LM4868MTX/NOPB

更新时间:2024-10-29 14:24:23
品牌:TI
描述:1.5W, 2 CHANNEL, AUDIO AMPLIFIER, PDSO20, 4.40 MM, PLASTIC, TSSOP-20

LM4868MTX/NOPB 概述

1.5W, 2 CHANNEL, AUDIO AMPLIFIER, PDSO20, 4.40 MM, PLASTIC, TSSOP-20 音频/视频放大器

LM4868MTX/NOPB 规格参数

是否Rohs认证:符合生命周期:Obsolete
包装说明:TSSOP,Reach Compliance Code:compliant
ECCN代码:EAR99HTS代码:8542.33.00.01
风险等级:5.84Is Samacsys:N
标称带宽:20 kHz商用集成电路类型:AUDIO AMPLIFIER
JESD-30 代码:R-PDSO-G20JESD-609代码:e3
长度:6.5 mm湿度敏感等级:1
信道数量:2功能数量:1
端子数量:20最高工作温度:85 °C
最低工作温度:-40 °C标称输出功率:1.5 W
封装主体材料:PLASTIC/EPOXY封装代码:TSSOP
封装形状:RECTANGULAR封装形式:SMALL OUTLINE, THIN PROFILE, SHRINK PITCH
峰值回流温度(摄氏度):260认证状态:Not Qualified
座面最大高度:1.2 mm最大压摆率:15 mA
最大供电电压 (Vsup):5.5 V最小供电电压 (Vsup):2 V
表面贴装:YES温度等级:INDUSTRIAL
端子面层:TIN端子形式:GULL WING
端子节距:0.65 mm端子位置:DUAL
处于峰值回流温度下的最长时间:NOT SPECIFIED宽度:4.4 mm
Base Number Matches:1

LM4868MTX/NOPB 数据手册

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August 2002  
LM4868  
Output-Transient-Free Dual 2.1W Audio Amplifier Plus  
No Coupling Capacitor Stereo Headphone Function  
General Description  
Key Specifications  
n PO at 1% THD+N  
The LM4868 is a dual bridge-connected audio power ampli-  
fier which, when connected to a 5V supply, will deliver 2.1W  
to a 4load (Note 1) or 2.4W to a 3load (Note 2) with less  
than 1.0% THD+N. The LM4868 uses advanced, latest gen-  
eration circuitry to eliminate all traces of clicks and pops  
when the supply voltage is first applied. The amplifier has a  
headphone-amplifier-select input pin. It is used to switch the  
amplifiers from bridge to single-ended mode for driving  
headphones. A new circuit topology eliminates headphone  
output coupling capacitors (patent pending). A MUX control  
pin allows selection between the two sets of stereo input  
signals. The MUX control can also be used to select be-  
tween two different customer-specified closed-loop re-  
sponses.  
n
n
n
n
LM4868LQ, 3load  
LM4868LQ, 4load  
LM4868MTE, 4Ω  
LM4868MT, 8Ω  
2.4W (typ)  
2.1W (typ)  
1.9W (typ)  
1.1W (typ)  
n Single-ended mode - THD+N at 75mW into 32Ω  
(max)  
n Shutdown current  
0.5%  
0.7µA (typ)  
Features  
n Advanced “click and pop” suppression circuitry  
n Eliminates headphone amplifier output coupling  
capacitors (patent pending)  
Boomer audio power amplifiers are designed specifically to  
provide high quality output power from a surface mount  
package and require few external components. To simplify  
audio system design, the LM4868 combines dual bridge  
speaker amplifiers and stereo headphone amplifiers in one  
package.  
n Stereo headphone amplifier mode  
n Input mux control and two separate inputs per channel  
n Thermal shutdown protection circuitry  
n LLP, TSSOP, and exposed-DAP TSSOP packaging  
available  
The LM4868 features an externally controlled power-saving  
micropower shutdown mode, a stereo headphone amplifier  
mode, and thermal shutdown protection.  
Note 1: An LM4868LQ or LM4868MTE that has been properly mounted to  
a circuit board will deliver 2.1W into 4. The Mux control can also be used to  
select two different closed-loop responses. LM4868MT will deliver 1.1W into  
8. See the Application Information sections for further information concern-  
ing the LM4868LQ and the LM4868MT.  
Applications  
n Multimedia monitors  
n Portable and desktop computers  
n Portable audio systems  
Note 2: An LM4868LQ or LM4868MTE that has been properly mounted to a  
circuit board and forced-air cooled will deliver 2.4W into 3.  
Connection Diagrams  
20026758  
Top View  
Order Number LM4868MT, LM4868MTE  
See NS Package Number MTC20 for TSSOP  
See NS Package Number MXA20A for Exposed-DAP TSSOP  
Boomer® is a registered trademark of National Semiconductor Corporation.  
© 2002 National Semiconductor Corporation  
DS200267  
www.national.com  
Connection Diagrams (Continued)  
20026738  
Top View  
Order Number LM4868LQ  
See NS Package Number LQA24A for Exposed-DAP LLP  
Typical Application  
20026731  
*
Refer to the Application Information section titled PROPER SELECTION OF EXTERNAL COMPONENTS for details concerning the value of C .  
B
FIGURE 1. Typical Audio Amplifier Application Circuit (Pin out shown for the 24-pin Exposed-DAP LLP package.  
Numbers in ( ) are for the 20-pin MTE and MT packages.)  
www.national.com  
2
Absolute Maximum Ratings (Note 3)  
If Military/Aerospace specified devices are required,  
please contact the National Semiconductor Sales Office/  
Distributors for availability and specifications.  
See AN-450 “Surface Mounting and their Effects on  
Product Reliablilty” for other methods of soldering  
surface mount devices.  
Thermal Resistance  
Supply Voltage  
Storage Temperature  
Input Voltage  
6.0V  
−65˚C to +150˚C  
−0.3V to VDD  
+0.3V  
θJC (typ)MTC20  
θJA (typ)MTC20  
θJC (typ)MXA20A  
θJA (typ)MXA20A  
θJA (typ)MXA20A  
θJA (typ)MXA20A  
θJC (typ)LQA24A  
θJA (typ)LQA24A  
20˚C/W  
80˚C/W  
2˚C/W  
41˚C/W (Note 7)  
51˚C/W (Note 8)  
90˚C/W (Note 9)  
3.0˚C/W  
Power Dissipation (Note 4)  
ESD Susceptibility (Note 5)  
Internally limited  
All pins except Pin 3 (MT, MTE), Pin 2 (LQ)  
Pin 3 (MT, MTE), Pin 2 (LQ)  
ESD Susceptibility (Note 6)  
Junction Temperature  
2000V  
8000V  
200V  
42˚C/W (Note 10)  
150˚C  
Operating Ratings  
Solder Information  
Temperature Range  
TMIN TA TMAX  
Supply Voltage  
Small Outline Package  
−40˚C TA 85˚C  
2.0V VDD 5.5V  
Vapor Phase (60 sec.)  
215˚C  
220˚C  
Infrared (15 sec.)  
Electrical Characteristics for Entire IC (Notes 3, 11)  
The following specifications apply for VDD= 5V unless otherwise noted. Limits apply for TA= 25˚C.  
Symbol  
Parameter  
Conditions  
LM4868  
Typical Limit  
Units  
(Limits)  
(Note 12) (Note 13)  
VDD  
IDD  
Supply Voltage  
2
V (min)  
V (max)  
5.5  
Quiescent Power Supply Current VIN = 0V, IO = 0A (Note 14), HP-IN = 0V  
VIN = 0V, IO = 0A (Note 14), HP-IN = 4V  
7.5  
3.0  
0.7  
25  
15  
6
mA (max)  
mA (max)  
µA (max)  
mV (min)  
mV (max)  
ISD  
Shutdown Current  
VDD applied to the SHUTDOWN pin  
GND applied to SHUTDOWN pin  
VIN applied to selected MUX channel  
2
THum  
Un-Mute Threshold Voltage  
10  
40  
Electrical Characteristics for Bridged-Mode Operation (Notes 3, 11)  
The following specifications apply for VDD= 5V unless otherwise specified. Limits apply for TA= 25˚C.  
Symbol  
Parameter  
Conditions  
LM4868  
Typical Limit  
(Note 12) (Note 13)  
Units  
(Limits)  
VOS  
PO  
Output Offset Voltage  
VIN = 0V  
5
50  
mV (max)  
Output Power (Note 15)  
THD = 1%, f = 1kHz  
(Note 16)  
LM4868MTE, RL = 3Ω  
LM4868LQ, RL = 3Ω  
LM4868MTE, RL = 4Ω  
LM4868LQ, RL = 4Ω  
LM4868, RL = 8Ω  
2.2  
2.4  
1.9  
2.1  
1.1  
W
W
W
W
1.0  
W (min)  
THD+N = 10%, f = 1kHz (Note 16)  
LM4868MTE, RL = 3Ω  
LM4868LQ, RL = 3Ω  
LM4868MTE, RL = 4Ω  
LM4868LQ, RL = 4Ω  
LM4868, RL = 8Ω  
3.0  
3.0  
2.6  
2.6  
1.5  
0.34  
W
W
W
W
W
W
THD+N = 1%, f = 1 kHz, RL = 32Ω  
3
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Electrical Characteristics for Bridged-Mode Operation (Notes 3, 11) (Continued)  
The following specifications apply for VDD= 5V unless otherwise specified. Limits apply for TA= 25˚C.  
Symbol  
Parameter  
Conditions  
LM4868  
Typical Limit  
Units  
(Limits)  
(Note 12) (Note 13)  
THD+N Total Harmonic Distortion+Noise 20Hz f 20kHz, AVD = 2  
LM4868MTE, RL = 4, PO = 2W  
0.3  
0.3  
0.3  
67  
%
%
LM4868LQ, RL = 4, PO = 2W  
LM4868, RL = 8, PO = 1W  
VDD = 5V, VRIPPLE = 200 mVRMS, RL = 8,  
CB = 2.2µF  
%
PSRR  
Power Supply Rejection Ratio  
dB  
XTALK  
SNR  
Channel Separation  
f = 1 kHz, CB = 2.2µF  
80  
97  
dB  
dB  
Signal To Noise Ratio  
VDD = 5V, PO = 1.1W, RL = 8Ω  
Electrical Characteristics for Single-Ended Operation (Notes 3, 11)  
The following specifications apply for VDD= 5V unless otherwise specified. Limits apply for TA= 25˚C.  
Symbol  
Parameter  
Conditions  
LM4868  
Units  
(Limits)  
Typical  
Limit  
(Note 13)  
50  
(Note 12)  
VOS  
PO  
Output Offset Voltage  
Output Power  
VIN = 0V  
5
mV (max)  
mW (min)  
mW  
THD = 0.5%, f = 1kHz, RL = 32Ω  
THD+N = 1%, f = 1kHz, RL = 8(Note  
17)  
85  
75  
180  
THD+N = 1%, f = 1kHz, RL = 16Ω  
THD+N = 1%, f = 1kHz, RL = 32Ω  
THD+N = 10%, f = 1kHz, RL = 16Ω  
THD+N = 10%, f = 1kHz, RL = 32Ω  
THD = 0.05%, RL = 5kΩ  
165  
88  
mW  
mW  
mW  
mW  
VP-P  
%
208  
114  
1
VOUT  
Output Voltage Swing  
THD+N  
Total Harmonic Distortion+Noise  
AV = −1, PO = 75mW, 20 Hz f 20kHz,  
RL = 32Ω  
0.2  
PSRR  
Power Supply Rejection Ratio  
CB = 2.2µF, VRIPPLE = 200mVRMS  
f = 1kHz  
,
52  
dB  
XTALK  
SNR  
Channel Separation  
f = 1kHz, CB = 2.2µF  
60  
94  
dB  
dB  
Signal To Noise Ratio  
VDD = 5V, PO = 340mW, RL = 8Ω  
Note 3: 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 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 4: The maximum power dissipation must be derated at elevated temperatures and is dictated by T  
, θ , and the ambient temperature T . The maximum  
A
JMAX JA  
allowable power dissipation is P  
= (T  
− T )/θ . For the LM4868, T  
= 150˚C. For the θ s for different packages, please see the Application  
DMAX  
JMAX  
A
JA  
JMAX  
JA  
Information section or the Absolute Maximum Ratings section.  
Note 5: Human body model, 100pF discharged through a 1.5kresistor.  
Note 6: Machine model, 220pF–240pF discharged through all pins.  
2
Note 7: The given θ is for an LM4868 packaged in an MXA20A with the Exposed-DAP soldered to an exposed 2in area of 1oz printed circuit board copper.  
JA  
2
Note 8: The given θ is for an LM4868 packaged in an MXA20A with the Exposed-DAP soldered to an exposed 1in area of 1oz printed circuit board copper.  
JA  
Note 9: The given θ is for an LM4868 packaged in an MXA20A with the Exposed-DAP not soldered to printed circuit board copper.  
JA  
2
Note 10: The given θ is for an LM4868 packaged in an LQA24A with the Exposed-DAP soldered to an exposed 2in area of 1oz printed circuit board copper.  
JA  
Note 11: All voltages are measured with respect to the ground (GND) pins, unless otherwise specified.  
Note 12: Typicals are measured at 25˚C and represent the parametric norm.  
Note 13: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level). Datasheet min/max specification limits are guaranteed by design, test, or  
statistical analysis.  
Note 14: The quiescent power supply current depends on the offset voltage when a practical load is connected to the amplifier.  
Note 15: Output power is measured at the device terminals.  
Note 16: When driving 3or 4loads and operating on a 5V supply, the LM4868LQ and LM4868MTE must be mounted to a circuit board that has a minimum of  
2
2.5in of exposed, uniterrupted copper area connected to the LLP or TSSOP package’s exposed DAP.  
Note 17: See Application Information section ’Single-Ended Output Power Performance and Measurement Considerations’ for more information.  
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4
Typical Performance Characteristics  
MTE- and LQ- Specific Characteristics  
LM4868MTE  
LM4868MTE  
THD+N vs Output Power  
THD+N vs Frequency  
20026734  
20026754  
20026755  
20026733  
LM4868LQ  
THD+N vs Output Power  
LM4868LQ  
THD+N vs Frequency  
20026753  
LM4868MTE  
THD+N vs Output Power  
LM4868LQ  
THD+N vs Output Power  
20026736  
5
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Typical Performance Characteristics  
MTE- and LQ- Specific Characteristics (Continued)  
LM4868LQ, LM4868MTE  
LM4868MTE (Note 18)  
Power Dissipation vs Power Output  
Power Derating Curve  
20026761  
20026759  
LM4868LQ  
Power Derating Curve  
20026769  
Note 18: This curve shows the LM4868MTE’s thermal dissipation ability at different ambient temperatures given these conditions:  
500LFPM + JEDEC board: The part is soldered to a 1S2P 20-lead exposed-DAP TSSOP test board with 500 linear feet per minute of forced-air flow across  
it. Board information - copper dimensions: 74x74mm, copper coverage: 100% (buried layer) and 12% (top/bottom layers), 16 vias under the exposed-DAP.  
2
2
500LFPM + 2.5in : The part is soldered to a 2.5in , 1 oz. copper plane with 500 linear feet per minute of forced-air flow across it.  
2
2
2.5in : The part is soldered to a 2.5in , 1oz. copper plane.  
Not Attached: The part is not soldered down and is not forced-air cooled.  
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6
Typical Performance  
Characteristics  
THD+N vs Frequency  
THD+N vs Frequency  
THD+N vs Output Power  
THD+N vs Output Power  
20026703  
20026705  
20026707  
20026704  
20026706  
20026708  
THD+N vs Frequency  
THD+N vs Output Power  
7
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Typical Performance Characteristics (Continued)  
THD+N vs Output Power  
THD+N vs Frequency  
20026765  
20026763  
THD+N vs Output Power  
THD+N vs Frequency  
20026766  
20026764  
Output Power vs  
Load Resistance  
Power Dissipation vs  
Supply Voltage  
20026762  
20026760  
www.national.com  
8
Typical Performance Characteristics (Continued)  
Output Power vs  
Supply Voltage  
Output Power vs  
Supply Voltage  
20026709  
20026710  
20026712  
20026714  
Output Power vs  
Supply Voltage  
Output Power vs  
Load Resistance  
20026711  
Output Power vs  
Load Resistance  
Power Dissipation vs  
Output Power  
20026713  
9
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Typical Performance Characteristics (Continued)  
Dropout Voltage vs  
Supply Voltage  
Power Derating Curve  
20026715  
20026716  
Power Dissipation vs  
Output Power  
Noise Floor  
20026717  
20026718  
Channel Separation  
Channel Separation  
20026719  
20026720  
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10  
Typical Performance Characteristics (Continued)  
Power Supply  
Rejection Ratio  
Open Loop  
Frequency Response  
20026721  
20026722  
Supply Current vs  
Supply Voltage  
20026723  
External Components Description  
( Refer to Figure 1. )  
Components  
Functional Description  
1.  
Ri This is the inverting input resistance that sets the closed-loop gain in conjunction with Rf. This resistor also  
forms a high pass filter with Ci at fc = 1/(2πRiCi).  
2.  
Ci This is the input coupling capacitor. It blocks the DC voltage at the amplifier’s input terminals. It also  
creates a highpass filter with Ri at fc = 1/(2πRiCi). Refer to the section, Selecting External Components,  
for an explanation of how to determine the value of Ci.  
3.  
4.  
Rf This is the feedback resistance. It sets the closed-loop gain in conjunction with Ri.  
Cs This is the supply bypass capacitor. It provides power supply filtering. Refer to the Power Supply  
Bypassing section for information concerning proper placement and selection of the supply bypass  
capacitor.  
5.  
CB This is the bypass pin capacitor. It provides half-supply filtering. Refer to the section, Proper Selection of  
External Components, for information concerning proper placement and selection of CB.  
Application Information  
ELIMINATING OUTPUT COUPLING CAPACITORS  
couples the audio signal to the headphones. The signal  
return to circuit ground is through the headphone jack’s  
sleeve.  
Typical single-supply audio amplifiers that can switch be-  
tween driving bridge-tied-load (BTL) speakers and  
single-ended (SE) headphones use a coupling capacitor on  
each SE output. This capacitor blocks the half-supply volt-  
age to which the output amplifiers are typically biased and  
The LM4868 eliminates these coupling capacitors. Amp2A is  
internally configured to apply VDD/2 to a stereo headphone  
jack’s sleeve. This voltage matches the quiescent voltage  
present on the Amp1A and Amp1B outputs that drive the  
11  
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shown in Trace A. Trace C is the VOUT- output signal and  
trace D is the VOUT+ output signal. The shutdown signal  
frequency is 1Hz with a 50% duty cycle. Figure 3 is gener-  
ated with the same conditions except that the output drives a  
32single-ended (SE) load. Again, no trace of output tran-  
sients is seen.  
Application Information (Continued)  
headphones. The headphones operate in a manner very  
similar to a bridge-tied-load (BTL). 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.  
When operating as a headphone amplifier, the headphone  
jack sleeve is not connected to circuit ground. Using the  
headphone output jack as a line-level output will place the  
LM4868’s one-half supply voltage on a plug’s sleeve con-  
nection. Driving  
a
portable notebook computer or  
audio-visual display equipment is possible. This presents no  
difficulty when the external equipment uses capacitively  
coupled inputs. For the very small minority of equipment that  
is DC-coupled, the LM4868 monitors the current supplied by  
the amplifier that drives the headphone jack’s sleeve. If this  
current exceeds 500mAPK, the amplifier is shutdown, pro-  
tecting the LM4868 and the external equipment. For more  
information, see the section titled ’Single-Ended Output  
Power Performance and Measurement Considerations’.  
20026757  
FIGURE 3. Single-ended output signal (Trace B) is  
devoid of transients. The SHUTDOWN pin is driven by  
a shutdown signal (Trace A). The inverting output  
(Trace C) and the VBYPASS output (Trace D) are applied  
across a 32BTL load.  
OUTPUT TRANSIENT (’POPS AND CLICKS’)  
ELIMINATED  
The LM4868 contains advanced circuitry that eliminates out-  
put transients (’pop and click’). This circuitry prevents all  
traces of transients when the supply voltage is first applied,  
when the part resumes operation after shutdown, or when  
switching between BTL speakers and SE headphones. Two  
circuits combine to eliminate pop and click. One circuit  
mutes the output when switching between speaker loads.  
Another circuit monitors the input signal. It maintains the  
muted condition until there is sufficient input signal magni-  
USING THE LM4868 TO UPGRADE LM4863 AND LM4873  
DESIGNS  
The  
LM4868’s  
noise-free  
operation  
plus  
coupling-capacitorless headphone operation and functional  
compatibility with the LM4873 and the LM4863 simplifies  
upgrading systems using these parts. Upgrading older de-  
signs that use either the LM4863 or the LM4873 is easy.  
Simply remove and short the coupling capacitors located  
between the LM4873’s or LM4863’s Amp1A and Amp1B  
outputs and the headphone connections. Also remove the  
1kresistor between each headphone connection and  
ground. Finally, remove any resistors connected to the  
HP-IN pin (typically two 100kresistors). Connect the HP-IN  
pin directly to the headphone jack control pin as shown in  
Figure 4.  
>
tude ( 25mVRMS, typ) to mask any remaining transient that  
may occur.  
The LM4868’s pin configuration simplifies the process of  
upgrading systems that use the LM4863. Except for its four  
MUX function pins, the LM4868’s pin configuration matches  
the LM4863’s pin configuration. If the LM4868’s MUX func-  
tionality is not needed when replacing an LM4863, connect  
the MUX CTRL pin to either VDD or ground. To ensure  
correct amplifier operation, unused MUX inputs should be  
tied to GND. As shown in Table 1, grounding the MUX CTRL  
pin selects stereo input 1 (−IN A1 and −IN B1), whereas  
applying VDD to the MUX CTRL pin selects stereo input 2  
(−IN A2 and −IN B2).  
20026756  
FIGURE 2. Differential output signal (Trace B) is devoid  
of transients. The SHUTDOWN pin is driven by a  
shutdown signal (Trace A). The inverting output (Trace  
C) and the non-inverting output (Trace D) are applied  
across an 8BTL load.  
The LM4868’s unique headphone sense circuit requires a  
dual switch headphone jack. Replace the four-terminal head-  
phone jack used with the LM4863 and LM4873 with the  
five-terminal headphone jack, such as the Switchcraft  
35RAPC4BH3, shown in Figure 4. Connect the +OUT A  
(Amp2A) pin to the five-terminal headphone jack’s sleeve  
pin.  
Figure 2 shows the LM4868’s lack of transients in the differ-  
ential signal (Trace B) across a BTL 8load. The LM4868’s  
active-high SHUTDOWN pin is driven by the logic signal  
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12  
Application Information (Continued)  
20026731  
FIGURE 4. Typical Audio Amplifier Application Circuit  
(Pin out shown for the 24-pin Exposed-DAP LLP package. Numbers in ( ) are for the 20-pin MTE and MT packages.)  
Typical LM4868 applications use the MUX to switch between  
two stereo input signals. Each stereo channel’s gain can be  
tailored to produce the required output signal level by choos-  
ing the appropriate input and feedback resistor ratio.  
Another configuration uses the MUX to select two different  
gains or frequency compensated gains that amplify a single  
pair of stereo input signals. Figure 5 shows two different  
feedback networks, Network 1 and Network 2. Network 1  
produces increasing gain as the input signal’s frequency  
decreases. This can be used to compensate a small,  
full-range speaker’s low frequency response roll-off. Network  
2 sets the gain for an alternate load such as headphones.  
The circuit in Figure 6 uses Network 1 when driving external  
speakers, switching to Network 2 when headphones are  
connected. The normally closed control switch in Figure 6’s  
headphone jack connects to the MUX CTRL pin. When  
headphones are connected, the LM4868’s internal pull-up  
20026728  
that applies VDD to the HP-IN and the external 100kresis-  
tor applies VDD to MUX CTRL pin. Simultaneously applying  
these control voltages automatically selects the amplifier  
FIGURE 5. Input MUX Example  
(headphone or bridge) and switches the gain (MUX channel  
selection). Alternatively, leaving the MUX CTRL pin indepen-  
dently accessible allows a user to select bass boost as  
needed. This alternative user-selectable bass-boost scheme  
requires connecting equal ratio resistor feedback networks  
to each MUX input channel. The value of the resistor in the  
RC network is chosen to give a gain that is necessary to  
achieve the desired bass-boost.  
STEREO-INPUT MULTIPLEXER (STEREO MUX)  
The LM4868 has two stereo inputs. The MUX CTRL Pin  
controls which stereo input is active. As shown in the Truth  
Table for Logic Inputs, applying 0V to the MUX CTRL input  
activates stereo input 1, whereas applying VDD to the MUX  
CTRL inputs activates stereo input 2. To ensure correct  
amplifier operation, unused MUX inputs should be tied to  
GND.  
Switching between the MUX channels may change the input  
signal source or the feedback resistor network. During the  
channel switching transition, the average voltage level  
13  
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with a 1.27mm pitch. Ensure efficient thermal conductivity by  
plugging and tenting the vias with plating and solder mask,  
respectively.  
Application Information (Continued)  
present on the internal amplifier’s input may change. This  
change can slew at a rate that may produce audible voltage  
transients or clicks in the amplifier’s output signal. Using the  
MUX to select between two vastly dissimilar gains is a typical  
transient-producing situation. As the MUX is switched, an  
audible click may occur as the gain suddenly changes.  
Best thermal performance is achieved with the largest prac-  
tical copper heat sink area. If the heatsink and amplifier  
share the same PCB layer, a nominal 2.5in2 (min) area is  
necessary for 5V operation with a 4load. Heatsink areas  
not placed on the same PCB layer as the LM4868 should be  
5in2 (min) for the same supply voltage and load resistance.  
The last two area recommendations apply for 25˚C ambient  
temperature. Increase the area to compensate for ambient  
temperatures above 25˚C. In systems using cooling fans, the  
LM4868MTE can take advantage of forced air cooling. With  
an air flow rate of 450 linear-feet per minute and a 2.5in2  
exposed copper or 5.0in2 inner layer copper plane heatsink,  
the LM4868MTE can continuously drive a 3load to full  
power. The LM4868LQ achieves the same output power  
level without forced-air cooling. In all circumstances and  
under all conditions, the junction temperature must be held  
below 150˚C to prevent activating the LM4868’s thermal  
shutdown protection. The LM4868’s power de-rating curve in  
the Typical Performance Characteristics shows the maxi-  
mum power dissipation versus temperature. Example PCB  
layouts for the exposed-DAP TSSOP and LQ packages are  
shown in the Demonstration Board Layout section. Further  
detailed and specific information concerning PCB layout and  
fabrication and mounting an LQ (LLP) is found in National  
Semiconductor’s AN1187.  
PIN OUT COMPATIBILITY WITH THE LM4863  
The LM4868 pin out was designed to simplify replacing the  
LM4863: except for the four Pins(-IN A2, MUX CTRL, -IN B2,  
and NC) that implement the LM4868’s extra functionality, the  
LM4868MT/MTE and LM4863MT/MTE pin outs match.  
(Note 19)  
Note 19: If the LM4868 replaces an LM4863 and the input MUX circuitry is  
not being used, the LM4868 MUX CTRL pin must be tied to V or GND and  
DD  
the unused MUX inputs must be connected to GND.  
PCB LAYOUT AND SUPPLY REGULATION  
CONSIDERATIONS FOR DRIVING 3AND 4LOADS  
Power dissipated by a load is a function of the voltage swing  
across the load and the load’s impedance. As load imped-  
ance decreases, load dissipation becomes increasingly de-  
pendent on the interconnect (PCB trace and wire) resistance  
between the amplifier output pins and the load’s connec-  
tions. Residual trace resistance causes a voltage drop,  
which results in power dissipated in the trace and not in the  
load as desired. For example, 0.1trace resistance reduces  
the output power dissipated by a 4load from 2.1W to 2.0W.  
The problem of decreased load dissipation is exacerbated  
as load impedance decreases. Therefore, to maintain the  
highest load dissipation and widest output voltage swing,  
PCB traces that connect the output pins to a load must be as  
wide as possible.  
20026739  
FIGURE 6. As configured, connecting headphones to  
this jack automatically selects the stereo headphone  
amplifier and, with the additional NC switch, changes  
MUX channels (Network 2 in Figure 5 )  
EXPOSED-DAP MOUNTING CONSIDERATIONS  
The LM4868’s exposed-DAP (die attach paddle) packages  
(MTE and LQ) 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 surround-  
ing PCB copper area heatsink, copper traces, ground plane,  
and finally, surrounding air. The result is a low voltage audio  
power amplifier that produces 2.4W dissipation in a 4load  
at 1% THD+N and over 3W in a 3load at 10% THD+N.  
This high power is achieved through careful consideration of  
necessary thermal design. Failing to optimize thermal design  
may compromise the LM4868’s high power performance and  
activate unwanted, though necessary, thermal shutdown  
protection.  
Poor power supply regulation adversely affects maximum  
output power. A poorly regulated supply’s output voltage  
decreases with increasing load current. Reduced supply  
voltage causes decreased headroom, output signal clipping,  
and reduced output power. Even with tightly regulated sup-  
plies, trace resistance creates the same effects as poor  
supply regulation. Therefore, making the power supply  
traces as wide as possible helps maintain full output voltage  
swing.  
BRIDGE CONFIGURATION EXPLANATION  
The MTE and LQ packages must have their DAPs soldered  
to a copper pad on the PCB. The DAP’s PCB copper pad is  
then, ideally, connected to a large plane of continuous un-  
broken 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 32 (4 X 8) (MTE) or 6 (3  
X 2) (LQ) vias. The via diameter should be 0.012in - 0.013in  
As shown in Figure 4, the LM4868 consists of two pairs of  
operational amplifiers, forming a two-channel (channel A and  
channel B) stereo amplifier. (Though the following discusses  
channel A, it applies equally to channel B.) External resistors  
Rf and Ri set the closed-loop gain of Amp1A, whereas two  
internal 20kresistors set Amp2A’s gain at -1. The LM4868  
drives a load, such as a speaker, connected between the two  
amplifier outputs, -OUTA and +OUTA.  
Figure 4 shows that Amp1A’s output serves as Amp2A’s  
input. This results in both amplifiers producing signals iden-  
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14  
soldered to a DAP pad that expands to a copper area of 2in2  
on a PCB, the LM4868’s θJA is 41˚C/W. At any given ambient  
temperature TA, use Equation (4) to find the maximum inter-  
nal power dissipation supported by the IC packaging. Rear-  
ranging Equation (4) and substituting PDMAX for PDMAX’ re-  
sults in Equation (5). This equation gives the maximum  
ambient temperature that still allows maximum stereo power  
dissipation without violating the LM4868’s maximum junction  
temperature.  
Application Information (Continued)  
tical in magnitude, but 180˚ out of phase. Taking advantage  
of this phase difference, a load is placed between -OUTA  
and +OUTA and driven differentially (’commonly referred to  
as bridge mode’). This results in a differential gain of  
*
AVD = 2 (Rf/R )  
(1)  
i
Bridge mode amplifiers are different from single-ended am-  
plifiers that drive loads connected between a single amplifi-  
er’s output and ground. For a given supply voltage, bridge  
mode has a distinct advantage over the single-ended con-  
figuration: 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 as-  
sumes that the amplifier is not current limited or that the  
output signal is not clipped. To ensure minimum output sig-  
nal clipping when choosing an amplifier’s closed-loop gain,  
refer to the Audio Power Amplifier Design section.  
TA = TJMAX − 2 X PDMAX θJA  
(5)  
For a typical application with a 5V power supply and an 4Ω  
load, the maximum ambient temperature that allows maxi-  
mum stereo power dissipation without exceeding the maxi-  
mum junction temperature is approximately 43˚C for the LQ  
package and 45˚C for the MTE package.  
TJMAX = PDMAX θJA + TA  
(6)  
Equation (6) gives the maximum junction temperature  
TJMAX. If the result violates the LM4868’s 150˚C, reduce the  
maximum junction temperature by reducing the power sup-  
ply voltage or increasing the load resistance. Further allow-  
ance should be made for increased ambient temperatures.  
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. Eliminating an output cou-  
pling 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.  
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.  
POWER DISSIPATION  
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. When adding a  
heat sink, the θJA is the sum of θJC, θCS, and θSA. (θJC is the  
junction−to−case thermal impedance, θCS is the  
case−to−sink thermal impedance, and θSA is the  
sink−to−ambient thermal impedance.) Refer to the Typical  
Performance Characteristics curves for power dissipation  
information at lower output power levels.  
Power dissipation is a major concern when designing a  
successful single-ended or bridged amplifier. Equation (2)  
states the maximum power dissipation point for  
single-ended amplifier operating at a given supply voltage  
and driving a specified output load.  
a
PDMAX = (VDD)2/(2π2RL): Single-Ended  
(2)  
However, a direct consequence of the increased power de-  
livered to the load by a bridge amplifier is higher internal  
power dissipation for the same conditions.  
The LM4868 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 ampli-  
fier. From Equation (3), assuming a 5V power supply and an  
4load, the maximum single channel power dissipation is  
1.27W or 2.54W for stereo operation.  
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 stabi-  
lize 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  
LM4868’s supply pins and ground. Do not substitute a ce-  
ramic capacitor for the tantalum. Doing so may cause oscil-  
lation. Keep the length of leads and traces that connect  
capacitors between the LM4868’s power supply pin and  
ground as short as possible. Connecting a 1µF 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  
PDMAX = 4 (VDD)2/(2π2RL): Bridge Mode (3)  
*
The LM4868’s power dissipation is twice that given by Equa-  
tion (2) or Equation (3) when operating in the single-ended  
mode or bridge mode, respectively. Twice the maximum  
power dissipation point given by Equation (3) must not ex-  
ceed the power dissipation given by Equation (4):  
PDMAX’ = (TJMAX − TA)/θJA  
(4)  
The LM4868’s TJMAX = 150˚C. In the LQ package soldered  
to a DAP pad that expands to a copper area of 5in2 on a  
PCB, the LM4868’s θJA is 42˚C/W. In the MTE package  
15  
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eration. To ensure that the output signal remains  
transient−free, do not cycle the shutdown function  
faster than 1Hz.  
Application Information (Continued)  
pop performance. The selection of bypass capacitor values,  
especially CB, depends on desired PSRR requirements,  
click and pop performance (as explained in the section,  
Proper Selection of External Components), system cost,  
and size constraints.  
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 100kpull−up resistor between the  
SHUTDOWN pin and VDD. Connect the switch between the  
SHUTDOWN pin and ground. Select normal amplifier opera-  
tion by closing the switch. Opening the switch connects the  
SHUTDOWN pin to VDD through the pull−up resistor, acti-  
vating micro−power shutdown. The switch and resistor guar-  
antee that the SHUTDOWN pin will not float. This prevents  
unwanted state changes. In a system with a microprocessor  
or a microcontroller, use a digital output to apply the control  
voltage to the SHUTDOWN pin. Driving the SHUTDOWN pin  
with active circuitry eliminates the pull up resistor.  
MICRO−POWER SHUTDOWN  
The voltage applied to the SHUTDOWN pin controls the  
LM4868’s shutdown function. Activate micro−power shut-  
down by applying VDD to the SHUTDOWN pin. When active,  
the LM4868’s micro−power shutdown feature turns off the  
amplifier’s bias circuitry, reducing the supply current. The  
logic threshold is typically VDD/2. The low 0.7µA typical  
shutdown current is achieved by applying a voltage that is as  
near as VDD as possible to the SHUTDOWN pin. A voltage  
that is less than VDD may increase the shutdown current.  
Table 1 shows the logic signal levels that activate and deac-  
tivate micro−power shutdown and headphone amplifier op-  
Truth Table for Logic Inputs  
SHUTDOWN  
PIN  
HP-IN  
PIN  
MUX CHANNEL  
INPUT SELECT  
Logic Low  
Logic High  
Logic Low  
Logic High  
X
OPERATIONAL MODE (MUX  
INPUTCHANNEL #)  
Logic Low  
Logic Low  
Logic Low  
Logic Low  
Logic High  
= −OUTB signal  
= −OUTB signal  
Bridged amplifiers (1)  
Bridged amplifiers (2)  
−OUTB signal  
−OUTB signal  
X
Single-ended amplifiers (1)  
Single-ended amplifiers (2)  
Micro−power shutdown  
provides the return to Amp2A. A headphone jack with one  
control pin contact is sufficient to drive the HP−IN pin when  
connecting headphones.  
HEADPHONE (SINGLE-ENDED) AMPLIFIER  
OPERATION  
A switch can replace the headphone jack contact pin. When  
a switch shorts the HP−IN pin to VDD, bridge−connected  
speakers are muted and Amp1A and Amp2A drive a pair of  
headphones. When a switch shorts the HP−IN pin to GND,  
the LM4868 operates in bridge mode. If headphone drive is  
not needed, short the HP−IN pin to the −OUTB pin.  
An internal pull−up circuit is connected to the HP−IN (pin 20)  
headphone amplifier control pin. When this pin is left uncon-  
nected, VDD is applied to the HP−IN. This turns off Amp2B  
and switches Amp2A’s input signal from an audio signal to  
the VDD/2 voltage present on pin 14. The result is muted  
bridge-connected loads. Quiescent current consumption is  
reduced when the IC is in this single−ended mode.  
Figure 7 shows an optional resistor connected between the  
amplifier output that drives the headphone jack sleeve and  
ground. This resistor provides a ground path that supressed  
power supply hum. This hum may occur in applications such  
as notebook computers in a shutdown condition and con-  
nected to an external powered speaker. The resistor’s 100Ω  
value is a suggested starting point. Its final value must be  
determined based on the tradeoff between the amount of  
noise suppression that may be needed and minimizing the  
additional current drawn by the resistor (25mA for a 100Ω  
resistor and a 5V supply).  
Figure 8 shows the implementation of the LM4868’s head-  
phone control function. An internal comparator with a nomi-  
nal 400mV offset monitors the signal present at the −OUTB  
output. It compares this signal against the signal applied to  
the HP−IN pin. When these signals are equal, as is the case  
when a BTL is connected to the amplifier, the comparator  
forces the LM4868 to maintain bridged−amplifier operation.  
When the HP−IN pin is externally floated, such as when  
headphones are connected to the jack shown in Figure 8,  
and internal pull−up forces VDD on the internal comparator’s  
HP−IN inputs. This changes the comparator’s output state  
and enables the headphone function: it turns off Amp2B,  
switches Amp2A’s input signal from an audio signal to the  
VDD/2 voltage present on pin 14, and mutes the  
bridge-connected loads. Amp1A and Amp1B drive the head-  
phones.  
ESD Protection  
As stated in the Absolute Maximum Ratings, pin 28 on the  
MT and MH packages have a maximum ESD susceptibility  
rating of 8000V. For higher ESD voltages, the addition of a  
PCDN042 dual transil (from California Micro Devices), as  
shown in Figure 7, will provide additional protection.  
Figure 8 also shows the suggested headphone jack electri-  
cal 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  
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16  
Application Information (Continued)  
20026724  
FIGURE 8. Headphone Circuit  
(Pin numbers in ( ) are for the 20-pin MTE and MT  
packages.)  
200267A1  
FIGURE 7. The PCDN042 provides additional ESD  
protection beyond the 8000V shown in the Absolute  
Maximum Ratings for the AMP2A output  
SELECTING EXTERNAL COMPONENTS  
Input Capacitor Value Selection  
Single-Ended Output Power Performance and  
Measurement Considerations  
Amplifying the lowest audio frequencies requires high value  
input coupling capacitor (Ci in Figure 4). A high value capaci-  
tor 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.  
The LM4868 delivers clean, low distortion SE output power  
into loads that are greater than 10. As an example, output  
power for 16and 32loads are shown in the Typical  
Performance Characteristic curves. For loads less than  
10, the LM4876 can typically supply 180mW of low distor-  
tion power. However, when higher dissipation is desired in  
loads less than 10, a dramatic increase in THD+N may  
occur. This is normal operation and does not indicate that  
proper functionality has ceased. When a jump from moder-  
ate to excessively high distortion is seen, simply reducing  
the output voltage swing will restore the clean, low distortion  
SE operation.  
Besides effecting system cost and size, Ci has an affect on  
the LM4868’s click and pop performance. When the supply  
voltage is first applied, a transient (pop) is created as the  
charge on the input capacitor changes from zero to a quies-  
cent state. The magnitude of the pop is directly proportional  
to the input capacitor’s size. Higher value capacitors need  
more time to reach a quiescent DC voltage (usually VDD/2)  
when charged with a fixed current. The amplifier’s output  
charges the input capacitor through the feedback resistor,  
Rf. Thus, pops can be minimized by selecting an input  
capacitor value that is no higher than necessary to meet the  
desired −3dB frequency and is between 0.14CB and 0.20CB.  
The dramatic jump in distortion for loads less than 10Ω  
occurs when current limiting circuitry activates. During SE  
operation, AMP2A (refer to Figure 4) drives the headphone  
sleeve. An on-board circuit monitors this amplifier’s output  
current. The sudden increase in THD+N is caused by the  
current limit circuitry forcing AMP2A into a high−impedance  
output mode. When this occurs, the output waveform has  
discontinuities that produce large amounts of distortion. It  
has been observed that as the output power is steadily  
increased, the distortion may jump from 5% to greater than  
35%. Indeed, 10% THD+N may not actually be achievable.  
A shown in Figure 4, the input resistor (RI) and the input  
capacitor, CI produce a −3dB high pass filter cutoff frequency  
that is found using Equation (7).  
f−3dB = 1/(2πRINCI)  
(7)  
Using the Single−Ended Output for Line Level  
Applications  
As an example when using a speaker with a low frequency  
limit of 150Hz, Ci, using Equation (4) is 0.063µF. The 1.0µF  
Ci shown in Figure 4 allows the LM4868 to drive high effi-  
ciency, full range speaker whose response extends below  
30Hz.  
Some samples of the LM4868 may exhibit small amplitude,  
high frequency oscillation when the SE output is connected  
to a line-level input. This oscillation can be eliminated by  
connecting a 5%, 300resistor between Amp2A’s output pin  
and each amplifier, AMP1A and AMP1B, output.  
Bypass Capacitor Value Selection  
Besides minimizing the input capacitor size, careful consid-  
eration should be paid to value of CB, the capacitor con-  
nected to the BYPASS pin. Since CB determines how fast  
the LM4868 settles to quiescent operation, its value is critical  
when minimizing turn-on pops. The slower the LM4868’s  
outputs ramp to their quiescent DC voltage (nominally 1/2  
17  
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Application Information (Continued)  
Input Level:  
Input Impedance:  
Bandwidth:  
1 VRMS  
20 kΩ  
V
DD), the smaller the turn-on pop. Choosing CB equal to  
100 Hz−20 kHz 0.25 dB  
1.0µF along with a small value of Ci (in the range of 0.1µF to  
0.39µF), produces a click-less and pop-less shutdown func-  
tion. As discussed above, choosing Ci no larger than neces-  
sary for the desired bandwidth helps minimize clicks and  
pops. CB’s value should be in the range of 5 times to 7 times  
the value of Ci. This ensures that output transients are  
eliminated when power is first applied or the LM4868 re-  
sumes operation after shutdown.  
The design begins by specifying the minimum supply voltage  
necessary to obtain the specified output power. One way to  
find the minimum supply voltage is to use the Output Power  
vs Supply Voltage curve in the Typical Performance Char-  
acteristics section. Another way, using Equation (8), is to  
calculate the peak output voltage necessary to achieve the  
desired output power for a given load impedance. To ac-  
count for the amplifier’s dropout voltage, two additional volt-  
ages, based on the Dropout Voltage vs Supply Voltage in the  
Typical Performance Characteristics curves, must be  
added to the result obtained by Equation (8). The result is  
Equation (9).  
OPTIMIZING CLICK AND POP REDUCTION  
PERFORMANCE  
The LM4868 contains circuitry that eliminates turn-on and  
shutdown transients (“clicks and pops“) and transients that  
could occur when switching between BTL speakers and  
single-ended headphones. For this discussion, turn-on re-  
fers to either applying the power supply voltage or when the  
shutdown mode is deactivated. While the power supply is  
ramping to its final value, the LM4868’s internal amplifiers  
are configured as unity gain buffers and are disconnected  
from the -OUT and +OUT pins. An internal current source  
changes the voltage of the BYPASS pin in a controlled,  
linear manner. Ideally, the input and outputs track the voltage  
applied to the BYPASS pin. The gain of the internal amplifi-  
ers remains unity until the voltage on the bypass pin reaches  
1/2 VDD. As soon as the voltage on the bypass pin is stable,  
the device becomes fully operational and the amplifier out-  
puts are reconnected to the -OUT and +OUT pins. Although  
the BYPASS pin current cannot be modified, changing the  
size of CB alters the device’s turn-on time. There is a linear  
relationship between the size of CB and the turn-on time.  
Here are some typical turn-on times for various values of CB:  
(8)  
VDD (VOUTPEAK+ (VOD  
+ VODBOT))  
(9)  
TOP  
The Output Power vs Supply Voltage graph for an 8load  
indicates a minimum supply voltage of 4.6V. This is easily  
met by the commonly used 5V supply voltage. The additional  
voltage creates the benefit of headroom, allowing the  
LM4868 to produce peak output power in excess of 1W  
without clipping or other audible distortion. The choice of  
supply voltage must also not create a situation that violates  
of maximum power dissipation as explained above in the  
Power Dissipation section.  
After satisfying the LM4868’s power dissipation require-  
ments, the minimum differential gain needed to achieve 1W  
dissipation in an 8load is found using Equation (10).  
CB  
TON  
3ms  
0.01µF  
0.1µF  
0.22µF  
0.47µF  
1.0µF  
2.2µF  
30ms  
63ms  
134ms  
300ms  
630ms  
(10)  
Thus, a minimum gain of 2.83 allows the LM4868’s to reach  
full output swing and maintain low noise and THD+N perfor-  
mance. For this example, let AVD = 3.  
In order eliminate “clicks and pops“, all capacitors must be  
discharged before turn-on. Rapidly switching VDD may not  
allow the capacitors to fully discharge, which may cause  
“clicks and pops“.  
The amplifier’s overall gain is set using the input (Ri) and  
feedback (Ri) resistors. With the desired input impedance  
set at 20k, the feedback resistor is found using Equation  
(11).  
NO LOAD STABILITY  
Rf/Ri = AVD/2  
(11)  
The LM4868 may exhibit low level oscillation when the load  
resistance is greater than 10k. This oscillation only occurs  
as the output signal swings near the supply voltages. Pre-  
vent this oscillation by connecting a 5kbetween the output  
pins and ground.  
The value of Rf is 30k.  
The last step in this design example is setting the amplifier’s  
−3dB frequency bandwidth. To achieve the desired 0.25dB  
pass band magnitude variation limit, the low frequency re-  
sponse must extend to at least one-fifth the lower bandwidth  
limit and the high frequency response must extend to at least  
five times the upper bandwidth limit. The gain variation for  
both response limits is 0.17dB, well within the 0.25dB  
desired limit. The results are an  
AUDIO POWER AMPLIFIER DESIGN  
Audio Amplifier Design: Driving 1W into an 8Load  
The following are the desired operational parameters:  
Power Output:  
1 WRMS  
Load Impedance:  
8Ω  
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18  
upper passband response limit. With AVD = 3 and fH  
=
Application Information (Continued)  
100kHz, the closed-loop gain bandwidth product (GBWP) is  
300kHz. This is less than the LM4868’s 3.5MHz GBWP. With  
this margin, the amplifier can be used in designs that require  
more differential gain while avoiding performance,restricting  
bandwidth limitations.  
fL = 100Hz/5 = 20Hz  
(12)  
(13)  
and an  
fH = 20kHz x 5 = 100kHz  
RECOMMENDED PRINTED CIRCUIT BOARD LAYOUT  
Figures 9 through 13 show the recommended four-layer PC  
board layout that is optimized for the 24-pin LQ-packaged  
LM4868 and associated external components. Figures 14  
through 18 show the recommended four-layer PC board  
layout that is optimized for the 24-pin MTE-packaged  
LM4868 and associated external components. Figures 19  
through 21 show the recommended two-layer PC board  
layout that is optimized for the 20-pin MT-packaged LM4868  
and associated external components. These circuits are de-  
signed for use with an external 5V supply and 4speakers  
(or greater) for the MT-packaged LM4868 or 3speakers (or  
greater) for the LQ- and MTE-packaged LM4868.  
As mentioned in the Selecting Proper External Compo-  
nents section, Ri and Ci create a highpass filter that sets the  
amplifier’s lower bandpass frequency limit. Find the coupling  
capacitor’s value using Equation (12).  
Ci1/(2πR ifL)  
(14)  
The result is  
*
*
1/(2π 20k20Hz) = 0.397µF  
(15)  
These circuit boards are easy to use. Apply 5V and ground to  
the board’s VDD and GND pads, respectively. Connect 4Ω  
speakers between the board’s −OUTA and +OUTA and  
OUTB and +OUTB pads.  
Use a 0.39µF capacitor, the closest standard value.  
The product of the desired high frequency cutoff (100kHz in  
this example) and the differential gain AVD, determines the  
20026778  
FIGURE 9. Recommended LQ PC Board Layout:  
Component-Side Silkscreen  
19  
www.national.com  
Application Information (Continued)  
20026741  
FIGURE 10. Recommended LQ PC Board Layout:  
Component-Side Layout  
20026742  
FIGURE 11. Recommended LQ PC Board Layout:  
Upper Inner-Layer Layout  
www.national.com  
20  
Application Information (Continued)  
20026743  
FIGURE 12. Recommended LQ PC Board Layout:  
Lower Inner-Layer Layout  
20026744  
FIGURE 13. Recommended LQ PC Board Layout:  
Bottom-Side Layout  
21  
www.national.com  
Application Information (Continued)  
20026770  
FIGURE 14. Recommended MTE PC Board Layout:  
Component-Side Silkscreen  
20026771  
FIGURE 15. Recommended MTE PC Board Layout:  
Component-Side Layout  
www.national.com  
22  
Application Information (Continued)  
20026772  
FIGURE 16. Recommended MTE PC Board Layout:  
Upper Inner-Layer Layout  
20026773  
FIGURE 17. Recommended MTE PC Board Layout:  
Lower Inner-Layer Layout  
23  
www.national.com  
Application Information (Continued)  
20026774  
FIGURE 18. Recommended MTE PC Board Layout:  
Bottom-Side Layout  
20026775  
FIGURE 19. Recommended MT PC Board Layout:  
Component-Side Silkscreen  
www.national.com  
24  
Application Information (Continued)  
20026776  
FIGURE 20. Recommended MT PC Board Layout:  
Component-Side Layout  
20026777  
FIGURE 21. Recommended MT PC Board Layout:  
Bottom-Side Layout  
25  
www.national.com  
Physical Dimensions inches (millimeters) unless otherwise noted  
24-Lead MOLDED PKG, Leadless Leadframe Package LLP  
Order Number LM4868LQ  
NS Package Number LQA24A  
20-Lead MOLDED PKG, TSSOP, JEDEC, 4.4mm BODY WIDTH  
Order Number LM4868MT  
NS Package Number MTC20  
www.national.com  
26  
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)  
20-Lead MOLDED TSSOP, EXPOSED PAD, 6.5x4.4x0.9mm  
Order Number LM4868MTE  
NS Package Number MXA20A  
27  
www.national.com  
Notes  
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.  
National Semiconductor  
Corporation  
Americas  
National Semiconductor  
Europe  
National Semiconductor  
Asia Pacific Customer  
Response Group  
Tel: 65-2544466  
Fax: 65-2504466  
National Semiconductor  
Japan Ltd.  
Tel: 81-3-5639-7560  
Fax: 81-3-5639-7507  
Fax: +49 (0) 180-530 85 86  
Email: support@nsc.com  
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  
Email: ap.support@nsc.com  
www.national.com  
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.  

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