MAX9725EEBC+TG45 ,1V, Low-Power, DirectDrive, Stereo Headphone Amplifier with ShutdownFeatures♦ Low Quiescent CurrentThe MAX9725A–MAX9725D fixed-gain, stereo head-2.1mA (MAX9725A–MAX972 ..
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MAX9726AETP+T ,DirectDrive, Headphone Amplifier with BassMax, I²C, Volume and Gain ControlApplicationsBassMaxBMLSCL2I C SDAINTERFACEFBLINLOUTLΣVOLUMECONTROLOUTRΣINRFBRMAX9726BMRBassMaxSMBus ..
MAX9726BETP+T ,DirectDrive, Headphone Amplifier with BassMax, I²C, Volume and Gain Controlfeatures Maxim’s♦ Short-Circuit and Thermal-Overload Protectionindustry-leading click-and-pop suppr ..
MAX9728AETC+T ,60mW, DirectDrive, Stereo Headphone Amplifier with ShutdownFeaturesThe MAX9728A/MAX9728B stereo headphone ampli- ♦ No Bulky DC-Blocking Capacitors Requiredfie ..
MAX9728AEUD , 60mW, DirectDrive, Stereo Headphone Amplifiers with Shutdown
MB89475 ,F2MC-8L/Low Power/Low Voltage Microcontrollersapplications forconsumer product.2* : F MC stands for FUJITSU Flexible Microcontroller.n
MB89535A ,F2MC-8L/Low Power/Low Voltage MicrocontrollersFEATURES• Wide range of package options• Two types of QFP packages (1 mm pitch, 0.65 mm pitch) • LQ ..
MB89535A ,F2MC-8L/Low Power/Low Voltage MicrocontrollersFUJITSU SEMICONDUCTORDS07-12547-4EDATA SHEET8-bit Original Microcontroller CMOS2F MC-8L MB89530A Se ..
MB89537A , 8-bit Original Microcontroller CMOS, F-2MC-8L MB89530A Series
MB89537A , 8-bit Original Microcontroller CMOS, F-2MC-8L MB89530A Series
MB89537AC , 8-bit Original Microcontroller CMOS, F-2MC-8L MB89530A Series
MAX9725CETC+T-MAX9725EEBC+TG45
1V, Low-Power, DirectDrive, Stereo Headphone Amplifier with Shutdown
DirectDrive OUTPUTS
ELIMINATE DC-BLOCKING
CAPACITORS.
C1N
C1P
OUTL
SINGLE
1.5V CELL
AA OR AAA
BATTERY
OUTR
SGNDPGND
MAX9725A–MAX9725D
INVERTING
CHARGE PUMP
INR
INL
VDD
PVSS
VSS
General DescriptionThe MAX9725A–MAX9725D fixed-gain, stereo head-
phone amplifiers are ideal for portable equipment where
board space is at a premium. The MAX9725E offers the
flexibility to adjust the gain with external input and feed-
back resistors. The MAX9725A–MAX9725E 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. Fixed gains of -2V/V (MAX9725A),
-1.5V/V (MAX9725B), -1V/V (MAX9725C), and -4V/V
(MAX9725D) further reduce external component count.
The adjustable gain of the MAX9725E DirectDrive head-
phone amplifier allows for any gain down to -1V/V using
external resistors.
The MAX9725 delivers up to 20mW per channel into a
32Ωload and achieves 0.006% THD+N. An 80dB at 1kHz
power-supply rejection ratio (PSRR) allows the MAX9725
to operate from noisy digital supplies without an additional
linear regulator. The MAX9725 includes ±8kV ESD protec-
tion on the headphone output. Comprehensive click-and-
pop circuitry suppresses audible clicks and pops at
startup and shutdown. A low-power shutdown mode
reduces supply current to 0.6µA (typ).
The MAX9725 operates from a single 0.9V to 1.8V supply,
allowing the device to be powered directly from a single
AA or AAA battery. The MAX9725 consumes only
2.1mA of supply current, provides short-circuit protection,
and is specified over the extended -40°C to +85°C tem-
perature range. The MAX9725 is available in a tiny
(1.54mm x 2.02mm x 0.6mm) 12-bump chip-scale
package (UCSP™) and a 12-pin thin QFN package
(4mm x 4mm x 0.8mm).
Applications
FeaturesLow Quiescent Current2.1mA (MAX9725A–MAX9725D) 2.3mA (MAX9725E)Single-Cell, 0.9V to 1.8V Single-Supply OperationFixed Gain Eliminates External Feedback NetworkMAX9725A: -2V/VMAX9725B: -1.5V/VMAX9725C: -1V/VMAX9725D: -4V/VAdjustable Gain with External Input and FeedbackResistors MAX9725E: Minimum Stable Gain of -1V/VGround-Referenced Outputs Eliminate DC BiasNo Degradation of Low-Frequency Response Dueto Output Capacitors20mW per Channel into 32ΩLow 0.006% THD+NHigh PSRR (80dB at 1kHz)Integrated Click-and-Pop SuppressionLow-Power Shutdown ControlShort-Circuit Protection±8kV ESD-Protected Amplifier Outputs
Available in Space-Saving Packages12-Bump UCSP (1.54mm x 2.02mm x 0.6mm)12-Pin Thin QFN (4mm x 4mm x 0.8mm)
MAX9725, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown19-3465; Rev 4; 3/09
EVALUATION KITAVAILABLE
Ordering Information
PA RT PIN- PA CK A G ET O PA RK A IN V/V) MAX9725AEBC +TG4512 U C S P+ ACK- 2AX9725AE TC+ 12 TQFN- EP*+ AAE W- 2
MAX9725BEBC +TG4512 U C S P+ ACL- 1.5
UCSP is a trademark of Maxim Integrated Products, Inc.
Ordering Information continued at end of data sheet.
Note:All devices are specified over the -40°C to +85°C operating
temperature range.
+Denotes a lead(Pb)-free/RoHS-compliant package.
T = Tape and reel.
*EP = Exposed pad.
Block Diagrams continued at end of data sheet.
MP3 Players
Cellular Phones
PDAs
Smart Phones
Portable Audio Equipment
Block Diagrams
MAX9725, Low-Power, DirectDrive, Stereo Headphone
Amplifier 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.
SGND to PGND.....................................................-0.3V to +0.3V
VDDto SGND or PGND............................................-0.3V to +2V
VSSto PVSS...........................................................-0.3V to +0.3V
C1P to PGND..............................................-0.3V to (VDD+ 0.3V)
C1N to PGND............................................(PVSS- 0.3V) to +0.3V
VSS, PVSSto GND....................................................+0.3V to -2V
OUTR, OUTL, INR, INL to SGND
(MAX9725A–MAX9725D)..............(VSS- 0.3V) to (VDD+ 0.3V)
OUTR, OUTL to SGND
(MAX9725E)..................................(VSS- 0.3V) to (VDD+ 0.3V)
INR, INL to SGND (MAX9725E)...................................-4V to +4V
SHDNto SGND or PGND.........................................-0.3V to +4V
Output Short-Circuit Current......................................Continuous
Continuous Power Dissipation (TA= +70°C)
12-Bump UCSP (derate 6.5mW/°C above +70°C)....518.8mW
12-Pin Thin QFN (derate 16.9mW/°C above +70°C)..1349.1mW
Junction Temperature......................................................+150°C
Operating Temperature Range...........................-40°C to +85°C
Storage Temperature Range.............................-65°C to +150°C
Bump Temperature (soldering) Reflow............................+230°C
Lead Temperature (soldering, 10s).................................+300°C
ELECTRICAL CHARACTERISTICS (MAX9725A–MAX9725D)(VDD= 1.5V, VPGND= VSGND= 0V, VSHDN= 1.5V, VSS= VPVSS, C1 = C2 = 1µF, CIN= 1µF, RL= ∞, TA= TMINto TMAX, unless other-
wise noted. Typical values are at TA= +25°C.) (See the Functional Diagrams.) (Note 1)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSSupply Voltage RangeVDDGuaranteed by PSRR test0.91.8V
Quiescent Supply CurrentIDDBoth channels active2.13.3mA
TA = +25°C0.610Shutdown CurrentISHDNVSHDN = 0VTA = -40°C to +85°C30µA
Shutdown to Full OperationtON180µs
VIHVDD = 0.9V to 1.8V0.7 x VDDSHDN ThresholdsVILVDD = 0.9V to 1.8V0.3 x VDDV
SHDN Input Leakage CurrentILEAKVDD = 0.9V to 1.8V (Note 2)±1µA
CHARGE PUMPOscillator FrequencyfOSC493580667kHz
AMPLIFIERSMAX9725A-2.04-2.00-1.96
MAX9725B-1.53-1.5-1.47
MAX9725C-1.02-1.00-0.98Voltage GainAV
MAX9725D-4.08-4.00-3.92
V/V
Gain MatchΔAV±0.5%
MAX9725A/MAX9725D±0.3±1.05
MAX9725B±0.45±1.58Total Output Offset VoltageVOS
Input AC-coupled,
RL = 32Ω to GND,
TA = +25°CMAX9725C±0.6±2.1
Input ResistanceRIN152535kΩ
VDD = 0.9V to 1.8V, TA = +25°C6080
fIN = 1kHz70Power-Supply Rejection RatioPSRR100mVP-P ripplefIN = 20kHz62
RL = 32Ω1020VDD = 1.5VRL = 16Ω25
VDD = 1.0V, RL = 32Ω7Output Power (Note 3)POUT
VDD = 0.9V, RL = 32Ω6
MAX9725, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
ELECTRICAL CHARACTERISTICS (MAX9725A–MAX9725D) (continued)(VDD= 1.5V, VPGND= VSGND= 0V, VSHDN= 1.5V, VSS= VPVSS, C1 = C2 = 1µF, CIN= 1µF, RL= ∞, TA= TMINto TMAX, unless other-
wise noted. Typical values are at TA= +25°C.) (See the Functional Diagrams.) (Note 1)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSRL = 32Ω, POUT = 12mW, f = 1kHz0.006Total Harmonic Distortion Plus
NoiseTHD+NRL = 16Ω, POUT = 15mW, f = 1kHz0.015%
BW = 22Hz to 22kHz89Signal-to-Noise RatioSNRRL = 32Ω, POUT = 12mWA-weighted filter92dB
Slew RateSR0.2V/µs
Maximum Capacitive LoadCLNo sustained oscillations150pF
CrosstalkXTALKfIN = 1.0kHz, RL = 32Ω, POUT = 5mW100dB
Into shutdown72.8
Click-and-Pop LevelKCP
RL = 32Ω, peak voltage,
A-weighted, 32 samples per
second (Note 4)Out of shutdown72.8
dBV
ESD ProtectionVESDHuman Body Model (OUTR, OUTL)±8kV
ELECTRICAL CHARACTERISTICS (MAX9725E)(VDD= 1.5V, VPGND= VSGND= 0V, VSHDN= 1.5V, VSS= VPVSS, C1 = C2 = 1µF, CIN= 1µF, RL= 32Ω, RF= 60kΩ, RIN= 10kΩ, TA=
TMINto TMAX, unless otherwise noted. Typical values are at TA= +25°C.) (See the Functional Diagrams.) (Note 1)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSSupply Voltage RangeVDDGuaranteed by PSRR test0.91.8V
Quiescent Supply CurrentIDDBoth channels active2.33.7mA
TA = +25°C0.61Shutdown CurrentISHDNVSHDN = 0VTA = -40°C to +85°C10µA
Shutdown to Full OperationtON180µs
VIHVDD = 0.9V to 1.8V0.7 x VDDSHDN ThresholdsVILVDD = 0.9V to 1.8V0.3 x VDDV
SHDN Input Leakage CurrentILEAKVDD = 0.9V to 1.8V (Note 2)±1µA
CHARGE PUMPOscillator FrequencyfOSC483592687kHz
AMPLIFIERSVoltage GainAV(Note 5)-6.11-6.07-6.00V/V
Minimum Stable GainΔAV-1.0V/V
Total Output Offset VoltageVOSInput AC-coupled, RL = 32Ω to GND,
TA = +25°C (Note 6)±0.63±2.1mV
Input ResistanceRIN6.39.7814kΩ
INR, INL Input Leakage
CurrentILK±100nA
Maximum Input Parasitic
CapacitanceCPAR5pF
VDD = 0.9V to 1.8V, TA = +25°C52.967.8
fIN = 1kHz70Power-Supply Rejection RatioPSRR100mVP-P ripple
(Note 5)fIN = 20kHz62
MAX9725, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
Note 1:All specifications are 100% tested at TA= +25°C; temperature limits are guaranteed by design.
Note 2:Input leakage current measurements limited by automated test equipment.
Note 3:fIN= 1kHz, TA= +25°C, THD+N < 1%, both channels driven in-phase.
Note 4:Testing performed with 32Ωresistive load connected to outputs. Mode transitions controlled by SHDN. KCPlevel calculated
as 20 log [peak voltage under normal operation at rated power level / peak voltage during mode transition]. Inputs are AC-
grounded.
Note 5:Using existing resistors with 1% precision.
Note 6:RIN= 10Ω, RF=10kΩ.
Typical Operating Characteristics(VDD= 1.5V, VPGND= VSGND= 0V, VSHDN= 1.5V, VSS= VPVSS, C1 = C2 = 1µF, CIN= 1µF, THD+N measurement bandwidth =
22Hz to 22kHz, TA= +25°C, unless otherwise noted.) (See the Functional Diagrams.)10k1k100100k
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. FREQUENCYMAX9725 toc01
THD+N (%)
VDD = 1.5V
RL = 16Ω
AV = -2V/V
POUT = 15mW
POUT = 2mW10k1k100100k
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. FREQUENCYMAX9725 toc02
THD+N (%)
VDD = 1.5V
RL = 32Ω
AV = -2V/V
POUT = 12mW
POUT = 2mW10k1k100100k
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. FREQUENCYMAX9725 toc03
THD+N (%)
VDD = 1V
RL = 16Ω
AV = -2V/V
POUT = 0.7mW
POUT = 4mW
ELECTRICAL CHARACTERISTICS (MAX9725E) (continued)(VDD= 1.5V, VPGND= VSGND= 0V, VSHDN= 1.5V, VSS= VPVSS, C1 = C2 = 1µF, CIN= 1µF, RL= 32Ω, RF= 60kΩ, RIN= 10kΩ, TA=
TMINto TMAX, unless otherwise noted. Typical values are at TA= +25°C.) (See the Functional Diagrams.) (Note 1)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSRL = 32Ω1024VDD = 1.5VRL = 16Ω25
VDD = 1.0V, RL = 32Ω7Output Power (Note 3)POUT
VDD = 0.9V, RL = 32Ω6
RL = 32Ω, POUT = 12mW, f = 1kHz0.006Total Harmonic Distortion Plus
Noise (Note 5)THD+NRL = 16Ω, POUT = 15mW, f = 1kHz0.015%
BW = 22Hz to 22kHz89Signal-to-Noise RatioSNRRL = 32Ω, POUT = 12mWA-weighted filter92dB
Slew RateSR0.3V/µs
Maximum Capacitive LoadCLNo sustained oscillations150pF
CrosstalkXTALKfIN = 1.0kHz, RL = 32Ω, POUT = 5mW100dB
Into shutdown72.8
Click-and-Pop LevelKCP
RL = 32Ω, peak voltage,
A-weighted, 32 samples per
second (Note 4)Out of shutdown72.8
dBV
ESD ProtectionVESDHuman Body Model (OUTR, OUTL)±8kV
RL = 32Ω-120Attenuation in ShutdownATT(SD)VSHDN = 0VRL = 10kΩ-75dB
MAX9725, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
Typical Operating Characteristics (continued)(VDD= 1.5V, VPGND= VSGND= 0V, VSHDN= 1.5V, VSS= VPVSS, C1 = C2 = 1µF, CIN= 1µF, THD+N measurement bandwidth = 22Hz
to 22kHz, TA= +25°C, unless otherwise noted.) (See the Functional Diagrams.)
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. OUTPUT POWER
MAX9725 toc07
OUTPUT POWER (mW)
THD+N (%)
VDD = 1V
RL = 16Ω
AV = -2V/VfIN = 20Hz
fIN = 1kHz
fIN = 10kHz
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. OUTPUT POWER
MAX9725 toc08
OUTPUT POWER (mW)
THD+N (%)
VDD = 1V
RL = 32Ω
AV = -2V/V
fIN = 20Hz
fIN = 1kHz
fIN = 10kHz10010k1k100k
POWER-SUPPLY REJECTION RATIO
vs. FREQUENCYMAX9725 toc09
FREQUENCY(Hz)
PSRR (dB)
VDD = 1.5V
RL = 32Ω10010k1k100k
POWER-SUPPLY REJECTION RATIO
vs. FREQUENCYMAX9725 toc10
PSRR (dB)
VDD = 1V
RL = 32Ω10010k1k100k
CROSSTALK vs. FREQUENCYMAX9725 toc11
PSRR (dB)
VDD = 1.5V
POUT = 5mW
RL = 32Ω
RIGHT TO LEFT
LEFT TO RIGHT
OUTPUT POWER vs. SUPPLY VOLTAGE
MAX9725 toc12
OUTPUT POWER (mW)
fIN = 1kHz
RL = 16Ω
BOTH INPUTS
DRIVEN IN-PHASE
THD+N = 10%
THD+N = 1%10k1k100100k
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. FREQUENCYMAX9725 toc04
FREQUENCY (Hz)
THD+N (%)
POUT = 0.7mW
POUT = 4mW
VDD = 1V
RL = 32Ω
AV = -2V/V
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. OUTPUT POWER
MAX9725 toc05
OUTPUT POWER (mW)
THD+N (%)
VDD = 1.5V
RL = 16Ω
AV = -2V/VfIN = 20Hz
fIN = 1kHz
fIN = 10kHz
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. OUTPUT POWER
MAX9725 toc06
OUTPUT POWER (mW)
THD+N (%)
VDD = 1.5V
RL = 32Ω
AV = -2V/V
fIN = 20Hz
fIN = 1kHz
fIN = 10kHz
MAX9725, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown1020304050
POWER DISSIPATION
vs. OUTPUT POWERMAX9725 toc16
OUTPUT POWER (mW)
POWER DISSIPATION (mW)VDD = 1.5V
fIN = 1kHz
POUT = POUTL + POUTR
OUTPUTS IN-PHASE
RL = 16Ω
RL = 32Ω5101520
POWER DISSIPATION
vs. OUTPUT POWERMAX9725 toc17
OUTPUT POWER (mW)
POWER DISSIPATION (mW)VDD = 1V
fIN = 1kHz
POUT = POUTL + POUTR
OUTPUTS IN-PHASE
RL = 16Ω
RL = 32Ω10010k1k100k
GAIN FLATNESS
vs. FREQUENCYFREQUENCY(Hz)
AMPLITUDE (dB)
MAX9725 toc1820304050
OUTPUT POWER vs. CHARGE-PUMP
CAPACITANCE AND LOAD RESISTANCEMAX9725 toc19
OUTPUT POWER (mW)
VDD = 1.5V
fIN = 1kHz
THD+N = 1%
C1 = C2 = 2.2μF
C1 = C2 = 1μF
C1 = C2 = 0.47μF
C1 = C2 = 0.68μF
OUTPUT SPECTRUM
vs. FREQUENCY
MAX9725 toc20
AMPLITUDE (dB)
fIN = 1kHz
RL = 32Ω
VOUT = -60dBV
VDD = 1.5V
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX9725 toc21
SUPPLY CURRENT (mA)
NO LOAD
Typical Operating Characteristics (continued)(VDD= 1.5V, VPGND= VSGND= 0V, VSHDN= 1.5V, VSS= VPVSS, C1 = C2 = 1µF, CIN= 1µF, THD+N measurement bandwidth = 22Hz
to 22kHz, TA= +25°C, unless otherwise noted.) (See the Functional Diagrams.)
OUTPUT POWER
vs. SUPPLY VOLTAGE
MAX9725 toc13
SUPPLY VOLTAGE (V)
OUTPUT POWER (mW)
fIN = 1kHz
RL = 32Ω
BOTH INPUTS
DRIVEN IN-PHASE
THD+N = 10%
THD+N = 1%1001k
OUTPUT POWER
vs. LOAD RESISTANCEMAX9725 toc14
LOAD RESISTANCE (Ω)
OUTPUT POWER (mW)
VDD = 1.5V
fIN = 1kHz
BOTH INPUTS
DRIVEN IN-PHASE
THD+N = 10%
THD+N = 1%1001k
OUTPUT POWER
vs. LOAD RESISTANCEMAX9725 toc15
LOAD RESISTANCE (Ω)
OUTPUT POWER (mW)
VDD = 1V
fIN = 1kHz
BOTH INPUTS
DRIVEN IN-PHASE
THD+N = 10%
THD+N = 1%
MAX9725, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
PINBUMP
THIN
QFNUCSPNAMEFUNCTIONA1C1NFlying Capacitor Negative Terminal. Connect a 1µF capacitor from C1P to C1N.
2A2PVSSInverting Charge-Pump Output. Bypass with 1µF from PVSS to PGND. PVSS must be connected to
VSS.A3INLLeft-Amplifier Inverting Input. Connect input resistor RIN from input capacitor C1N to INL (MAX9725E
only).A4INRRight-Amplifier Inverting Input. Connect input resistor RIN from input capacitor C1N to INR
(MAX9725E only).
5B4VSSAmplifier Negative Power Supply. Must be connected to PVSS.B3SGNDSignal Ground. SGND must be connected to PGND. SGND is the ground reference for the input and
output signal.C4OUTRRight-Channel Output. Connect feedback resistor RFB between OUTR and INR (MAX9725E only).C3OUTLLeft-Channel Output. Connect feedback resistor RFB between OUTL and INL (MAX9725E only).
9C2VDDPositive Power-Supply Input. Bypass with a 1µF capacitor to PGND.C1C1PFlying Capacitor Positive Terminal. Connect a 1µF capacitor from C1P to C1N.B1PGNDPower Ground. Ground reference for the internal charge pump. PGND must be connected to SGND.B2SHDNActive-Low Shutdown. Connect to VDD for normal operation. Pull low to disable the amplifier and
charge pump.—EPExposed Paddle. Internally connected to VSS. Leave paddle unconnected or solder to VSS.
Pin Description
SHUTDOWN CURRENT
vs. SUPPLY VOLTAGEMAX9725 toc22
SUPPLY VOLTAGE (V)
SHUTDOWN CURRENT (
EXITING SHUTDOWN
MAX9725 toc23
200μs/div
OUT_
1V/div
SHDN
500mV/div
POWER-UP/-DOWN WAVEFORMMAX9725toc24
200ms/div
OUT_
10mV/div
VDD
1V/div
Typical Operating Characteristics (continued)(VDD= 1.5V, VPGND= VSGND= 0V, VSHDN= 1.5V, VSS= VPVSS, C1 = C2 = 1µF, CIN= 1µF, THD+N measurement bandwidth = 22Hz
to 22kHz, TA= +25°C, unless otherwise noted.) (See the Functional Diagrams.)
MAX9725, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
Detailed DescriptionThe MAX9725 stereo headphone driver features Maxim’s
DirectDrive architecture, eliminating the large output-cou-
pling capacitors required by conventional single-supply
headphone drivers. The MAX9725 consists of two 20mW
class AB headphone drivers, shutdown control, inverting
charge pump, internal gain-setting resistors, and compre-
hensive click-and-pop suppression circuitry (see the
Functional Diagrams). A negative power supply (PVSS) is
created by inverting the positive supply (VDD). Powering
the drivers from VDDand PVSSincreases the dynamic
range of the drivers to almost twice that of other 1V sin-
gle-supply drivers. This increase in dynamic range allows
for higher output power.
The outputs of the MAX9725 are biased about GND
(Figure 1). The benefit of this GND bias is that the driver
outputs do not have a DC component, thus large DC-
blocking capacitors are unnecessary. Eliminating the
DC-blocking capacitors on the output saves board
space, system cost, and improves frequency response.
DirectDriveConventional single-supply headphone drivers have their
outputs biased about a nominal DC voltage (typically half
the supply) for maximum dynamic range. Large coupling
capacitors are needed to block the DC bias from the
headphones. Without these capacitors, a significant
amount of DC current flows to the headphone, resulting
in unnecessary power dissipation and possible damage
to both headphone and headphone driver.
Maxim’s DirectDrive architecture uses a charge pump
to create an internal negative supply voltage. This
allows the MAX9725 outputs to be biased about GND,
increasing the dynamic range while operating from a
single supply. A conventional amplifier powered from
1.5V ideally provides 18mW to a 16Ωload. The
MAX9725 provides 25mW to a 16Ωload. The
DirectDrive architecture eliminates the need for two
large (220µF, typ) DC-blocking capacitors on the out-
put. The MAX9725 charge pump requires two small
ceramic capacitors, conserving board space, reducing
cost, and improving the frequency response of the
headphone driver. See the Output Power vs. Charge-
Pump Capacitance and Load Resistance graph in the
Typical Operating Characteristics for details of the possi-
ble capacitor sizes.
Previous attempts to eliminate the output-coupling
capacitors involved biasing the headphone return
(sleeve) to the DC-bias voltage of the headphone
amplifiers. This method raises some issues:The sleeve is typically grounded to the chassis.
Using this biasing approach, the sleeve must be
isolated from system ground, complicating product
design.During an ESD strike, the driver’s ESD structures
are the only path to system ground. The driver must
be able to withstand the full ESD strike.When using the headphone jack as a line out to
other equipment, the bias voltage on the sleeve may
conflict with the ground potential from other equip-
ment, resulting in possible damage to the drivers.
Figure 1. Traditional Driver Output Waveform vs. MAX9725
Output Waveform (Ideal Case)
VDD
-VDD
GNDVOUT
CONVENTIONAL DRIVER-BIASING SCHEME
DirectDrive BIASING SCHEME
VDD / 2
VDD
GND
VOUT
MAX9725
Low-Frequency ResponseLarge DC-blocking capacitors limit the amplifier’s low-
frequency response and can distort the audio signal:The impedance of the headphone load and the DC-
blocking capacitor forms a highpass filter with the
-3dB point set by:
where RLis the impedance of the headphone and
COUTis the value of the DC-blocking capacitor. The
highpass filter is required by conventional single-
ended, single power-supply headphone drivers to
block the midrail DC-bias component of the audio
signal from the headphones. The drawback to the
filter is that it can attenuate low-frequency signals.
Larger values of COUTreduce this effect but result
in physically larger, more expensive capacitors.
Figure 2 shows the relationship between the size of
COUTand the resulting low-frequency attenuation.
Note that the -3dB point for a 16Ωheadphone with
a 100µF blocking capacitor is 100Hz, well within the
normal audio band, resulting in low-frequency
attenuation of the reproduced signal.The voltage coefficient of the DC-blocking capacitor
contributes distortion to the reproduced audio signal
as the capacitance value varies when the function of
the voltage across the capacitor changes. At low
frequencies, the reactance of the capacitor domi-
nates at frequencies below the -3dB point and the
voltage coefficient appears as frequency-dependent
distortion. Figure 3 shows the THD+N introduced by
two different capacitor dielectric types. Note that
below 100Hz, THD+N increases rapidly.
The combination of low-frequency attenuation and fre-
quency-dependent distortion compromises audio
reproduction in portable audio equipment that empha-
sizes low-frequency effects such as multimedia lap-
tops, as well as MP3, CD, and DVD players. These
low-frequency, capacitor-related deficiencies are elimi-
nated by using DirectDrive technology.
Charge PumpThe MAX9725 features a low-noise charge pump. The
580kHz switching frequency is well beyond the audio
range, and does not interfere with the audio signals.
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 parasitic bond
wire and trace inductance is minimized by limiting the
turn-on/off speed of the charge pump. Additional high-
frequency noise attenuation can be achieved by
increasing the size of C2 (see the Functional Diagrams).
Extra noise attenuation is not typically required.
ShutdownThe MAX9725’s low-power shutdown mode reduces
supply current to 0.6µA. Driving SHDNlow disables the
amplifiers and charge pump. The driver’s output imped-
ance is typically 50kΩ(MAX9725A), 37.5kΩ
(MAX9725B), 25kΩ(MAX9725C), 100kΩ(MAX9725D),
or RF(MAX9725E) when in shutdown mode.2RC-3dBLOUT1
Figure 2. Low-Frequency Attenuation for Common DC-Blocking
Capacitor Values
LF ROLLOFF (16Ω LOAD)FREQUENCY (Hz)
ATTENUATION (dB)
-10-3dB CORNER FOR
100μF IS 100Hz-15
-351k
33μF
330μF
220μF
100μF
Figure 3. Distortion Contributed By DC-Blocking Capacitors
ADDITIONAL THD+N DUE
TO DC-BLOCKING CAPACITORSFREQUENCY (Hz)
THD+N (%)
10k1k100
0.0001100k
TANTALUM
ALUM/ELEC
, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
