LM4985
Stereo 135mW Low Noise Headphone Amplifier with
Selectable Capacitively Coupled or Output
Capacitor-less (OCL) Output and Digitally Controlled
(I
2
C) Volume Control
General Description
The LM4985 is a stereo audio power amplifier with internal
digitally controlled volume control. This amplifier is capable
of delivering 68mW
RMS
per channel into a 16Ωload or
38mW
RMS
per channel into a 32Ωload at 1% THD when
powered by a 3.6V power supply and operating in the OCL
mode.
Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components. To that end, the LM4985 features two
functions that optimize system cost and minimize PCB area:
an integrated, digitally controlled (I
2
C bus) volume control
and an operational mode that eliminates output signal cou-
pling capacitors (OCL mode). Since the LM4985 does not
require bootstrap capacitors, snubber networks, or output
coupling capacitors, it is optimally suited for low-power, bat-
tery powered portable systems. For added design flexibility,
the LM4985 can also be configured for single-ended capaci-
tively coupled outputs.
The LM4985 features a current shutdown mode for mi-
cropower dissipation and thermal shutdown protection.
Key Specifications (V
DD
= 3.6V)
jPSRR: 217Hz and 1kHz
Output Capacitor-less (OCL)
f
RIPPLE
= 217Hz 77dB (typ)
f
RIPPLE
= 1kHz 76dB (typ)
Capacitor Coupled (C-CUPL)
f
RIPPLE
= 217Hz 63dB (typ)
f
RIPPLE
= 1kHz 62dB (typ)
jOutput Power per channel
(f
IN
= 1kHz, THD+N = 1%),
R
L
=16Ω,OCL
V
DD
= 2.5V 31mW (typ)
V
DD
= 3.6V 68mW (typ)
V
DD
= 5.0V 135mW (typ)
jTHD+N (f = 1kHz)
R
LOAD
=16Ω, OCL, P
OUT
= 60mW 0.60
R
LOAD
=32Ω, OCL, P
OUT
= 33mW 0.031
jShutdown Current 0.1µA (typ)
Features
nOCL or capacitively coupled outputs (patent pending)
nI
2
C Digitally Controlled Volume Control
nAvailable in space-saving 0.4mm lead-pitch micro SMD
package
nVolume control range: –76dB to +18dB
nUltra low current shutdown mode
n2.3V - 5.5V operation
nUltra low noise
Applications
nMobile Phones
nPDAs
nPortable electronics devices
nMP3 Players
Boomer®is a registered trademark of National Semiconductor Corporation.
May 2006
LM4985 Stereo 135mW Low Noise Headphone Amplifier with Selectable Capacitive Coupled or
Capacitor-less (OCL) Output and Digitally Controlled (I
2
C) Volume Control
© 2006 National Semiconductor Corporation DS201697 www.national.com
Block Diagram
201697E9
FIGURE 1. Block Diagram
LM4985
www.national.com 2
Typical Application
201697F0
FIGURE 2. Typical Capacitively Coupled Output Configuration Circuit
201697F1
FIGURE 3. Typical OCL Output Configuration Circuit
LM4985
www.national.com3
Connection Diagrams
micro SMD Package micro SMD Marking
20169715
Top View
Order Number LM4985TM
See NS Package Number TMD12AAA
20169730
Top View
X – Date Code
T – Die Traceability
G – Boomer Family
H2 – LM4985TM
Pin Reference, Name, and Function
Reference Name Function
A1 ADR I
2
C serial interface address input.
A2 IN2 Analog signal input two.
A3 OUT2 Power amplifier two output.
B1 SDA I
2
C serial interface data input.
B2 BYPASS The internal V
DD
/2 ac bypass node.
B3 CNTGND In OCL mode, this is the ac ground
return. It is biased to V
DD
/2. Leave
unconnected for C-CUPL mode.
C1 SCL I2C serial interface clock input.
C2 GND The LM4985’s power supply ground
input.
C3 V
DD
The LM4985’s power supply voltage
input.
D1 I
2
CV
DD
I
2
C serial interface power supply
input. Can be connected to the same
supply that is connected to the V
DD
pin.
D2 IN1 Analog signal input one.
D3 OUT1 Power amplifier one output.
LM4985
www.national.com 4
Absolute Maximum Ratings (Notes 1, 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage (V
DD
,I
2
CV
DD
) 6.0V
Storage Temperature −65˚C to +150˚C
Input Voltage (IN1, IN2, OUT1,
OUT2, BYPASS, CNTGND, GND
pins relative to the V
DD
pin) -0.3V to V
DD
+ 0.3V
Input Voltage (ADR, SDA, SCL
pins, relative to the I
2
CV
DD
pin) -0.3V to I
2
CV
DD
+ 0.3V
Power Dissipation (Note 3) Internally Limited
ESD Susceptibility (Note 4) 2000V
ESD Susceptibility (Note 5) 200V
Junction Temperature 150˚C
Thermal Resistance
θ
JA
109˚C/W
Operating Ratings
Temperature Range
T
MIN
≤T
A
≤T
MAX
−40˚C ≤T
A
≤85˚C
Supply Voltage
V
DD
2.3V ≤V
CC
≤5.5V
I
2
CV
DD
1.7V ≤I
2
CV
DD
≤5.5V
Electrical Characteristics V
DD
=5V(Notes 1, 2)
The following specifications apply for R
L
=16Ω, f = 1kHz, and C
B
= 4.7µF unless otherwise specified. Limits apply to T
A
=
25˚C.
Symbol Parameter Conditions LM4985 Units
(Limits)
Typ
(Note 6)
Limit
(Notes 7,
8)
I
DD
Quiescent Power Supply Current
V
IN
= 0V, I
OUT
=0A
Single-Channel no load OCL
Single-Channel no load C-CUPL
Dual-Channel no load OCL
Dual-Channel no load C-CUPL
2
1.5
3
2.3
4.9
3.8
mA (max)
I
SD
Shutdown Current V
SHUTDOWN
= GND 0.1 1.0 µA (max)
V
SDIH
Logic Voltage Input High 3.5 V (min)
V
SDIL
Logic Voltage Input Low 1.5 V (max)
P
O
Output Power
THD ≤1%; f
IN
= 1kHz
R
LOAD
=16ΩOCL 135 115
R
LOAD
=16ΩC-CUPL 135 mW (min)
R
LOAD
=32ΩOCL 79 70
R
LOAD
=32ΩC-CUPL 80
THD+N Total Harmonic Distortion + Noise
R
LOAD
=16ΩOCL, P
O
= 100mW
R
LOAD
=16ΩC-CUPL, P
O
= 100mW
R
LOAD
=32ΩOCL, P
O
= 60mW
R
LOAD
=32ΩC-CUPL, P
O
= 70mW
0.08
0.02
0.04
0.01
%
V
ON
Output Noise Voltage V
IN
= AC GND, A
V
= 0dB, A-weighted 15 µV
PSRR Power Supply Rejection Ratio
V
RIPPLE
= 200mVp-p (Note 9)
f
IN
= 217Hz sinewave
OCL
C-CUPL
77
65
57
dB (min)
f
IN
= 1kHz sinewave
OCL
C-CUPL
77
65
60
Xtalk Channel-to-channel Crosstalk
P
out
= 40mW. OCL
R
LOAD
=16Ω
R
LOAD
=32Ω
51
56
dB
P
out
= 50mW. C-CUPL
R
LOAD
=16Ω
R
LOAD
=32Ω
58
68
dB
LM4985
www.national.com5
Electrical Characteristics V
DD
=5V(Notes 1, 2) (Continued)
The following specifications apply for R
L
=16Ω, f = 1kHz, and C
B
= 4.7µF unless otherwise specified. Limits apply to T
A
=
25˚C.
Symbol Parameter Conditions LM4985 Units
(Limits)
Typ
(Note 6)
Limit
(Notes 7,
8)
T
WU
Wake Up Time form Shutdown
C
BYPASS
= 4.7µF (Note 11)
WT1=0,WT0=0
OCL
C-CUPL
75
285
msec
WT1=0,WT0=1
OCL
C-CUPL
110
530
WT1=1,WT0=0
OCL
C-CUPL
180
1030
WT1=1,WT0=1
OCL
C-CUPL
320
2050
R
IN
Input Resistance Stereo mode
Mono mode
20
10 kΩ
A
VMIN
Minimum Gain Code = 00000 –76 dB (min)
A
VMAX
Maximum Gain Code = 11111 18 dB (min)
∆A
V
Gain Accuracy per Step 18dB ≥A
V
≥–44dB
–44dB ≥A
V
>–76dB
±0.5
±1.0 dB
V
OS
Output Offset Voltage
OCL
R
LOAD
=32Ω
V
IN
=ACGND
2.0 20 mV (max)
Electrical Characteristics V
DD
= 3.6V (Notes 1, 2)
The following specifications apply for R
L
=16Ω, f = 1kHz, and C
B
= 4.7µF unless otherwise specified. Limits apply to T
A
=
25˚C.
Symbol Parameter Conditions LM4985 Units
(Limits)
Typ
(Note 6)
Limit
(Notes 7,
8)
I
DD
Quiescent Power Supply Current
V
IN
= 0V, I
OUT
=0A
Single-Channel no load OCL 1.8 3.1
mA (max)
Single-Channel no load C-CUPL 1.0
Dual-Channel no load OCL 2.1 4
Dual-Channel no load C-CUPL 2.3 3
I
SD
Shutdown Current V
SHUTDOWN
= GND 0.1 1.0 µA (max)
V
SDIH
Logic Voltage Input High 2.52 V (min)
V
SDIL
Logic Voltage Input Low 1.08 V (max)
P
O
Output Power
THD+N <1%, f
IN
= 1kHz
R
LOAD
=16ΩOCL 68 60
R
LOAD
=16ΩC-CUPL 70 mW (min)
R
LOAD
=32ΩOCL 38 34
R
LOAD
=32ΩC-CUPL 41
LM4985
www.national.com 6
Electrical Characteristics V
DD
= 3.6V (Notes 1, 2) (Continued)
The following specifications apply for R
L
=16Ω, f = 1kHz, and C
B
= 4.7µF unless otherwise specified. Limits apply to T
A
=
25˚C.
Symbol Parameter Conditions LM4985 Units
(Limits)
Typ
(Note 6)
Limit
(Notes 7,
8)
THD+N Total Harmonic Distortion + Noise
R
LOAD
=16ΩOCL, P
O
= 60mW
R
LOAD
=16ΩC-CUPL, P
O
= 60mW
R
LOAD
=32ΩOCL, P
O
= 33mW
R
LOAD
=32ΩC-CUPL, P
O
= 38mW
0.06
0.03
0.03
0.03
%
V
ON
Output Noise Voltage V
IN
= AC GND, A
V
= 0dB, A-weighted 15 µV
PSRR Power Supply Rejection Ratio
V
RIPPLE
= 200mVp-p (Note 9)
f
IN
= 217Hz sinewave
OCL
C-CUPL
77
63
55
dB (min)
f
IN
= 1kHz sinewave
OCL
C-CUPL
76
62
57
Xtalk Channel-to-Channel Crosstalk
P
out
= 40mW. OCL
R
LOAD
=16Ω
R
LOAD
=32Ω
51
56
dB
P
out
= 50mW. C-CUPL
R
LOAD
=16Ω
R
LOAD
=32Ω
58
69
dB
T
WU
Wake Up Time from Shutdown
C
BYPASS
= 4.7µF (Note 11)
WT1=0,WT0=0
OCL
C-CUPL
66
222
93
msec
WT1=0,WT0=1
OCL
C-CUPL
92
405
WT1=1,WT0=0
OCL
C-CUPL
143
774
WT1 = 1, WT0 =1
OCL
C-CUPL
246
1532
R
IN
Input Resistance Stereo mode
Mono mode
20
10 kΩ
A
VMIN
Minimum Gain Code = 00000 –76 –72 dB (max)
A
VMAX
Maximum Gain Code = 11111 18 17 dB (min)
∆A
V
Gain Accuracy per Step 18dB ≥A
V
≥–44dB
–44dB ≥A
V
>–76dB
±0.5
±1.0
±1.0
±2.0 dB
V
OS
Output Offset Voltage
OCL
R
LOAD
=32Ω
V
IN
=ACGND
2.0 20 mV (max)
LM4985
www.national.com7
Electrical Characteristics V
DD
= 2.5V (Notes 1, 2)
The following specifications apply for R
L
=16Ω, f = 1kHz, and C
B
= 4.7µF unless otherwise specified. Limits apply to T
A
=
25˚C.
Symbol Parameter Conditions LM4985 Units
(Limits)
Typ
(Note 6)
Limit
(Notes 7,
8)
I
DD
Quiescent Power Supply Current
V
IN
= 0V, I
OUT
=0A
Single-Channel no load OCL
Single-Channel no load C-CUPL
Dual-Channel no load OCL
Dual-Channel no load C-CUPL
1.6
1
2.1
1.6
mA
I
SD
Shutdown Current V
SHUTDOWN
= GND 0.1 µA
V
SDIH
Logic Voltage Input High 1.75 V (min)
V
SDIL
Logic Voltage Input Low 0.75 V (max)
P
O
Output Power
THD+N <1%, f
IN
= 1kHz
R
LOAD
=16ΩOCL
R
LOAD
=16ΩC-CUPL
R
LOAD
=32ΩOCL
R
LOAD
=32ΩC-CUPL
31
33
19
19
mW
THD+N Total Harmonic Distortion + Noise
R
LOAD
=16ΩOCL, P
O
= 26mW
R
LOAD
=16ΩC-CUPL, P
O
= 20mW
R
LOAD
=32ΩOCL, P
O
= 16mW
R
LOAD
=32ΩC-CUPL, P
O
= 15mW
0.07
0.05
0.06
0.04
%
V
ON
Output Noise Voltage V
IN
= AC GND, A
V
= 0dB, A-weighted 10 µV
PSRR Power Supply Rejection Ratio
V
RIPPLE
= 200mVp-p (Note 9)
f
IN
= 217Hz sinewave
OCL
C-CUPL
75
59 dB
f
IN
= 1kHz sinewave
OCL
C-CUPL
75
59
Xtalk Channel-to-Channel Crosstalk
P
out
= 20mW, OCL
R
LOAD
=16Ω
R
LOAD
=32Ω
50
55
dB
P
out
= 20mW. C-CUPL
R
LOAD
=16Ω
R
LOAD
=32Ω
58
67
dB
T
WU
Wake Up Time from Shutdown
C
BYPASS
= 4.7µF (Note 11)
WT1=0,WT0=0
OCL
C-CUPL
66
214
msec
WT1=0,WT0=1
OCL
C-CUPL
92
544
WT1=1,WT0=0
OCL
C-CUPL
145
1053
WT1=1,WT0=1
OCL
C-CUPL
250
2098
R
IN
Input Resistance Stereo mode
Mono mode
20
10 kΩ
A
VMIN
Minimum Gain Code = 00000 –76 dB
LM4985
www.national.com 8
Electrical Characteristics V
DD
= 2.5V (Notes 1, 2) (Continued)
The following specifications apply for R
L
=16Ω, f = 1kHz, and C
B
= 4.7µF unless otherwise specified. Limits apply to T
A
=
25˚C.
Symbol Parameter Conditions LM4985 Units
(Limits)
Typ
(Note 6)
Limit
(Notes 7,
8)
A
VMAX
Maximum Gain Code = 11111 18 dB
∆A
V
Gain Accuracy per Step 18dB ≥A
V
≥–44dB
–44dB ≥A
V
>–76dB
±0.5
±1.0 dB
V
OS
Output Offset Voltage
OCL
R
LOAD
=32Ω
V
IN
=ACGND
2.0 mV
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 which
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 =(T
JMAX -T
A)/ θJA or the number given in Absolute Maximum Ratings, whichever is lower. For the LM4985, see power derating
currents for more information.
Note 4: Human Body Model: 100pF discharged through a 1.5kΩresistor.
Note 5: Machine Model: 200pF ≤Cmm ≤220pF 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: 10Ωterminated input.
Note 10: The LDA10A package has its exposed-DAP soldered to an exposed 1.2in2area of 1oz. Printed circuit board copper.
Note 11: The wake-up time (TWU) is calculated using the following formula; TWU =[C
BYPASS (VDD)/2(I
BYPASS)] + 40ms.
External Components Description (Figure 2)
Components Functional Description
1. C
I
Input coupling capacitor which blocks the DC voltage at the amplifier’s input terminals. Also creates a
high-pass filter with R
i
at f
c
= 1/(2πR
i
C
i
). Refer to the section Proper Selection of External Components, for
an explanation of how to determine the value of C
i
.
2. C
S
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.
3. C
B
Bypass pin capacitor which provides half-supply filtering. Refer to the section, Proper Selection of Proper
Components, for information concerning proper placement and selection of C
B
6. C
o
Output coupling capacitor which blocks the DC voltage at the amplifier’s output. Forms a high pass filter with
R
L
at f
o
= 1/(2πR
L
C
o
)
LM4985
www.national.com9
Typical Performance Characteristics
T
A
= 25˚C, A
V
= 0dB, f
IN
= 1kHz unless otherwise stated.
THD+N vs Frequency
V
DD
= 2.5V, R
L
=16Ω
P
OUT
= 20mW, C-CUPL
THD+N vs Frequency
V
DD
= 3.6V, R
L
=16Ω
P
OUT
= 50mW, C-CUPL
201697D1 201697D2
THD+N vs Frequency
V
DD
= 5V, R
L
=16Ω
P
OUT
= 50mW, C-CUPL
THD+N vs Frequency
V
DD
= 2.5V, R
L
=32Ω
P
OUT
= 15mW, C-CUPL
201697D3 201697D4
THD+N vs Frequency
V
DD
= 3.6V, R
L
=32Ω
P
OUT
= 35mW, C-CUPL
THD+N vs Frequency
V
DD
= 5.0V, R
L
=32Ω
P
OUT
= 60mW, C-CUPL
201697D5 201697D6
LM4985
www.national.com 10
Typical Performance Characteristics (Continued)
THD+N vs Frequency
V
DD
= 2.5V, R
L
=16Ω
P
OUT
= 20mW, OCL
THD+N vs Frequency
V
DD
= 3.6V, R
L
=16Ω
P
OUT
= 50mW, OCL
20169764 20169765
THD+N vs Frequency
V
DD
= 5.0V, R
L
=16Ω
P
OUT
= 50mW, OCL
THD+N vs Frequency
V
DD
= 2.5V, R
L
=32Ω
P
OUT
= 15mW, OCL
20169766
20169767
THD+N vs Frequency
V
DD
= 3.6V, R
L
=32Ω
P
OUT
= 35mW, OCL
THD+N vs Frequency
V
DD
= 5.0V, R
L
=32Ω
P
OUT
= 60mW, OCL
20169768 20169769
LM4985
www.national.com11
Typical Performance Characteristics (Continued)
THD+N vs Output Power
V
DD
= 2.5V, R
L
=16Ω
C-CUPL
THD+N vs Output Power
V
DD
= 3.6V, R
L
=16Ω
C-CUPL
201697H3 201697C6
THD+N vs Output Power
V
DD
= 5.0V, R
L
=16Ω
C-CUPL
THD+N vs Output Power
V
DD
= 2.5V, R
L
=32Ω
C-CUPL
201697C7
201697F2
THD+N vs Output Power
V
DD
= 3.6V, R
L
=32Ω
C-CUPL
THD+N vs Output Power
V
DD
= 5.0V, R
L
=32Ω
C-CUPL
201697F3
201697H4
LM4985
www.national.com 12
Typical Performance Characteristics (Continued)
THD+N vs Output Power
V
DD
= 2.5V, R
L
=16Ω
OCL
THD+N vs Output Power
V
DD
= 3.6V, R
L
=16Ω
OCL
20169758 20169759
THD+N vs Output Power
V
DD
= 5.0V, R
L
=16Ω
OCL
THD+N vs Output Power
V
DD
= 2.5V, R
L
=32Ω
OCL
20169760 20169761
THD+N vs Output Power
V
DD
= 3.6V, R
L
=32Ω
OCL
THD+N vs Output Power
V
DD
= 5.0V, R
L
=32Ω
OCL
20169762
20169763
LM4985
www.national.com13
Typical Performance Characteristics (Continued)
PSRR vs Frequency
V
DD
= 2.5V, R
L
=16Ω
V
RIPPLE
= 200mVpp, OCL
PSRR vs Frequency
V
DD
= 3.6V, R
L
=16Ω
V
RIPPLE
= 200mVpp, OCL
20169776 201697H5
PSRR vs Frequency
V
DD
= 5.0V, R
L
=16Ω
V
RIPPLE
= 200mVpp, OCL
PSRR vs Frequency
V
DD
= 2.5V, R
L
=32Ω
V
RIPPLE
= 200mVpp, OCL
201697H6 201697H7
PSRR vs Frequency
V
DD
= 3.6V, R
L
=32Ω
V
RIPPLE
= 200mVpp, OCL
PSRR vs Frequency
V
DD
= 5.0V, R
L
=32Ω
V
RIPPLE
= 200mVpp, OCL
201697H8 201697H9
LM4985
www.national.com 14
Typical Performance Characteristics (Continued)
PSRR vs Frequency
V
DD
= 2.5V, R
L
=16Ω
V
RIPPLE
= 200mVpp, C-CUPL
PSRR vs Frequency
V
DD
= 3.6V, R
L
=16Ω
V
RIPPLE
= 200mVpp, C-CUPL
201697I0 201697I1
PSRR vs Frequency
V
DD
= 5.0V, R
L
=16Ω
V
RIPPLE
= 200mVpp, C-CUPL
PSRR vs Frequency
V
DD
= 2.5V, R
L
=32Ω
V
RIPPLE
= 200mVpp, C-CUPL
201697I2 201697I3
PSRR vs Frequency
V
DD
= 3.6V, R
L
=32Ω
V
RIPPLE
= 200mVpp, C-CUPL
PSRR vs Frequency
V
DD
= 5.0V, R
L
=32Ω
V
RIPPLE
= 200mVpp, C-CUPL
201697I4 201697I5
LM4985
www.national.com15
Typical Performance Characteristics (Continued)
Crosstalk vs Frequency
V
DD
= 2.5V, R
L
=16Ω
P
OUT
= 20mW. OCL
Crosstalk vs Frequency
V
DD
= 3.6V, R
L
=16Ω
P
OUT
= 40mW, OCL
201697I6 201697G8
Crosstalk vs Frequency
V
DD
= 5.0V, R
L
=16Ω
P
OUT
= 40mW, OCL
Crosstalk vs Frequency
V
DD
= 2.5V, R
L
=32Ω
P
OUT
= 20mW, OCL
201697G9 201697H0
Crosstalk vs Frequency
V
DD
= 3.6V, R
L
=32Ω
P
OUT
= 40mW, OCL
Crosstalk vs Frequency
V
DD
= 5.0V, R
L
=32Ω
P
OUT
= 50mW, OCL
201697H1 201697H2
LM4985
www.national.com 16
Typical Performance Characteristics (Continued)
Crosstalk vs Frequency
V
DD
= 2.5V, R
L
=16Ω
P
OUT
= 20mW, C-CUPL
Crosstalk vs Frequency
V
DD
= 3.6V, R
L
=16Ω
P
OUT
= 50mW, C-CUPL
201697D7 201697D8
Crosstalk vs Frequency
V
DD
= 5.0V, R
L
=16Ω
P
OUT
= 50mW, C-CUPL
Crosstalk vs Frequency
V
DD
= 2.5V, R
L
=32Ω
P
OUT
= 20mW, C-CUPL
201697D9 201697E0
Crosstalk vs Frequency
V
DD
= 3.6V, R
L
=32Ω
P
OUT
= 50mW, C-CUPL
Crosstalk vs Frequency
V
DD
= 5.0V, R
L
=32Ω
P
OUT
= 50mW, C-CUPL
201697E1 201697E2
LM4985
www.national.com17
Typical Performance Characteristics (Continued)
Load Dissipation vs Amplifier Dissipation
V
DD
= 2.5V, C-CUPL
Load Dissipation vs Amplifier Dissipation
V
DD
= 3.6V, C-CUPL
20169755 20169756
Load Dissipation vs Amplifier Dissipation
V
DD
= 5.0V, C-CUPL
Load Dissipation vs Amplifier Dissipation
V
DD
= 2.5V, OCL
20169757 20169738
Load Dissipation vs Amplifier Dissipation
V
DD
= 3.6V, OCL
Load Dissipation vs Amplifier Dissipation
V
DD
= 5.0V, OCL
20169739 20169740
LM4985
www.national.com 18
Typical Performance Characteristics (Continued)
Output Power vs Load Resistance
V
DD
= 2.5V, C-CUPL
Output Power vs Load Resistance
V
DD
= 3.6V, C-CUPL
20169741 20169742
Output Power vs Load Resistance
V
DD
= 5.0V, C-CUPL
Output Power vs Load Resistance
V
DD
= 2.5V, OCL
20169743 20169744
Output Power vs Load Resistance
V
DD
= 3.6V, OCL
Output Power vs Load Resistance
V
DD
= 5.0V, OCL
20169745 20169746
LM4985
www.national.com19
Typical Performance Characteristics (Continued)
Output Power vs Supply Voltage
R
L
=16Ω, C-CUPL
Output Power vs Supply Voltage
R
L
=32Ω, C-CUPL
20169747 20169748
Output Power vs Supply Voltage
R
L
=16Ω, OCL
Output Power vs Supply Voltage
R
L
=32Ω, OCL
20169749 20169750
Supply Current vs Supply Voltage
R
L
=16Ω, C-CUPL
Supply Current vs Supply Voltage
R
L
=32Ω, C-CUPL
20169751 20169752
LM4985
www.national.com 20
Typical Performance Characteristics (Continued)
Supply Current vs Supply Voltage
R
L
=16Ω, OCL
Supply Current vs Supply Voltage
R
L
=32Ω, OCL
20169753 20169754
Gain vs Volume Steps
V
CC
= 2.5V, R
L
=16Ω, OCL
Gain vs Volume Steps
V
CC
= 3.6V, R
L
=16Ω, OCL
201697F7 201697F5
Gain vs Volume Steps
V
CC
= 5V, R
L
=16Ω, OCL
Gain vs Volume Steps
V
CC
= 2.5V, R
L
=16Ω, C-CUPL
201697G4 201697F6
LM4985
www.national.com21
Typical Performance Characteristics (Continued)
Gain vs Volume Steps
V
CC
= 3.6V, R
L
=16Ω, C-CUPL
Gain vs Volume Steps
V
CC
= 5V, R
L
=16Ω, C-CUPL
201697G0 201697G3
Gain vs Volume Steps
V
CC
= 2.5V, R
L
=32Ω, OCL
Gain vs Volume Steps
V
CC
= 3.6V, R
L
=32Ω, OCL
201697F9 201697G2
Gain vs Volume Steps
V
CC
= 5V, R
L
=32Ω, OCL
Gain vs Volume Steps
V
CC
= 2.5V, R
L
=32Ω, C-CUPL
201697G6 201697F8
LM4985
www.national.com 22
Typical Performance Characteristics (Continued)
Gain vs Volume Steps
V
CC
= 3.6V, R
L
=32Ω, C-CUPL
Gain vs Volume Steps
V
CC
= 5V, R
L
=32Ω, C-CUPL
201697G1 201697G5
LM4985
www.national.com23
Application Information
AMPLIFIER CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4985 has three internal power
amplifiers. Two of the amplifiers which amplify signals ap-
plied to their inputs, have internally configurable gain. The
remaining third amplifier provides both half-supply output
bias and AC ground return.
Loads, such as a headphone speaker, are connected be-
tween OUT1 and CNTGND or OUT2 and CNTGND. This
configuration does not require an output coupling capacitor.
The classical single-ended amplifier configuration, where
one side of the load is connected to ground, requires large,
expensive output coupling capacitors.
A configuration such as the one used in the LM4985 has a
major advantage over single supply, single-ended amplifiers.
Since the outputs OUT1, OUT2, and CNTGND are all biased
at 1/2 V
DD
, no net DC voltage exists across each load. This
eliminates the need for output coupling capacitors which are
required in a single-supply, single-ended amplifier configura-
tion. Without output coupling capacitors in a typical single-
supply, single-ended amplifier, the bias voltage is placed
across the load resulting in both increased internal IC power
dissipation and possible loudspeaker damage.
The LM4985 eliminates these output coupling capacitors
when operating in Output Capacitor-less (OCL) mode. Un-
less shorted to ground, VoC is internally configured to apply
a 1/2 V
DD
bias voltage to a stereo headphone jack’s sleeve.
This voltage matches the bias voltage present on VoA and
VoB outputs that drive the headphones. The headphones
operate in a manner similar to a bridge-tied load (BTL).
Because the same DC voltage is applied to both headphone
speaker terminals this results in no net DC current flow
through the speaker. AC current flows through a headphone
speaker as an audio signal’s output amplitude increases on
the speaker’s terminal.
The headphone jack’s sleeve is not connected to circuit
ground when used in OCL mode. Using the headphone
output jack as a line-level output will place the LM4985’s 1/2
V
DD
bias voltage on a plug’s sleeve connection. This pre-
sents no difficulty when the external equipment uses capaci-
tively coupled inputs. For the very small minority of equip-
ment that is DC coupled, the LM4985 monitors the current
supplied by the amplifier that drives the headphone jack’s
sleeve. If this current exceeds 500mA
PEAK
, the amplifier is
shutdown, protecting the LM4985 and the external equip-
ment.
POWER DISSIPATION
Power dissipation is a major concern when using any power
amplifier. When operating in capacitor-coupled mode (C-
CUPL), 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.
P
DMAX
= 2(V
DD
)
2
/(2π
2
R
L
) (1)
When operating in the OCL mode, the LM4985’s three op-
erational amplifiers produce a maximum power dissipation
given in Equation 2:
P
DMAX
= [2(V
DD
)
2
/(2π
2
R
L
)]+[V
DD2
/(4πR
L
)] (2)
The maximum power dissipation point obtained from Equa-
tion 1 or Equation 2 must not be greater than the power
dissipation that results from Equation 3:
P
DMAX
=(T
JMAX
-T
A
)/θ
JA
(3)
For package TMD12AAA, θ
JA
= 190˚C/W. T
JMAX
= 150˚C for
the LM4985. Depending on the ambient temperature, T
A
,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 2 is greater than that of
Equation 3, then either the supply voltage must be de-
creased, the load impedance increased or T
A
reduced.
For a typical application using a 3.6V power supply, with a
32Ωload, the maximum ambient temperature possible with-
out violating the maximum junction temperature is approxi-
mately 144˚C provided that device operation is around the
maximum power dissipation point. Thus, for typical applica-
tions, power dissipation is not an issue. 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. Refer to the
Typical Performance Characteristics curves for power dissi-
pation information for lower output powers.
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is important
for low noise performance and high power supply rejection.
The capacitor location on the power supply pins should be
as close to the device as possible.
Typical applications employ a regulator with 10µF tantalum
or electrolytic capacitor and a ceramic bypass capacitor
which aid in supply stability. This does not eliminate the need
for bypassing the supply nodes of the LM4985. A bypass
capacitor value in the range of 0.1µF to 1µF is recommended
for C
S
.
MICRO POWER SHUTDOWN
The LM4985’s micropower shutdown is activated or deacti-
vated through its I
2
C digital interface . Please refer to Table
1 for the I
2
C Address, Register Select, and Mode Control
registers. Each amplifier within the LM4985 can be shut-
down individually.
Please observe the following protocol when placing an indi-
vidual amplifier channel in shutdown while the other channel
remains active. The protocol requires activating both chan-
nels’ shutdown simultaneously, then deactivating the shut-
down of the channel whose output is desired (or leaving the
desire channel in shutdown mode). Also, when operating in
the C-CUPL mode, a short delay time is required between
activating one channel after placing both channels in shut-
down. If the user finds that both channels activate when only
one was chosen, increase the delay.
SELECTION OF INPUT CAPACITOR SIZE
Amplifying the lowest audio frequencies requires a high
value input coupling capacitor, C
i
. A high value capacitor can
be expensive and may compromise space efficiency in por-
table designs. In many cases, however, the headphones
used in portable systems have little ability to reproduce
signals below 60Hz. Applications using headphones with this
limited frequency response reap little improvement by using
a high value input capacitor.
In addition to system cost and size, turn on time is affected
by the size of the input coupling capacitor C
i
. A larger input
LM4985
www.national.com 24
Application Information (Continued)
coupling capacitor requires more charge to reach its quies-
cent DC voltage. This charge comes from the output via the
feedback Thus, by minimizing the capacitor size based on
necessary low frequency response, turn-on time can be
minimized. A small value of C
i
(in the range of 0.22µF to
0.68µF), is recommended.
MAXIMIZING OCL MODE CHANNEL-to-CHANNEL
SEPARATION
The OCL mode AC ground return (CNT_GND pin) is shared
by both amplifiers. As such, any resistance between the
CNT_GND pin and the load will create a voltage divider with
respect to the load resistance. In a typical circuit, the amount
of CNT_GND resistance can be very small, but still signifi-
cant. It is significant because of the relatively low load im-
pedances for which the LM4985 was designed to drive: 16Ω
to 32Ω. The ratio of this voltage divider will determine the
magnitude of any residual signal present at the CNT_GND
pin. It is this residual signal that leads to channel-to-channel
separation (crosstalk) degradation.
For example, for a 60dB channel-to-channel separation
while driving a 16Ωload, the resistance between the
LM4985’s CNT_GND pin and the load must be less than
16mΩ. This is achieved by ensuring that the trace that
connects the CNT_GND pin to the headphone jack sleeve
should be as short and massive as possible, given the
physical constraints of any specific printed circuit board lay-
out and design.
DEMONSTRATION BOARD AND PCB LAYOUT
Information concerning PCB layout considerations and dem-
onstration board use and performance is found in Application
Note AN-1452.
LM4985
www.national.com25
I
2
C Control Register
Table 1 shows the actions that are implemented by manipulating the bits within the two internal I
2
C control registers.
Table 1. LM4985 I
2
C Control Register Addressing and Data Format Chart
LM4985 I2C Contorl Register Addressing and Data Chart
I2C
Address
A6 A5 A4 A3 A2 A1 A0 Function
1100 1 1 A0
Register
Select
D7 D6 D5 D4 D3 D2 RS1 RS0
0 0 0 0 0 0 0 0 Read and write the mode
control register
0 0 0 0 0 0 0 1 Read and write the volume
control register
Mode
Control
Register
D7 D6 D5 D4 D3 D2 D1 D0
WT1 WT0 PHG SDCH1 SDCH2 CHSEL1 CHSEL2
0 X X X X X X X D7 must always be set to 0
– 0 0 X X X X X Wake-up time: 80ms (OCL),
250ms (C-CUPL)
– 0 1 X X X X X Wake-up time: 110ms (OCL),
450ms (C-CUPL)
– 1 0 X X X X X Wake-up time: 170ms (OCL),
850ms (C-CUPL)
– 1 1 X X X X X Wake-up time: 290ms (OCL),
1650ms (C-CUPL)
– X X 1 X X X X Output capacitor-less mode
active
– X X 0 X X X X Output capacitor-less mode
inactive
– X X X 0 0 X X Amplifier’s SHUTDOWN
mode active
– X X X 0 1 X X Illegal mode
– X X X 1 0 X X Illegal mode
– X X X 1 1 X X Amplifier’s SHUTDOWN
mode inactive
– X X X X X 0 02 Amplifier’s Chan. 1 is Input 1,
Chan 2. is Input 2
– X X X X X 0 1 Amplifier’s Chan. 1 is Input 1,
Chan 2. is Input 1
– X X X X X 1 0 Amplifier’s Chan. 1 is Input 2,
Chan 2. is Input 2
– X X X X X 1 1 Amplifier’s Chan. 1 is Input 2,
Chan 2. is Input 1
LM4985
www.national.com 26
Volume Control Settings Binary Values
The minimum volume setting is set to –76dB when 00000 is loaded into the volume control register. Incrementing the volume
control register in binary fashion increases the volume control setting, reaching full scale at 11111. Table C1 shows the value of
the gain for each of the 32 binary volume control settings.
Table C1. Binary Values for the Different Volume Control Gain Settings
Gain B4 B3 B2 B1 B0
18 11111
17 11110
16 11101
15 11100
14 11011
13 11010
12 11001
10 11000
8 10111
6 10110
4 10101
2 10100
0 10011
–2 10010
–4 10001
–6 10000
–8 01111
–1001110
–1201101
–1401100
–1601011
–1801010
–2101001
–2401000
–2700111
–3000110
–3400101
–3800100
–4400011
–5200010
–6200001
–7600000
LM4985
www.national.com27
Revision History
Rev Date Description
1.0 05/17/06 Initial WEB release.
LM4985
www.national.com 28
Physical Dimensions inches (millimeters) unless otherwise noted
micro SMD
Order Number LM4985TM
NS Package Number TMD12AAA
X
1
= 1.215mm ±0.03mm X
2
= 1.615mm ±0.03mm X
3
= 0.600mm ±0.075mm
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
For the most current product information visit us at www.national.com.
LIFE SUPPORT POLICY
NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS
WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR
CORPORATION. As used herein:
1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body, or
(b) support or sustain life, and whose failure to perform when
properly used in accordance with instructions for use
provided in the labeling, can be reasonably expected to result
in a significant injury to the user.
2. A critical component is any component of a life support
device or system whose failure to perform can be reasonably
expected to cause the failure of the life support device or
system, or to affect its safety or effectiveness.
BANNED SUBSTANCE COMPLIANCE
National Semiconductor follows the provisions of the Product Stewardship Guide for Customers (CSP-9-111C2) and Banned Substances
and Materials of Interest Specification (CSP-9-111S2) for regulatory environmental compliance. Details may be found at:
www.national.com/quality/green.
Lead free products are RoHS compliant.
National Semiconductor
Americas Customer
Support Center
Email: new.feedback@nsc.com
Tel: 1-800-272-9959
National Semiconductor
Europe Customer Support Center
Fax: +49 (0) 180-530 85 86
Email: europe.support@nsc.com
Deutsch Tel: +49 (0) 69 9508 6208
English Tel: +44 (0) 870 24 0 2171
Français Tel: +33 (0) 1 41 91 8790
National Semiconductor
Asia Pacific Customer
Support Center
Email: ap.support@nsc.com
National Semiconductor
Japan Customer Support Center
Fax: 81-3-5639-7507
Email: jpn.feedback@nsc.com
Tel: 81-3-5639-7560
www.national.com
LM4985 Stereo 135mW Low Noise Headphone Amplifier with Selectable Capacitive Coupled or
Capacitor-less (OCL) Output and Digitally Controlled (I
2
C) Volume Control
