supply Voltage
DS101042-32
Supply Current vs Supply Voltage
DS101042-33
LM4875
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5
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