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MAX9728AETC+T-MAX9728AEUD+
60mW, DirectDrive, Stereo Headphone Amplifier with Shutdown
General DescriptionThe MAX9728A/MAX9728B stereo headphone ampli-
fiers are designed for display and notebook applica-
tions or portable equipment where board space is at a
premium. These devices use a unique DirectDrive®
architecture to produce a ground-referenced output
from a single supply, eliminating the need for large DC-
blocking capacitors, saving cost, board space, and
component height. The MAX9728A offers an externally
adjustable gain, while the MAX9728B has an internally
preset gain of -1.5V/V. The MAX9728A/MAX9728B
deliver up to 60mW per channel into a 32Ωload and
have low 0.02% THD+N. An 80dB at 1kHz power-sup-
ply rejection ratio (PSRR) allows these devices to oper-
ate from noisy digital supplies without an additional
linear regulator. Comprehensive click-and-pop circuitry
suppresses audible clicks and pops on startup and
shutdown.
The MAX9728A/MAX9728B operate from a single 4.5V
to 5.5V supply, consume only 3.5mA of supply current,
feature short-circuit and thermal-overload protection,
and are specified over the extended -40°C to +85°C
temperature range. The devices are available in tiny 12-
pin Thin QFN (3mm x 3mm x 0.8mm) and 14-pin
TSSOP packages (5mm x 4.4mm x 1.1mm).
FeaturesNo Bulky DC-Blocking Capacitors RequiredLow-Power Shutdown Mode, < 0.1µAAdjustable Gain (MAX9728A) or Fixed -1.5V/V
Gain (MAX9728B)Low 0.02% THD+NHigh PSRR (80dB at 1kHz) Eliminates LDOIntegrated Click-and-Pop Suppression4.5V to 5.5V Single-Supply OperationLow Quiescent Current (3.5mA)Available in Space-Saving Packages
12-Pin Thin QFN (3mm x 3mm x 0.8mm)
14-Pin TSSOP (5mm x 4.4mm x 1.1mm)
MAX9728A/MAX9728B
60mW, DirectDrive, Stereo
Headphone Amplifiers with Shutdown
Ordering Information19-3963; Rev 1; 7/09
EVALUATION KIT
AVAILABLE
PARTGAIN (V/V)PIN-PACKAGETOP MARKMAX9728AETC+Adj.12 TQFN-EP*ABC
MAX9728AEUD+Adj.14 TSSOP—
MAX9728BETC+-1.512 TQFN-EP*ABD
MAX9728BEUD+-1.514 TSSOP—
Note:All devices specified over the -40°C to +85°C operating
range.
+Denoteslead(Pb)-free/RoHS-compliant package.
*EP = Exposed pad.
Notebook PCs
DVD Players
LCD/PDP Displays
CRT TVs
Multimedia Monitors
LEFT
AUDIO
INPUT
RIGHT
AUDIO
INPUT
SHDN
MAX9728A
LEFT
AUDIO
INPUT
RIGHT
AUDIO
INPUT
MAX9728B
SHDN
DirectDrive OUTPUTS
ELIMINATE DC-BLOCKING
CAPACITORS
DirectDrive OUTPUTS
ELIMINATE DC-BLOCKING
CAPACITORS
FIXED GAIN ELIMINATES
EXTERNAL RESISTOR
NETWORK
Block Diagrams
Pin Configurations appear at end of data sheet.
ApplicationsDirectDrive is a registered trademark of Maxim Integrated
Product, Inc.
MAX9728A/MAX9728B
60mW, DirectDrive, Stereo Headphone
Amplifiers with Shutdown
ABSOLUTE MAXIMUM RATINGSStresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
VDDto PGND............................................................-0.3V to +6V
PVSSto SVSS.........................................................-0.3V to +0.3V
PGND to SGND.....................................................-0.3V to +0.3V
C1P to PGND..............................................-0.3V to (VDD+ 0.3V)
C1N to PGND............................................(PVSS- 0.3V) to +0.3V
PVSSand SVSSto PGND..........................................-6V to +0.3V
IN_ to SGND (MAX9728A)..........................-0.3V to (VDD+ 0.3V)
IN_ to SGND (MAX9728B).............(SVSS- 0.3V) to (VDD+ 0.3V)
OUT_ to SVSS(Note 1)....-0.3V to Min (VDD- SVSS+ 0.3V, +9V)
OUT_ to VDD(Note 2)......+0.3V to Max (SVSS- VDD- 0.3V, -9V)
SHDNto _GND.........................................................-0.3V to +6V
OUT_ Short Circuit to SGND......................................Continuous
Short Circuit between OUTL and OUTR....................Continuous
Continuous Input Current into PVSS..................................260mA
Continuous Input Current (any other pin).........................±20mA
Continuous Power Dissipation (TA= +70°C)
12-Pin TQFN (derate 14.7mW/°C above +70°C).........1177mW
14-Pin TSSOP (derate 9.1mW/°C above +70°C)...........727mW
Operating Temperature Range...........................-40°C to +85°C
Storage Temperature Range.............................-65°C to +150°C
Junction Temperature......................................................+150°C
Lead Temperature (soldering, 10s).................................+300°C
ELECTRICAL CHARACTERISTICS(VDD= 5V, PGND = SGND, SHDN= 5V, C1 = C2 = 1µF, RL= ∞, resistive load reference to ground; for MAX9728A gain = -1.5V/V
(RIN= 20kΩ, RF= 30kΩ); for MAX9728B gain = -1.5V/V (internally set), TA= -40°C to +85°C, unless otherwise noted. Typical values
are at TA= +25°C, unless otherwise noted.) (Note 3)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITS
GENERALSupply Voltage RangeVDD4.55.5V
Quiescent CurrentICC3.55.5mA
Shutdown CurrentISHDNSHDN = SGND = PGND< 0.11µA
Shutdown to Full OperationtSON180µs
Input ImpedanceRINMAX9728B, measured at IN_151925kΩ
Output Offset VoltageVOS±1.5±10mV
VDD = 4.5V to 5.5V86
f = 1kHz, 100mVP-P80Power-Supply Rejection RatioPSRR
f = 20kHz, 100mVP-P65
RL = 32Ω, THD+N = 1%3063Output PowerPOUTRL = 16Ω, THD+N = 1%42mW
Voltage GainAVMAX9728B (Note 4)-1.52-1.5-1.48V/V
Channel-to-Channel Gain
TrackingMAX9728B±0.15%
RL = 1kΩ, VOUT = 2VRMS, fIN = 1kHz0.003
RL = 32Ω, POUT = 50mW, fIN = 1kHz0.02Total Harmonic Distortion Plus
NoiseTHD+N
RL = 16Ω, POUT = 35mW, fIN = 1kHz0.04
Note 1:OUTR and OUTL should be limited to no more than 9V above SVSS, or above VDD+ 0.3V, whichever limits first.
Note 2:OUTR and OUTL should be limited to no more than 9V below VDD, or below SVSS- 0.3V, whichever limits first.
MAX9728A/MAX9728B
60mW, DirectDrive, Stereo Headphone
Amplifier with Shutdown
ELECTRICAL CHARACTERISTICS (continued)(VDD= 5V, PGND = SGND, SHDN= 5V, C1 = C2 = 1µF, RL= ∞, resistive load reference to ground; for MAX9728A gain = -1.5V/V
(RIN= 20kΩ, RF= 30kΩ); for MAX9728B gain = -1.5V/V (internally set), TA= -40°C to +85°C, unless otherwise noted. Typical values
are at TA= +25°C, unless otherwise noted.) (Note 3)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSBW = 22Hz to 22kHz102RL = 1kΩ,
VOUT = 2VRMSA-weighted105
BW = 22Hz to 22kHz98Signal-to-Noise RatioSNR
RL = 32Ω,
POUT = 50mWA-weighted101
Slew RateSR0.5V/µs
Capacitive DriveCLNo sustained oscillations100pF
CrosstalkL to R, R to L, f = 10kHz, RL = 16Ω, POUT =
15mW-70dB
Charge-Pump Oscillator
FrequencyfOSC190270400kHz
Into shutdown-67
Click-and-Pop LevelKCP
RL = 32Ω, peak voltage,
A-weighted, 32 samples per
second (Note 5)
Out of
shutdown-64dB
DIGITAL INPUTS (SHDN)Input Voltage HighVINH2V
Input Voltage LowVINL0.8V
Input Leakage Current±1µA
Note 3:All specifications are 100% tested at TA= +25°C; temperature limits are guaranteed by design.
Note 4:Gain for the MAX9728A is adjustable.
Note 5:Test performed with a 32Ωresistive load connected to SGND. Mode transitions are controlled by SHDN. KCPlevel is calcu-
lated as 20log[(peak voltage during mode transition, no input signal)/(peak voltage under normal operation at rated power
level)]. Units are expressed in dB.
MAX9728A/MAX9728B
60mW, DirectDrive, Stereo Headphone
Amplifiers with ShutdownTOTAL HARMONIC DISTORTION PLUS
NOISE vs. OUTPUT POWER
MAX9728A/28B toc01
OUTPUT POWER (mW)
THD+N (%)
VDD = 5V
RL = 16Ω
fIN = 1kHz
fIN = 20Hz
fIN = 10kHz
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. OUTPUT POWER
MAX9728A/28B toc02
OUTPUT POWER (mW)
THD+N (%)
VDD = 5V
RL = 32Ω
fIN = 1kHz
fIN = 20Hz
fIN = 10kHz1k10010k100k
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. FREQUENCY MAX9728A/28B toc03
FREQUENCY (Hz)
THD+N (%)
VDD = 5V
RL = 16Ω
POUT = 20mW
POUT = 37mW1k10010k100k
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. FREQUENCY MAX9728A/28B toc04
FREQUENCY (Hz)
THD+N (%)
VDD = 5V
RL = 32Ω
POUT = 50mW
POUT = 30mW
OUTPUT POWER vs. SUPPLY VOLTAGE
MAX9728A/28B toc05
SUPPLY VOLTAGE (V)
OUTPUT POWER (mW)
fIN = 1kHz
RL = 16Ω
THD+N = 1%
THD+N = 10%
OUTPUT POWER vs. SUPPLY VOLTAGE
MAX9728A/28B toc06
SUPPLY VOLTAGE (V)
OUTPUT POWER (mW)
fIN = 1kHz
RL = 32Ω
THD+N = 1%
THD+N = 10%
Typical Operating Characteristics(VDD= 5V, PGND = SGND = 0V, SHDN= VDD, C1 = C2 = 1µF, RL= ∞, gain = -1.5V/V (RIN= 20kΩ, RF= 30kΩfor the MAX9728A),
THD+N measurement bandwidth = 22Hz to 22kHz, both outputs driven in phase, TA= +25°C, unless otherwise noted.)
MAX9728A/MAX9728B-12010010k100k
POWER-SUPPLY REJECTION
RATIO vs. FREQUENCYMAX9728A/28B toc08
FREQUENCY (Hz)
PSRR (dB)
RL = 32Ω
VDD = 5V
-12010010k100k
CROSSTALK vs. FREQUENCYMAX9728A/28B toc09
FREQUENCY (Hz)
CROSSTALK (dB)
RIGHT TO LEFT
LEFT TO RIGHT
POUT = 15mW
RL = 16Ω
60mW, DirectDrive, Stereo Headphone
Amplifiers with Shutdown20030010050150250
OUTPUT POWER vs. LOAD RESISTANCE
AND CHARGE-PUMP CAPACITOR SIZEMAX9728A/28B toc10
LOAD RESISTANCE (Ω)
OUTPUT POWER (mW)
C1 = C2 = 2.2μF
C1 = C2 = 1μF
C1 = C2 = 0.47μF
VDD = 5V
fIN = 1kHz
THD+N = 1%
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX9728A/28B toc11
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (mA)
NO LOAD INPUTS GROUND
SHUTDOWN CURRENT
vs. SUPPLY VOLTAGE
MAX9728A/28B toc12
SUPPLY VOLTAGE (V)
SHUTDOWN CURRENT (nA)
NO LOAD INPUTS GROUND
OUTPUT POWER
vs. LOAD RESISTANCE
MAX9728A/28B toc07
LOAD RESISTANCE (Ω)
OUTPUT POWER (mW)30
VDD = 5V
fIN = 1kHz
THD+N = 10%
THD+N = 1%
Typical Operating Characteristics (continued)(VDD= 5V, PGND = SGND = 0V, SHDN= VDD, C1 = C2 = 1µF, RL= ∞, gain = -1.5V/V (RIN= 20kΩ, RF= 30kΩfor the MAX9728A),
THD+N measurement bandwidth = 22Hz to 22kHz, both outputs driven in phase, TA= +25°C, unless otherwise noted.)
MAX9728A/MAX9728B
60mW, DirectDrive, Stereo Headphone
Amplifiers with Shutdown
Typical Operating Characteristics (continued)(VDD= 5V, PGND = SGND = 0V, SHDN= VDD, C1 = C2 = 1µF, RL= ∞, gain = -1.5V/V (RIN= 20kΩ, RF= 30kΩfor the MAX9728A),
THD+N measurement bandwidth = 22Hz to 22kHz, both outputs driven in phase, TA= +25°C, unless otherwise noted.)
Pin Description
PIN
TQFNTSSOPNAMEFUNCTION3C1PFlying Capacitor Positive Terminal. Connect a 1µF ceramic capacitor from C1P to C1N.4PGNDPower Ground. Connect to SGND.5C1NFlying Capacitor Negative Terminal. Connect a 1µF ceramic capacitor from C1P to C1N.PVSSCharge-Pump Output. Connect to SVSS and bypass with a 1µF ceramic capacitor to PGND.SHDNActive-Low Shutdown Input9INLLeft-Channel Input10SGNDSignal Ground. Connect to PGND.11INRRight-Channel Input
912SVSSAmplifier Negative Supply. Connect to PVSS.14OUTRRight-Channel Output1OUTLLeft-Channel Output2VDDPositive Power-Supply Input. Bypass with a 1µF capacitor to PGND.6,13N.C.No Connection. Not internally connected.—EPExposed Paddle. Leave this connection floating or connect it to SVSS.
EXITING SHUTDOWN MAX9728A/28B toc13
VSHDN
5V/div
VOUT_
500mV/div
VIN_
1V/div
40μs/div
ENTERING SHUTDOWNMAX9728A/28B toc14
VSHDN
5V/div
VOUT_
500mV/div
VIN_
1V/div
20μs/div
MAX9728A/MAX9728B
Detailed DescriptionThe MAX9728A/MAX9728B stereo headphone ampli-
fiers feature Maxim’s DirectDrive architecture, eliminat-
ing the large output-coupling capacitors required by
conventional single-supply headphone amplifiers.
These devices consist of two 60mW Class AB head-
phone amplifiers, undervoltage lockout (UVLO)/shut-
down control, charge pump, and comprehensive
click-and-pop suppression circuitry (see the Functional
Diagram/Typical Operating Circuits). The charge pump
inverts the positive supply (VDD), creating a negative
supply (PVSS). The headphone amplifiers operate from
these bipolar supplies with their outputs biased about
SGND (Figure 1). The benefit of this SGND bias is that
the amplifier outputs do not have a DC component. The
large DC-blocking capacitors required with convention-
al headphone amplifiers are unnecessary, conserving
board space, reducing system cost, and improving fre-
quency response. The MAX9728A/MAX9728B feature
an undervoltage lockout that prevents operation from
an insufficient power supply and click-and-pop sup-
pression that eliminates audible transients on startup
and shutdown. The MAX9728A/MAX9728B also feature
thermal-overload and short-circuit protection.
DirectDriveConventional single-supply headphone amplifiers have
their outputs biased about a nominal DC voltage (typical-
ly half the supply) for maximum dynamic range. Large-
coupling capacitors are needed to block this DC bias
from the headphone. Without these capacitors, a signifi-
cant amount of DC current flows to the headphone,
resulting in unnecessary power dissipation and possible
damage to both headphone and headphone amplifier.
Maxim’s DirectDrive architecture uses a charge pump
to create an internal negative supply voltage, allowing
the MAX9728A/MAX9728B outputs to be biased about
SGND. With no DC component, there is no need for the
large DC-blocking capacitors. The MAX9728A/
MAX9728B charge pumps require two small ceramic
capacitors, conserving board space, reducing cost,
and improving the frequency response of the head-
phone amplifier. See the Output Power vs. Load
Resistance and Charge-Pump Capacitor Size graph in
the Typical Operating Characteristicsfor details of the
possible capacitor sizes. There is a low DC voltage on
the amplifier outputs due to amplifier offset. However,
the offsets of the MAX9728A/MAX9728B are typically
1.5mV, which, when combined with a 32Ωload, results
in less than 47µA of DC current flow to the head-
phones.
Charge PumpThe MAX9728A/MAX9728B feature a low-noise charge
pump. The 270kHz switching frequency is well beyond
the audio range and does not interfere with audio sig-
nals. The switch drivers feature a controlled switching
speed that minimizes noise generated by turn-on and
turn-off transients. The di/dt noise caused by the para-
sitic bond wire and trace inductance is minimized by
limiting the switching speed of the charge pump.
Although not typically required, additional high-fre-
quency noise attenuation can be achieved by increas-
ing the value of C2 (see the Functional Diagram/Typical
Operating Circuits).
Click-and-Pop SuppressionIn conventional single-supply audio amplifiers, the out-
put-coupling capacitor contributes significantly to audi-
ble clicks and pops. Upon startup, the amplifier charges
the coupling capacitor to its bias voltage, typically half
the supply. Likewise, on shutdown, the capacitor is dis-
charged. This results in a DC shift across the capacitor,
which appears as an audible transient at the speaker.
60mW, DirectDrive, Stereo Headphone
Amplifier with ShutdownVDD
-VDD
SGND
VOUT
CONVENTIONAL DRIVER-BIASING SCHEME
DirectDrive BIASING SCHEME
VDD/2
VDD
VDD
SGND
VOUT
2VDD
Figure1. Conventional Driver Output Waveform vs.
MAX9728A/MAX9728B Output Waveform
MAX9728A/MAX9728BSince the MAX9728A/MAX9728B do not require output-
coupling capacitors, this problem does not arise.
Additionally, the MAX9728A/MAX9728B feature exten-
sive click-and-pop suppression that eliminates any
audible transient sources internal to the device.
Typically, the output of the device driving the
MAX9728A/MAX9728B has a DC bias of half the supply
voltage. At startup, the input-coupling capacitor is
charged to the preamplifier’s DC-bias voltage through
the input and feedback resistors of the MAX9728A/
MAX9728B, resulting in a DC shift across the capacitor
and an audible click/pop. Delay the rise of SHDN4 to 5
time constants based on RINand CIN, relative to the
startup of the preamplifier, to eliminate clicks-and-pops
caused by the input filter.
ShutdownThe MAX9728A/MAX9728B feature a < 0.1µA, low-
power shutdown mode that reduces quiescent current
consumption and extends battery life for portable appli-
cations. Drive SHDNlow to disable the amplifiers and
the charge pump. In shutdown mode, the amplifier out-
put impedance is set to 14kΩ||RF(RFis 30kΩfor the
MAX9728B). The amplifiers and charge pump are
enabled once SHDNis driven high.
Applications Information
Power DissipationUnder normal operating conditions, linear power ampli-
fiers can dissipate a significant amount of power. The
maximum power dissipation for each package is given
in the Absolute Maximum Ratingssection under
Continuous Power Dissipation or can be calculated by
the following equation:
where TJ(MAX) is +150°C, TAis the ambient tempera-
ture, and θJAis the reciprocal of the derating factor in
°C/W as specified in the Absolute Maximum Ratings
section. For example, θJAof the Thin QFN package is
+68°C/W, and +110°C/W for the TSSOP package.
The MAX9728A/MAX9728B have two power dissipation
sources: a charge pump and the two output amplifiers.
If power dissipation for a given application exceeds the
maximum allowed for a particular package, reduce
VDD, increase load impedance, decrease the ambient
temperature, or add heatsinking to the device. Large
output, supply, and ground traces decrease θJA, allow-
ing more heat to be transferred from the package to the
surrounding air.
Thermal-overload protection limits total power dissipa-
tion in the MAX9728A/MAX9728B. When the junction
temperature exceeds +150°C, the thermal-protection
circuitry disables the amplifier output stage. The ampli-
fiers are enabled once the junction temperature cools
by approximately 12°C. This results in a pulsing output
under continuous thermal-overload conditions.
Output Dynamic RangeDynamic range is the difference between the noise floor
of the system and the output level at 1% THD+N.
Determine the system’s dynamic range before setting
the maximum output gain. Output clipping occurs if the
output signal is greater than the dynamic range of the
system. The DirectDrive architecture of the MAX9728A/
MAX9728B has increased the dynamic range compared
to other single-supply amplifiers.
Maximum Output SwingInternal device structures limit the maximum voltage
swing of the MAX9728A/MAX9728B. The output must
not be driven such that the peak output voltage exceeds
the opposite supply voltage by 9V. For example, if VDD
= 5V, the charge pump sets PVSS= -5V. Therefore, the
peak output swing must be less than ±4V to prevent
exceeding the absolute maximum ratings.
Component Selection
Input-Coupling CapacitorThe input capacitor (CIN), in conjunction with the input
resistor (RIN), forms a highpass filter that removes the
DC bias from an incoming signal (see the Functional
Diagram/Typical Operating Circuits). The AC-coupling
capacitor allows the device to bias the signal to an
optimum DC level. Assuming zero-source impedance,
the -3dB point of the highpass filter is given by:
Choose the CINsuch that f-3dBis well below the lowest
frequency of interest. Setting f-3dBtoo high affects the
device’s low-frequency response. Use capacitors
whose dielectrics have low-voltage coefficients, such
as tantalum or aluminum electrolytic. Capacitors with
high-voltage coefficients, such as ceramics, can result
in increased distortion at low frequencies.RCdBININ=31TT
DISSPKGMAXJMAXA()=
60mW, DirectDrive, Stereo Headphone
Amplifiers with Shutdown
MAX9728A/MAX9728B
60mW, DirectDrive, Stereo Headphone
Amplifier with Shutdown
Charge-Pump Capacitor SelectionUse ceramic capacitors with a low ESR for optimum
performance. For optimal performance over the extend-
ed temperature range, select capacitors with an X7R
dielectric. Table 1 lists suggested manufacturers.
Flying Capacitor (C1)The value of the flying capacitor (see the Functional
Diagram/Typical Operating Circuits) affects the charge
pump’s load regulation and output resistance. A C1
value that is too small degrades the device’s ability to
provide sufficient current drive, which leads to a loss of
output voltage. Increasing the value of C1 improves load
regulation and reduces the charge-pump output resis-
tance to an extent. See the Output Power vs. Load
Resistance and Charge-Pump Capacitor Sizegraph in
the Typical Operating Characteristics. Above 1µF, the
on-resistance of the switches and the ESR of C1 and C2
dominate.
Hold Capacitor (C2)The hold capacitor value (see the Functional
Diagram/Typical Operating Circuits) and ESR directly
affect the ripple at PVSS. Increasing the value of C2
reduces output ripple. Likewise, decreasing the ESR of
C2 reduces both ripple and output resistance. Lower
capacitance values can be used in systems with low
maximum output power levels. See the Output Power
vs. Load Resistance and Charge-Pump Capacitor Size
graph in the Typical Operating Characteristics.
Power-Supply Bypass Capacitor (C3)The power-supply bypass capacitor (see the Functional
Diagram/Typical Operating Circuits) lowers the output
impedance of the power supply, and reduces the
impact of the MAX9728A/MAX9728Bs’ charge-pump
switching transients. Bypass VDDwith C3, the same
value as C1, and place it physically close to the VDD
and PGND pins.
Amplifier GainThe gain of the MAX9728B amplifier is internally set to
-1.5V/V. All gain-setting resistors are integrated into the
device, reducing external component count. The inter-
nally set gain, in combination with DirectDrive, results in
a headphone amplifier that requires only five small
capacitors to complete the amplifier circuit: two for the
charge pump, two for audio input coupling, and one for
power-supply bypassing (see the Functional
Diagram/Typical Operating Circuits).
The gain of the MAX9728A amplifier is set externally as
shown in Figure 2, the gain is:= -RF/RIN (V/V)
Choose feedback resistor values in the tens of kΩ
range. Lower values may cause excessive power dissi-
pation and require impractically small values of RINfor
large gain settings. The high-impedance state of the
outputs can also be degraded during shutdown mode if
an inadequate feedback resistor is used since the
equivalent output impedance during shutdown is
14kΩ||RF(RFis equal to 30kΩfor the MAX9728B). The
source resistance of the input device may also need to
be taken into consideration. Since the effective value of
RINis equal to the sum of the source resistance of the
input device and the value of the input resistor connect-
ed to the inverting terminal of the headphone amplifier
(20kΩfor the MAX9728B), the overall closed-loop gain
of the headphone amplifier can be reduced if the input
resistor is not significantly larger than the source resis-
tance of the input device.
SUPPLIERPHONEFAXWEBSITETaiyo Yuden800-348-2496847-925-0899www.t-yuden.com
TDK847-803-6100847-390-4405www.component.tdk.com
Murata770-436-1300770-436-3030www.murata.com
Table 1. Suggested Capacitor Manufacturers
MAX9728A/MAX9728B
Lineout Amplifier and Filter BlockThe MAX9728A can be used as an audio line driver
capable of providing 2VRMSinto 10kΩloads with a sin-
gle 5V supply (see Figure 3 for the RMS Output Voltage
vs. Supply Voltage plot). 2VRMSis a popular audio line
level, first used in CD players, but now common in DVD
and set-top box (STB) interfacing standards. A 2VRMS
sinusoidal signal equates to approximately 5.7VP-P,
which means that the audio system designer cannot
simply run the lineout stage from a (typically common)
5V supply—the resulting output swing would be inade-
quate. A common solution to this problem is to use op
amps driven from split supplies (±5V typically), or to use
a high-voltage supply rail (9V to 12V). This can mean
adding extra cost and complexity to the system power
supply to meet this output level requirement. Having the
ability to derive 2VRMSfrom a 5V supply can often sim-
plify power-supply design in some systems.
When the MAX9728A is used as a line driver to provide
outputs that feed stereo equipment (receivers, STBs,
notebooks, and desktops) with a digital-to-analog con-
verter (DAC) used as an audio input source, it is often
desirable to eliminate any high-frequency quantization
noise produced by the DAC output before it reaches
the load. This high-frequency noise can cause the input
stages of the line-in equipment to exceed slew-rate lim-
itations or create excessive EMI emissions on the
cables between devices.
To suppress this noise, and to provide a 2VRMSstan-
dard audio output level from a single 5V supply, the
MAX9728A can be configured as a line driver and
active lowpass filter. Figure 4 shows the MAX9728A
connected as 2-pole Rauch/multiple feedback filter with
a passband gain of 6dB and a -3dB (below passband)
cutoff frequency of approximately 27kHz (see Figure 5
for the Gain vs. Frequency plot).
Layout and GroundingProper layout and grounding are essential for optimum
performance. Connect PGND and SGND together at a
single point on the PC board. Connect PVSSto SVSS
and bypass with a 1µF capacitor. Place the power-sup-
ply bypass capacitor and the charge-pump hold
capacitor as close to the MAX9728 as possible. Route
PGND and all traces that carry switching transients
away from SGND and the audio signal path. The thin
QFN package features an exposed paddle that
improves thermal efficiency. Ensure that the exposed
paddle is electrically isolated from PGND, SGND,
and VDD. Connect the exposed paddle to SVSSonly
when the board layout dictates that the exposed
paddle cannot be left floating.
60mW, DirectDrive, Stereo Headphone
Amplifiers with ShutdownLEFT
AUDIO
INPUT
RIGHT
AUDIO
INPUT
OUTL
INL
INR
OUTR
MAX9728A
RIN
RIN
Figure2. Gain Setting for the MAX9728A
RMS OUTPUT VOLTAGE
vs. SUPPLY VOLTAGEMAX9728A/28B fig03
SUPPLY VOLTAGE (V)
RMS OUTPUT VOLTAGE (V)
fIN = 1kHz
RL = 10kΩ
1% THD+N
RL = 1kΩ
1% THD+N
Figure3. RMS Output Voltage vs. Supply Voltage
