LM4918 Stereo Audio Amp with AGC Control (Rev. C)

LM4918 Stereo Audio Amp with AGC Control (Rev. C)
OBSOLETE
LM4918
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LM4918
Stereo Audio Amp with AGC Control
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FEATURES
DESCRIPTION
•
•
•
•
•
•
The LM4918 is a monolithic integrated circuit that
provides a automatic gain control (AGC), and stereo
bridged audio power amplifiers capable of producing
1W into 8Ω with less than 1.0% THD.
1
23
0.75dB per Step/32Step AGC Control Interface
Automatic Gain Control Circuitry
Stereo Bridged Power Amplifiers
“Click and Pop” Suppression Circuitry
Thermal Shutdown Protection Circuitry
Selectable Auto Detect Std-By Mode and Logic
Control
KEY SPECIFICATIONS
•
•
•
THD+N at 1kHz, 1W, 8Ω: 0.3% (typ)
Total Quiescent Power Supply Current: 18mA
(typ)
Total Shutdown Power Supply Current : 1µA
(typ)
Boomer™ audio integrated circuits were designed
specifically to provide high quality audio while
requiring a minimum amount of external components.
The LM4918 incorporates a AGC and stereo bridged
audio power amplifiers making it optimally suited for
multimedia monitors, portable radios, desktop and
portable computer applications.
The LM4918 features an externally controlled, lowpower consumption shutdown mode, and a power
amplifier mute for maximum system flexibility and
performance.
APPLICATIONS
•
•
•
Portable Computers
Desktop Computers
Multimedia Monitors
1
2
3
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
Boomer is a trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
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OBSOLETE
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Typical Application
Vol Out L
Headphone
Sense Pin
VDDS
0.39 PF
20 k:
20 k:
Stand-by
Mode Select
+
Left In
Input Vth
GND
+
-
Right In
Vol Out L
Stand-by
Mode
+
-Left Out
20 k:
Volume
Control
32 Steps
0.75 dB
per Step
20 k:
+
+Left Out
Vol Out R
Wait Time
1 PF
VDD A Left
PA Shutdown
4.7 PF
2.2 PF GND
ypass
V Bypass
Click and Pop
Suppression
Circuitry
PA GND
GND
VDD A Right
AGC Control In
VDD
+
-
+Right Out
AGC Ref In
20 k:
DIGITAL AGC BLOCK
Idisch
20 k:
+
-
-Right Out
(Peak Hold Detector and
Automatic Gain Control)
GND
Hold
Cap
Hold
Out
GND
AGC
OUT
Amp 1-
Signal
Ground
0.39 PF
20 k:
20 k:
Out R
0.39 PF
Figure 1. Typical Audio Amplifier Application Circuit
2
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29
VpaInL
30
VGA OutL
HP Sense
31
VGA InL
MODE
32
VDD
SD
Connection Diagrams
28
27
26
NC
C_STDBY
3
23
VoL-
Input Vth
4
22
VoL+
AGC Ref In
5
21
GND
R_Discharge
6
20
VoR+
C_Hold
7
19
VoR-
Vpk_In
8
18
NC
Vpk_Bfr
9
17
VDD
10
11
12
13
14
15
16
VpaInR
24
VGA OutR
2
VGA InR
NC
Bypass
VDD
GND
25
Volcontrol In
1
Vagc Out
STDBY
Figure 2. WQFN - Top View
Package Number NJF
WQFN Marking
NS
UZXYTT
L4918LQ
NS - Std NS Logo, U - Wafer Fab Code, Z - Assembly Plant Code, XY - 2 Digit Date Code, TT - Die Run Traceability, L4918LQ LM4918LQ
Figure 3. Top View
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
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(1) (2)
Absolute Maximum Ratings
Supply Voltage
6.0V
−65°C to +150°C
Storage Temperature
−0.3V to VDD +0.3V
Input Voltage
Power Dissipation
(3)
Internally Limited
ESD Susceptibility
(4)
2000V
ESD Susceptibility
(5)
200V
Junction Temperature
150°C
Thermal Resistance
θJA (WQFN)
51°C/W
See AN-1187 'Leadless Leadframe Packaging (LLP).'
(1)
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 ensure specific performance limits. Electrical Characteristics state DC and AC electrical
specifications under particular test conditions which ensure specific performance limits. This assumes that the device is within the
Operating Ratings. Specifications are not ensured for parameters where no limit is given, however, the typical value is a good indication
of device performance.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and
specifications.
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.
Human body model, 100pF discharged through a 1.5kΩ resistor.
Machine Model, 220pF–240pF discharged through all pins.
(2)
(3)
(4)
(5)
Operating Ratings
Temperature Range
TMIN ≤ TA ≤ TMAX
−40°C ≤ TA ≤ +85°C
2.7V ≤ VDD ≤ 5.5V
Supply Voltage
Electrical Characteristics
Unless otherwise specified, all limits are specified to Tj = 25°C, VDD = 5.0V.
LM4918
Symbol
Parameter
Conditions
Typical
Limit
Units
(Limits)
2.7
5.5
V(min)
V(max)
Common Portion
VDD
Supply Voltage
IDD
Quiescent Supply Current
Vin = 0V, Io = 0A
18
25
mA (max)
ISDIH
Shutdown Current
VSD = VDD
1
2
µA (max)
Istd-by
Stand-By Current
Vstd-by = Vdd
1
2
mA (max)
Vih
Logic High
0.8xVd
d
V (min)
Vil
Logic Low
0.2xVd
d
V (max)
AGC Volume
AGC Vol max
Max gain
AGC Vol min
Min gain
Vol control in ≧ 4.5V
0
0
0.5
-0.5
db(max)
dB(min)
Vol control in ≦ 0.5V
-24
-22
-26
dB(max)
dB(min)
AGC Step Size
.75
dB
AGC Control Block
Vdisch
Voltage on Rdisch
1.2
V
Idisch
Discharge Current on Chold
320
nA
Vos
Offset Voltage
VIN = 0V
10
Av Inv
Inverting amp gain
Vagcref = 4.5V
0
4
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mV
±0.5
dB
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Electrical Characteristics (continued)
Unless otherwise specified, all limits are specified to Tj = 25°C, VDD = 5.0V.
LM4918
Symbol
Parameter
Conditions
Typical
Limit
Units
(Limits)
Std-by Detect
14
50
mVpk
mVpk
C_STBY = 10μF
10
sec
Output Offset Voltage
Vin = 0V
10
40
mV(max)
Output Power
THD = 1%, F = 1kHz, Rl = 8Ω
1.1
1.0
W(min)
THD+N
Total Harmonic Distortion+Noise
20Hz ≦ f ≧ 20kHz, Avd = 2,
Rl = 8Ω, Po = 1W
0.3
PSRR
Power Supply Rejection Ratio
Vdd = 5V, Vripple = 200mVrms,
Rl = 8Ω, Cb = 1.0μF
67
Xtalk
Channel Separation in SE
F = 1kHz, Cb = 1.0μF
60
dB
Xtalk
Channel Separation in BTL
F = 1kHz, Cb = 1.0μF
76
dB
SNR
Signal to Noise Ratio
Vdd = 5V, Po= 1.1W, Rl = 8Ω
109
dB
Vin
Input Sig Threshold Level
Twait
Wait time
Vos
Po
Power Amp Block
%
dB
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Typical Performance Characteristics
10
THD+N vs Frequency
VDD = 3V, RL = 4Ω, BTL, PO = 225mW
10
5
5
2
2
THD+N vs Frequency
VDD = 3V, RL = 8Ω, BTL, PO = 275mW
1
THD+N (%)
THD+N (%)
1
(1)
0.5
0.2
0.5
0.2
0.1
0.1
0.05
0.05
0.02
0.02
0.01
20
100
1k
0.01
20
10k 20k
100
FREQUENCY (Hz)
10
1k
10k 20k
FREQUENCY (Hz)
Figure 4.
Figure 5.
THD+N vs Frequency
VDD = 5V, RL = 8Ω, BTL, PO = 400mW
THD+N vs Frequency
VDD = 5V, RL = 32Ω, SE, PO = 40mW
10
5
2
1
THD+N (%)
THD+N (%)
1
0.5
0.2
0.1
0.1
0.01
0.05
0.02
0.01
20
100
1k
10k 20k
0.001
20
100
10
1k
10k 20k
FREQUENCY (Hz)
FREQUENCY (Hz)
Figure 6.
Figure 7.
THD+N vs Frequency
VDD = 3V, RL = 32Ω, SE, PO = 25mW
THD+N vs Output Power
VDD = 3V, RL = 4Ω, BTL
10
5
2
1
THD+N (%)
THD+N (%)
1
0.1
0.5
0.2
0.1
0.01
0.05
0.02
0.001
20
100
1k
10k 20k
FREQUENCY (Hz)
6
100m
1
2
OUTPUT POWER (W)
Figure 8.
(1)
0.01
10m
Figure 9.
Data taken at VagcRef = VDD and Power Amp Gain set to A = 2.
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Typical Performance Characteristics (1) (continued)
THD+N vs Output Power
VDD = 3V, RL = 8Ω, BTL
5
5
2
2
1
1
0.5
0.2
0.5
0.2
0.1
0.1
0.05
0.05
0.02
0.02
0.01
10m
1
100m
THD+N vs Output Power
VDD = 5V, RL = 8Ω, BTL
10
THD+N (%)
THD+N (%)
10
0.01
10m
2
THD+N vs Output Power
VDD = 3V, RL = 32Ω, SE
THD+N vs Output Power
VDD = 5V, RL = 32Ω, SE
10
5
5
2
2
1
1
THD+N (%)
THD+N (%)
Figure 11.
0.5
0.2
0.5
0.2
0.1
0.1
0.05
0.05
0.02
0.02
0.01
0.01
1
10
1
100
OUTPUT POWER (mW)
10
100
OUTPUT POWER (mW)
Figure 12.
Figure 13.
Output Power vs Load Resistance
BTL
Output Power vs Load Resistance
SE
1.4
250
5V, 10% THD+N
1.2
200
5V, 10% THD+N
OUTPUT POWER (W)
OUTPUT POWER (W)
2
OUTPUT POWER (W)
Figure 10.
10
1
100m
OUTPUT POWER (W)
1.0
5V, 1% THD+N
0.8
3V, 10% THD+N
0.6
0.4
5V, 1% THD+N
150
3V, 10% THD+N
100
3V, 1% THD+N
50
0.2
3V, 1% THD+N
0
0
0
10
20
30
40
LOAD RESISTANCE (:)
0
20
40
60
80
LOAD RESISTANCE (:)
Figure 14.
Figure 15.
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Typical Performance Characteristics (1) (continued)
Power Dissipation vs Output Power
VDD = 3V, BTL
Power Dissipation vs Output Power
VDD = 5V, RL = 8Ω, BTL
0.8
0.7
0.5
POWER DISSIPATION (W)
POWER DISSIPATION (W)
0.6
4:
0.4
0.3
8:
0.2
0.1
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0
100
200
300
400
500
0
0.5
OUTPUT POWER (mW)
1
1.5
OUTPUT POWER (W)
Figure 16.
Figure 17.
Power Dissipation vs Output Power
VDD = 5V and 3V, RL = 32Ω, SE
PSRR vs Frequency
VDD = 3V, RL = 8Ω, BTL, Full System
Inputs 10Ω Terminated to GND
0
140
120
-20
5V
-30
100
PSRR (dB)
POWER DISSIPATION (mW)
-10
80
60
3V
-40
-50
-60
-70
40
-80
20
-90
-100
20
0
0
100
50
150
100
100k
Figure 18.
Figure 19.
PSRR vs Frequency
VDD = 5V, RL = 8Ω, BTL, Full System
Inputs 10Ω Terminated to GND
PSRR vs Frequency
VDD = 3V, RL = 32Ω, SE, Full System
Inputs 10Ω Terminated to GND
0
0
-10
-10
-20
-20
-30
-30
PSRR (dB)
PSRR (dB)
10k
FREQUENCY (Hz)
OUTPUT POWER (mW)
-40
-50
-60
-40
-50
-60
-70
-70
-80
-80
-90
-90
-100
20
-100
20
100
1k
10k
100k
FREQUENCY (Hz)
100
1k
10k
100k
FREQUENCY (Hz)
Figure 20.
8
1k
Figure 21.
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Typical Performance Characteristics (1) (continued)
PSRR vs Frequency Power Amplifier Only
VDD = 5V, RL = 8Ω, BTL
Inputs 10Ω Terminated toGND
0
0
-10
-10
-20
-20
-30
-30
PSRR (dB)
PSRR (dB)
PSRR vs Frequency
VDD = 5V, RL = 32Ω, SE, Full System
Inputs 10Ω Terminated to GND
-40
-50
-60
-40
-50
-60
-70
-70
-80
-80
-90
-90
-100
20
-100
20
100
10k
1k
100k
100
Figure 23.
Supply Voltage vs Output Power
RL = 8Ω, BTL,10% and 1% THD+N
200
Supply Voltage vs Output Power
RL = 32Ω, SE,10% and 1% THD+N
180
OUTPUT POWER (W)
OUTPUT POWER (W)
1.2
1.0
0.8
0.6
0.4
0.2
160
140
120
100
80
0
2.7 3.0
3.5
4.0
4.5
5.0
0
2.7 3.0
5.5
SUPPLY VOLTAGE (V)
3.5
4.5
5.0
5.5
Figure 25.
Channel Separation
VDD = 5V and 3V, RL = 8Ω, BTL
Noise Floor
A-Weighted
100
OUTPUT NOISE VOLTAGE (PV)
-10
-20
-30
CROSSTALK (dB)
4.0
SUPPLY VOLTAGE (V)
Figure 24.
0
100k
FREQUENCY (Hz)
Figure 22.
1.4
10k
1k
FREQUENCY (Hz)
-40
-50
-60
-70
-80
-90
-100
5V, BTL, RL= 8:
3V, BTL, RL= 8:
10
3V and 5V SE, RL= 32:
-110
-120
20
100
1k
10k 20k
1
20
100
1k
10k 20k
FREQUENCY (Hz)
FREQUENCY (Hz)
Figure 26.
Figure 27.
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APPLICATION INFORMATION
Bridge Configuration Explanation
As shown in Figure 1, the LM4918 consists of two pairs of operational amplifiers, forming a two-channel (channel
L and channel R) stereo amplifier. (Though the following discusses channel L, it applies equally to channel R.)
External resistors Rf and Ri (set as R0 and R1, respectively for the L channel, and R7 and R6, respectively for
the R channel on the demo board circuit) set the closed-loop gain of the first op-amp, whereas two internal 20kΩ
resistors set the second op-amps gain at -1. The LM4918 drives a load, such as a speaker, connected between
the two amplifier outputs, VoL- and VoL+ (VoR-, and VoR+ for the R channel). Figure 1shows that the first opamp’s output serves as the second op-amp’s input. This results in both amplifiers producing signals identical in
magnitude, but 180° out of phase. Taking advantage of this phase difference, a load is placed between VoL- and
VoL+ and driven differentially (commonly referred to as “bridge mode”). This results in a differential gain of
AVD= 2 x (Rf / Ri)
(1)
Bridge mode amplifiers are different from single-ended amplifiers that drive loads connected between a single
amplifier’s output and ground. For a given supply voltage, bridge mode has a distinct advantage over the singleended configuration: its differential output doubles the voltage swing across the load. This produces four times
the output power when compared to a single-ended amplifier under the same conditions. This increase in
attainable output power assumes that the amplifier is not current limited or that the output signal is not clipped.
Another advantage of the differential bridge output is no net DC voltage across the load. This is accomplished by
biasing channel A’s and channel B’s outputs at half-supply. This eliminates the coupling capacitor that single
supply, single-ended amplifiers require (such as when using the LM4918 to drive single-ended headphone
loads). Eliminating an output coupling capacitor in a single-ended configuration forces a single-supply amplifier’s
half-supply bias voltage across the load. This increases internal IC power dissipation and may permanently
damage loads such as speakers.
Power Dissipation
Power dissipation is a major concern when designing a successful single-ended or bridged amplifier. Equation 2
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)
Single-Ended
(2)
However, a direct consequence of the increased power delivered to the load by a bridge amplifier is higher
internal power dissipation for the same conditions.
The LM4918 has two operational amplifiers per channel. The maximum internal power dissipation per channel
operating in the bridge mode is four times that of a single-ended amplifier. From Equation 2, assuming a 5V
power supply and a 8Ω load, the maximum single channel power dissipation is 0.158W or 0.317W for stereo
operation.
PDMAX = 4 x (VDD)2 / (2π2RL)
Bridge-Mode
(3)
The LM4918’s power dissipation is twice that given by Equation 2 or Equation 3 when operating in the singleended mode or bridge mode, respectively. Twice the maximum power dissipation point given by Equation 3 must
not exceed the power dissipation given by Equation 4:
PDMAX' = (TJMAX -TA) / θJA
(4)
The LM4918’s TJMAX = 150°C. In the WQFN package soldered to a DAP pad that expands to a copper area of
1in2 on a PCB, the LM4918’s θJA is 51°C/W. At any given ambient temperature TA, use Equation 3 to find the
maximum internal power dissipation supported by the IC packaging. Rearranging Equation 3 and substituting
PDMAX for PDMAX' results in Equation 5. This equation gives the maximum ambient temperature that still allows
maximum stereo power dissipation without violating the LM4918’s maximum junction temperature.
TA = TJMAX - 2*PDMAXθJA
(5)
For a typical application with a 5V power supply and an 8Ω load, the maximum ambient temperature that allows
maximum stereo power dissipation without exceeding the maximum junction temperature is approximately 85°C
for the WQFN package.
TJMAX = PDMAXθJA + TA
10
(6)
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Equation 6 gives the maximum junction temperature TJMAX. If the result violates the LM4918’s 150°C, reduce the
maximum junction temperature by reducing the power supply voltage or increasing the load resistance. Further
allowance should be made for increased ambient temperatures.
The above examples assume that a device is a surface mount part operating around the maximum power
dissipation point. Since internal power dissipation is a function of output power, higher ambient temperatures are
allowed as output power or duty cycle decreases.
If the result of Equation 2 is greater than that of Equation 3, then decrease the supply voltage, increase the load
impedance, or reduce the ambient temperature. If these measures are insufficient, a heat sink can be added to
reduce θJA. The heat sink can be created using additional copper area around the package, with connections to
the ground pin(s), supply pin and amplifier output pins. External, solder attached SMT heatsinks such as the
Thermalloy 7106D can also improve power dissipation. Refer to the Typical Performance Characteristics curves
for power dissipation information at lower output power levels.
Exposed-DAP Mounting Considerations
The LM4918's exposed-DAP (die attach paddle) packages (LD) provide a low thermal resistance between the die
and the PCB to which the part is mounted and soldered. This allows rapid heat transfer from the die to the
surrounding PCB copper area heatsink, copper traces, ground plane, and finally, surrounding air. The result is a
low voltage audio power amplifier that produces 1.1W dissipation in an 8Ω load at ≤ 1% THD+N. This power is
achieved through careful consideration of necessary thermal design. Failing to optimize thermal design may
compromise the LM4918's performance and activate unwanted, though necessary, thermal shutdown protection.
The LM4918LD must have its DAP soldered to a copper pad on the PCB. The DAP's PCB copper pad is then,
ideally, connected to a large plane of continuous unbroken copper. This plane forms a thermal mass, heat sink,
and radiation area. Place the heat sink area on either outside plane in the case of a two-sided or multi-layer
PCB. (The heat sink area can also be placed on an inner layer of a multi-layer board. The thermal resistance,
however, will be higher.) Connect the DAP copper pad to the inner layer or backside copper heat sink area with
2 vias. The via diameter should be 0.012in - 0.013in with a 1.27mm pitch. Ensure efficient thermal conductivity
by plugging and tenting the vias with plating and solder mask, respectively. Further detailed and specific
information concerning PCB layout, fabrication, and mounting an WQFN (NJF) package is available in Texas
Instrument’s AN1187.
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 5V regulator typically use a 10µF in parallel with a 0.1µF filter capacitors to
stabilize the regulator’s output, reduce noise on the supply line, and improve the supply’s transient response.
However, their presence does not eliminate the need for a local 1.0µF tantalum bypass capacitance connected
between the LM4918’s supply pins and ground. Keep the length of leads and traces that connect capacitors
between the LM4918’s power supply pin and ground as short as possible. Connecting a 2.2µF capacitor, CB,
between the BYPASS pin and ground improves the internal bias voltage’s stability and improves the amplifier’s
PSRR. The PSRR improvements increase as the bypass pin capacitor value increases. Too large, however,
increases turn-on time and can compromise the amplifier’s click and pop performance. The selection of bypass
capacitor values, especially CB, depends on desired PSRR requirements, click and pop performance system
cost, and size constraints.
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JP10
STDBY
JP9
Vpu
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MODE
JP11
VDD
JP2
3
2
1
3
2
1
3
2
1
Vpu
2
1
R14
100k
POWER
R16
100k
R15
100k
VDD
GND
System Current
Measurement Jumper
VDD
R12
100k
R13
100k
Vpu
C3
Rpu
20M
C10
LM4918
1 PF
C11
J1
220 PF
R3
120
1
R2
100k
10 PF
R10
1k
R5
S3
R11
10k
3
4
5
6
7
8
9
10
11
12
13
14
15
16
100k
S2
2.2 PF
C4
R4
120
C5
0.47 PF
30k
R8
S4
C2
32
31
30
29
28
27
26
25
220 PF
1 PF
VDD C0
IN L
C8
C1
R1
VDD
R0
0.47 PF
20k
R17
1k
17
R18
1k
20k 0.47 PF
23
22
21
20
19
L OUT VDD
L OUT
+
C9
R OUT +
1 PF
0.47 PF
C7
S1
R6
20k
STDBY
SD
MODE
Cstdby
HPsense
Input Vth
Vdd
Vagcref
VgaInL
Rdischarge VgaOutL
Chold
VpaInL
Vpkin
Vdd
Vpkbfr
Vagcout
VoLn
VolControlIn
VoLp
GND
GND
Vbypass
VoRp
VgaInR
VoRn
VgaOutR
VpaInR
Vdd
R7
IN R
20k
R OUT -
C6
33 pF
R9
20k
Figure 28. REFERENCE DESIGN BOARD SCHEMATIC
MODE
STDBY
SD
VDDS
HPSense
ViL
Vbp
Rthrshld
Prg Vth Det.
&
Stdby Tmr
Bias
&
TSD
VDDP
+
-
Vbp
VoL-
+
Cstdby
PopClick
& Vbypass
Vbp
-
VpkBfr
VagcRef
+
Vagc VolCntrl
5-bit
ADC
Vbp
Vbp
-
VoL+
+
+
VoR+
Vbp
VoLVoL+
PD
FWR
VoRVoR+
Vbp
Chold
Rdis
100 nA
ViR
GND
Vbp
+
VoR-
-
+
Figure 29. Block Diagram with external component for AGC closed loop
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Automatic Gain Control (AGC)
The AGC represents a closed loop gain control network in conjunction with the peak detect monitoring the
outputs of the power amplifier section. A reference voltage is put on the “AGC Ref In” pin to set the desired
dynamic range of the AGC. This voltage should be set to allow for maximum desired output level for the
expected range of the input signal. The AGC may provide anywhere from 0 to 24dB of attenuation. The peak
detect constantly updates the AGC block with information about the maximum level of the output signal.
Internally, if this peak value is higher than the set reference voltage, the AGC will reduce the gain of the circuit
until the output peak is lower than the reference voltage. This entire process is timed by the Chold capacitor (in
conjunction with Rdischarge), which determines the hold time between peaks while restoring gain (gain returns to
normal levels after being attenuated by large power amp output signal).
The circuit shown in Figure 1 best describes the AGC loop operation. Square nodes are external pins. Vbp is the
bypass voltage. As is shown below, the peak detect section (PD) monitors all outputs of the power amp (VoL+,
VoL-, VoR+, and VoR-). Peaks are fed into the peak buffer, regulated by Chold and Rdis. The output of the peak
buffer (VpkBfr) is then compared to VagcRef. The AGC amplifier then sets the ADC output to control the gain
setting in 32 discrete steps from 0 to –24dB. Gain will remain reduced until peak levels (Vpkbfr) return below the
VagcRef, as buffered by the peak buffer and Chold. This means gain will slowly increase (speed determined by
Chold) back to normal levels (maximum gain set) or until the peaks again exceed the reference level.
The reference voltage VagcRef should be set in conjunction with the gain settings on the power amplifier stage
to provide maximum gain (0dB) for the typical desired maximum output level of the part for a maximum input
level. This will ensure that both the full 24dB dynamic range of the AGC is used effectively and that maximum
effective audio levels may be reached without clipping. Typical AGC peak gain values for given VagcRef voltages
are shown in Table 1. These are taken with a 20MΩ pullup resistor on Chold (as used shown on the LM4918
WQFN Board Artwork). These are shown as typical values only – VagcRef level should be set with the factors
described above.
Table 1. Typical VAGCREF Values
STEP
AGC Peak Gain Level (dB)
VagcRef (V)
1
-0.1
4.1
2
-1.6
4.0
3
-2.4
3.9
4
-3.1
3.8
5
-3.8
3.7
6
-4.6
3.6
7
-5.4
3.5
8
-6.1
3.3
9
-6.9
3.2
10
-7.6
3.1
11
-8.4
3.0
12
-9.1
2.9
13
-9.8
2.7
14
-10.6
2.6
15
-11.4
2.4
16
-12.1
2.3
17
-12.8
2.1
18
-13.6
2.0
19
-14.4
1.8
20
-15.1
1.7
21
-15.9
1.6
22
-16.6
1.4
23
-17.3
1.3
24
-18.1
1.2
25
-18.8
1.1
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Table 1. Typical VAGCREF Values (continued)
STEP
AGC Peak Gain Level (dB)
VagcRef (V)
26
-19.6
1.0
27
-20.3
0.8
28
-21.1
0.7
29
-21.8
0.5
30
-22.6
0.3
31
-23.4
0.2
32
-24.2
0
Shutdown Function
In order to reduce power consumption while not in use, the LM4918 contains a shutdown pin to externally turn off
the amplifier’s bias circuitry. This shutdown feature turns the amplifier off when a logic high (0.8 x VDD) is placed
on the shutdown pin. By switching the shutdown pin to VDD, the LM4918 supply current draw will be minimized in
idle mode. While the device will be disabled with shutdown pin voltages less than 0.8VDD, the idle current may be
greater than the typical value of 1µA. (Idle current is measured with the shutdown pin at VDD).
In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry to
provide a quick, smooth transition into shutdown. Another solution is to use a single-pole, single-throw switch in
conjunction with an external pull-down resistor. When the switch is closed, the shutdown pin is connected to VDD
and disables the amplifier. If the switch is open, then the external pull-down resistor will enable the LM4918. This
scheme ensures that the shutdown pin will not float thus preventing unwanted state changes.
Standby
When in MODE 0, activating Standby with a logic high (0.8 x VDD) maintains the outputs at their bias level, but
with no signal. This allows for extremely fast (less than 1ms) turn off/on of the audio outputs with a minimum
current draw while in Standby mode (1mA typ).
When in MODE 1, enabling/disabling Standby is also dependant upon the input signal level and the input
threshold setting. In this mode, Standby is not enabled until both a logic high is placed on the Standby pin and
the input signal level has remained below the set input threshold level for a period of time determined by Chold
(typically a few seconds). This delay time set by Cstdby prevents the LM4918 from inadvertently triggering
Standby mode during quiet passages in music. Returning from Standby while in MODE 0 is also dependant upon
both a logic low on the Standby pin and the input signal level being above the set threshold. In this case,
however, return from Standby is immediate and does not require the input to remain above the threshold level for
any amount of time.
Standby Mode Select
The LM4918 has two user-selectable modes for standby usage. The first mode (MODE 0), valid when MODE is
equal to logic low (0.2 x VDD or less), is characterized by the immediate activation of Standby mode (outputs
remain at bias point, with no signal) when a logic high is applied to the Standby pin, and the immediate return
from Standby upon a logic low on the Standby pin.
The second mode (MODE 1), valid when MODE is equal to logic high (0.8 x VDD), is characterized by a delayed
activation of Standby mode. In this case Standby is delayed until both a logic high is placed on the Standby pin
and the input signal has maintained a value below the input threshold (as determined by the resistance on Input
Vth) for a given amount of time (determined by Cstdby). Returning from Standby also requires both a logic high
on the Standby pin and a sufficiently large input signal to overcome the set input threshold. No hold time applies
to return from Standby – deactivation is immediate upon both conditions being met.
14
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Input Threshold Detect
The LM4819 has an input threshold detect function that works in conjunction with the Standby mode and is only
enabled when the LM4918 is in MODE 1 (see Standby Mode Select). This threshold level may be set anywhere
from 14mV to 50mV (typical peak values) by adjusting the resistance placed from Input Vth to GND. Typical
values have about 50kΩ for a 14mVpk level and 15kΩ for a 50mVpk level. Care should be taken not to set the
threshold values too low, as the part may cycle in and out of shutdown if any spurious noise was present on the
inputs. For this reason, threshold values less than 14mVpk with corresponding resistance values greater than
50kΩ are not recommended.
This threshold value sets the peak input level that must be exceeded for the part to return from Standby when in
MODE 1. It also is the level which the input must remain below (for an amount of time determined by Cstdby) for
the part to enter Standby when in MODE 1.
HP Sense Function
Applying a voltage greater than 0.8VDD to the LM4918’s HP Sense pin turns off the second amplifier on each
channel, muting a bridged-connected load. Quiescent current consumption is reduced when the IC is in this
single-ended mode.
Figure 28 shows the implementation of the LM4918’s headphone sense function. With no headphones connected
to the headphone jack, the R14-R17 voltage divider sets the voltage applied to the HP Sense pin (pin 30) at
approximately 50mV. This 50mV enables the second amplifiers, placing the LM4918 in bridged mode operation.
The output coupling capacitor blocks the amplifier’s half supply DC voltage, protecting the headphones.
The HP Sense threshold is set at 0.8VDD. While the LM4918 operates in bridged mode, the DC potential across
the load is essentially 0V. Therefore, even in an ideal situation, the output swing cannot cause a false singleended trigger. Connecting headphones to the headphone jack disconnects the headphone jack contact pin from
VoR− and allows R14 to pull the HP Sense pin up to VDD. This enables the headphone function, turns off the
second amplifiers, and mutes the bridged speaker. The amplifier then drives the headphones, whose impedance
is in parallel with resistor R17 and R18. These resistors have negligible effect on the LM4918’s output drive
capability since the typical impedance of headphones is 32Ω.
Figure 28 also shows the suggested headphone jack electrical connections. The jack is designed to mate with a
three-wire plug. The plug’s tip and ring should each carry one of the two stereo output signals, whereas the
sleeve should carry the ground return. A headphone jack with one control pin contact is sufficient to drive the HP
Sense pin when connecting headphones. A microprocessor or a switch can replace the headphone jack contact
pin. When a microprocessor or switch applies a voltage greater than 4V to the HP-IN pin, a bridge-connected
speaker is muted and the first set of amplifiers drive a pair of headphones.
Peak Detect
The LM4918 also incorporates a peak-detect function to properly monitor output level in conjunction with the
AGC. This peak detect function may be set for positive or negative detect, based on the users preference. On
the demo board, shown in Figure 28, both circuits are available for testing. The normal, positive peak detect, is
engaged by inserting jumpers at S1 and S3, while removing jumpers at S2 and S4. The negative peak circuit is
described by jumpers in place on S2 and S4, while removed from S1 and S3. Operating in the negative manner
places the Vpk_Bfr directly to VagcRef, and makes Vpk_in the effective AGC gain reference voltage. Keep in
mind this will reverse the voltage values used for a given AGC gain level as compared to VagcRef values.
The peak detect period is set by the Chold capacitor as described in the Automatic Gain Control (AGC) section.
AGC gain will slowly increase to maximum set value at a rate determined by Chold. Decreasing Chold results in
faster peak hold times and faster gain increase after a peak-induced gain reduction.
Selecting External Components
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value input coupling capacitor (C7 and C8 in Figure 28). A
high value capacitor can be expensive and may compromise 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 large input capacitor.
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The LM4918 actually has two different sets of input coupling caps: one for the AGC block (C7 and C8) and one
for the power amplifier block (C1 and C5). These must both be in place to properly protect the inputs from DC
offsets and should match for predictable frequency response.
The internal input resistor (R1 and R6) and the input capacitor (C1 and C5) produce a high pass filter cutoff
frequency that is found using Equation 7.
fc = 1 / (2πRC)
(7)
As an example when using a speaker with a low frequency limit of 150Hz, Ci, using Equation 7 is 0.063µF. The
0.49µF Ci shown in Figure 28 allows the LM4918 to drive high efficiency, full range speaker whose response
extends below 20Hz.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consideration should be paid to value of CB, the capacitor
connected to the Vbypass pin. Since CB determines how fast the LM4918 settles to quiescent operation, its value
is critical when minimizing turn-on pops. The slower the LM4918’s outputs ramp to their quiescent DC voltage
(nominally VDD/2), the smaller the turn-on pop. Choosing CB equal to 2.2µF along with a small value of Ci (in the
range of 0.1µF to 0.49µF), produces a click-less and pop-less shutdown function.
Output Capacitor Value Selection
Amplifying the lowest audio frequencies also requires the use of a high value output coupling capacitor (C10 and
C11 in Figure 28). A high value output capacitor can be expensive and may compromise space efficiency in
portable design. The speaker load (R) and the output capacitor (C) form a high pass filter with a low cutoff
frequency determined using Equation 7.
When using a typical headphone load of RL = 32Ω with a low frequency limit of 50Hz, CO is 99µF. The 200µF CO
shown in Figure 28 allows the LM4857 to drive a headphone whose frequency response extends below 50Hz.
Cstdby Value Selection
Cstdby is set to provide a large enough delay time on the threshold detect such that the device does not
inadvertently toggle in and out of Standby mode when in MODE 1 and low level music passages are playing.
This should be set for delay periods of several seconds. The demo board uses a 10uF cap that results in about
10s (typ) of delay time before entering Standby (MODE 1 must be enabled, Standby must be enabled, and the
input level must remain below the threshold level for 10s). Smaller values may result in Standby activation during
extended low periods of music.
PCB Layout Guidelines
This section provides practical guidelines for mixed signal PCB layout that involves various digital/analog power
and ground traces. Designers should note that these are only "rule-of-thumb" recommendations and the actual
results will depend heavily on the final layout.
General Mixed Signal Layout Recommendation
Power and Ground Circuits
For 2 layer mixed signal design, it is important to isolate the digital power and ground trace paths from the
analog power and ground trace paths. Star trace routing techniques (bringing individual traces back to a central
point rather than daisy chaining traces together in a serial manner) can have a major impact on low level signal
performance. Star trace routing refers to using individual traces to feed power and ground to each circuit or even
device. This technique will require a greater amount of design time but will not increase the final price of the
board. The only extra parts required will be some jumpers.
Single-Point Power / Ground Connections
The analog power traces should be connected to the digital traces through a single point (link). A "Pi-filter" can
be helpful in minimizing High Frequency noise coupling between the analog and digital sections. It is further
recommended to put digital and analog power traces over the corresponding digital and analog ground traces to
minimize noise coupling.
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Placement of Digital and Analog Components
All digital components and high-speed digital signal traces should be located as far away as possible from analog
components and circuit traces.
Avoiding Typical Design / Layout Problems
Avoid ground loops or running digital and analog traces parallel to each other (side-by-side) on the same PCB
layer. When traces must cross over each other do it at 90 degrees. Running digital and analog traces at 90
degrees to each other from the top to the bottom side as much as possible will minimize capacitive noise
coupling and cross talk.
LM4918 WQFN Board Artwork
Figure 30. Top Overlay
Figure 31. Top Solder
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Figure 32. Top Layer
18
Figure 33. Bottom Layer
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REVISION HISTORY
Changes from Revision B (April 2013) to Revision C
•
Page
Changed layout of National Data Sheet to TI format .......................................................................................................... 17
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