DirectDrive OUTPUTS
ELIMINATE DC-BLOCKING
CAPACITORS.
C1N
C1P
OUTL
SINGLE
1.5V CELL
AA OR AAA
BATTERY
OUTR
SGND PGND
MAX9725A–MAX9725D
INVERTING
CHARGE PUMP
INR
INL
VDD
PVSS
C2
VSS
C3
General Description
The 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,
patented 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 headphone 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
Features
oLow Quiescent Current
2.1mA (MAX9725A–MAX9725D)
2.3mA (MAX9725E)
oSingle-Cell, 0.9V to 1.8V Single-Supply Operation
oFixed Gain Eliminates External Feedback Network
MAX9725A: -2V/V
MAX9725B: -1.5V/V
MAX9725C: -1V/V
MAX9725D: -4V/V
oAdjustable Gain with External Input and Feedback
Resistors
MAX9725E: Minimum Stable Gain of -1V/V
oGround-Referenced Outputs Eliminate DC Bias
oNo Degradation of Low-Frequency Response Due
to Output Capacitors
o20mW per Channel into 32Ω
oLow 0.006% THD+N
oHigh PSRR (80dB at 1kHz)
oIntegrated Click-and-Pop Suppression
oLow-Power Shutdown Control
oShort-Circuit Protection
o±8kV ESD-Protected Amplifier Outputs
oAvailable in Space-Saving Packages
12-Bump UCSP (1.54mm x 2.02mm x 0.6mm)
12-Pin Thin QFN (4mm x 4mm x 0.8mm)
MAX9725
1V, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
________________________________________________________________
Maxim Integrated Products
1
19-3465; Rev 4; 3/09
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
EVALUATION KIT
AVAILABLE
Ordering Information
PA RT PIN- PA CK A G ET O P
M A RK
G A IN
( V/V)
MAX9725AEBC +TG45 12 U C S P+ ACK - 2
M AX9725AE TC+ 12 TQFN- EP* + AAE W- 2
MAX9725BEBC +TG45 12 U C S P+ ACL - 1.5
UCSP is a trademark of Maxim Integrated Products, Inc.
DirectDrive is a registered 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.
Pin Configurations appear at end of data sheet.
Block Diagrams continued at end of data sheet.
MP3 Players
Cellular Phones
PDAs
Smart Phones
Portable Audio Equipment
Block Diagrams
MAX9725
1V, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
Stresses 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
VDD to SGND or PGND ............................................-0.3V to +2V
VSS to 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, PVSS to 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
SHDN to 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= TMIN to TMAX, unless other-
wise noted. Typical values are at TA= +25°C.) (See the
Functional Diagrams.
) (Note 1)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Supply Voltage Range VDD Guaranteed by PSRR test 0.9 1.8 V
Quiescent Supply Current IDD Both channels active 2.1 3.3 mA
TA = +25°C 0.6 10
Shutdown Current ISHDN VSHDN = 0V TA = -40°C to +85°C30
µA
Shutdown to Full Operation tON 180 µs
VIH VDD = 0.9V to 1.8V 0.7 x VDD
SHDN Thresholds VIL VDD = 0.9V to 1.8V 0.3 x VDD
V
SHDN Input Leakage Current ILEAK VDD = 0.9V to 1.8V (Note 2) ±1µA
CHARGE PUMP
Oscillator Frequency fOSC 493 580 667 kHz
AMPLIFIERS
MAX9725A -2.04 -2.00 -1.96
MAX9725B -1.53 -1.5 -1.47
MAX9725C -1.02 -1.00 -0.98
Voltage Gain AV
MAX9725D -4.08 -4.00 -3.92
V/V
Gain Match ΔAV±0.5 %
MAX9725A/MAX9725D ±0.3 ±1.05
MAX9725B ±0.45 ±1.58Total Output Offset Voltage VOS
Input AC-coupled,
RL = 32Ω to GND,
TA = +25°CMAX9725C ±0.6 ±2.1
mV
Input Resistance RIN 15 25 35 kΩ
VDD = 0.9V to 1.8V, TA = +25°C6080
fIN = 1kHz 70Power-Supply Rejection Ratio PSRR 100mVP-P ripple fIN = 20kHz 62
dB
RL = 32Ω10 20
VDD = 1.5V RL = 16Ω25
VDD = 1.0V, RL = 32Ω7
Output Power (Note 3) POUT
VDD = 0.9V, RL = 32Ω6
mW
MAX9725
1V, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (MAX9725A–MAX9725D) (continued)
(VDD = 1.5V, VPGND = VSGND = 0V, VSHDN = 1.5V, VSS = VPVSS, C1 = C2 = 1µF, CIN = 1µF, RL= ∞, TA= TMIN to TMAX, unless other-
wise noted. Typical values are at TA= +25°C.) (See the
Functional Diagrams.
) (Note 1)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
RL = 32Ω, POUT = 12mW, f = 1kHz 0.006
Total Harmonic Distortion Plus
Noise THD+N RL = 16Ω, POUT = 15mW, f = 1kHz 0.015 %
BW = 22Hz to 22kHz 89
Signal-to-Noise Ratio SNR RL = 32Ω, POUT = 12mW A-weighted filter 92 dB
Slew Rate SR 0.2 V/µs
Maximum Capacitive Load CLNo sustained oscillations 150 pF
Crosstalk XTALK fIN = 1.0kHz, RL = 32Ω, POUT = 5mW 100 dB
Into shutdown 72.8
Click-and-Pop Level KCP
RL = 32Ω, peak voltage,
A-weighted, 32 samples per
second (Note 4) Out of shutdown 72.8
dBV
ESD Protection VESD Human Body Model (OUTR, OUTL) ±8 kV
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=
TMIN to TMAX, unless otherwise noted. Typical values are at TA= +25°C.) (See the
Functional Diagrams.
) (Note 1)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Supply Voltage Range VDD Guaranteed by PSRR test 0.9 1.8 V
Quiescent Supply Current IDD Both channels active 2.3 3.7 mA
TA = +25°C 0.6 1
Shutdown Current ISHDN VSHDN = 0V TA = -40°C to +85°C10
µA
Shutdown to Full Operation tON 180 µs
VIH VDD = 0.9V to 1.8V 0.7 x VDD
SHDN Thresholds VIL VDD = 0.9V to 1.8V 0.3 x VDD
V
SHDN Input Leakage Current ILEAK VDD = 0.9V to 1.8V (Note 2) ±1µA
CHARGE PUMP
Oscillator Frequency fOSC 483 592 687 kHz
AMPLIFIERS
Voltage Gain AV(Note 5) -6.11 -6.07 -6.00 V/V
Minimum Stable Gain ΔAV-1.0 V/V
Total Output Offset Voltage VOS Input AC-coupled, RL = 32Ω to GND,
TA = +25°C (Note 6) ±0.63 ±2.1 mV
Input Resistance RIN 6.3 9.78 14 kΩ
INR, INL Input Leakage
Current ILK ±100 nA
Maximum Input Parasitic
Capacitance CPAR 5pF
VDD = 0.9V to 1.8V, TA = +25°C 52.9 67.8
fIN = 1kHz 70
Power-Supply Rejection Ratio PSRR 100mVP-P ripple
(Note 5) fIN = 20kHz 62
dB
MAX9725
1V, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
4 _______________________________________________________________________________________
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. KCP level 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 =
10 10k1k100 100k
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. FREQUENCY
MAX9725 toc01
FREQUENCY (Hz)
THD+N (%)
1
0.1
0.001
0.01
VDD = 1.5V
RL = 16Ω
AV = -2V/V
POUT = 15mW
POUT = 2mW
10 10k1k100 100k
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. FREQUENCY
MAX9725 toc02
FREQUENCY (Hz)
THD+N (%)
1
0.1
0.001
0.01
VDD = 1.5V
RL = 32Ω
AV = -2V/V
POUT = 12mW
POUT = 2mW
10 10k1k100 100k
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. FREQUENCY
MAX9725 toc03
FREQUENCY (Hz)
THD+N (%)
1
0.1
0.001
0.01
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=
TMIN to TMAX, unless otherwise noted. Typical values are at TA= +25°C.) (See the
Functional Diagrams.
) (Note 1)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
RL = 32Ω10 24
VDD = 1.5V RL = 16Ω25
VDD = 1.0V, RL = 32Ω7
Output Power (Note 3) POUT
VDD = 0.9V, RL = 32Ω6
mW
RL = 32Ω, POUT = 12mW, f = 1kHz 0.006
Total Harmonic Distortion Plus
Noise (Note 5) THD+N RL = 16Ω, POUT = 15mW, f = 1kHz 0.015 %
BW = 22Hz to 22kHz 89
Signal-to-Noise Ratio SNR RL = 32Ω, POUT = 12mW A-weighted filter 92 dB
Slew Rate SR 0.3 V/µs
Maximum Capacitive Load CLNo sustained oscillations 150 pF
Crosstalk XTALK fIN = 1.0kHz, RL = 32Ω, POUT = 5mW 100 dB
Into shutdown 72.8
Click-and-Pop Level KCP
RL = 32Ω, peak voltage,
A-weighted, 32 samples per
second (Note 4) Out of shutdown 72.8
dBV
ESD Protection VESD Human Body Model (OUTR, OUTL) ±8 kV
RL = 32Ω-120
Attenuation in Shutdown ATT
(
SD
)
VSHDN = 0V RL = 10kΩ-75 dB
MAX9725
1V, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
_______________________________________________________________________________________
5
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.
)
100
0 5 10 15
10
1
0.1
0.01
0.001
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. OUTPUT POWER
MAX9725 toc07
OUTPUT POWER (mW)
THD+N (%)
VDD = 1V
RL = 16Ω
AV = -2V/V
fIN = 20Hz
fIN = 1kHz
fIN = 10kHz
100
0 5 10 15
10
1
0.1
0.01
0.001
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 = 10kHz
10 100 10k1k 100k
POWER-SUPPLY REJECTION RATIO
vs. FREQUENCY
MAX9725 toc09
FREQUENCY (Hz)
PSRR (dB)
-10
-110
-60
-50
-40
-30
-20
-70
-80
-90
-100
VDD = 1.5V
RL = 32Ω
10 100 10k1k 100k
POWER-SUPPLY REJECTION RATIO
vs. FREQUENCY
MAX9725 toc10
FREQUENCY (Hz)
PSRR (dB)
0
-100
-50
-40
-30
-20
-10
-60
-70
-80
-90
VDD = 1V
RL = 32Ω
10 100 10k1k 100k
CROSSTALK vs. FREQUENCY
MAX9725 toc11
FREQUENCY (Hz)
PSRR (dB)
0
-20
-40
-60
-80
-100
-120
VDD = 1.5V
POUT = 5mW
RL = 32Ω
RIGHT TO LEFT
LEFT TO RIGHT
0
10
20
30
40
50
60
70
80
0.9 1.1 1.3 1.5
OUTPUT POWER vs. SUPPLY VOLTAGE
MAX9725 toc12
SUPPLY VOLTAGE (V)
OUTPUT POWER (mW)
fIN = 1kHz
RL = 16Ω
BOTH INPUTS
DRIVEN IN-PHASE
THD+N = 10%
THD+N = 1%
10 10k1k100 100k
TOTAL HARMONIC DISTORTION PLUS
NOISE vs. FREQUENCY
MAX9725 toc04
FREQUENCY (Hz)
THD+N (%)
1
0.1
0.001
0.01
POUT = 0.7mW
POUT = 4mW
VDD = 1V
RL = 32Ω
AV = -2V/V
100
010203040
10
1
0.1
0.01
0.001
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
1V, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
6 _______________________________________________________________________________________
0
10
20
30
40
50
60
70
80
0 1020304050
POWER DISSIPATION
vs. OUTPUT POWER
MAX9725 toc16
OUTPUT POWER (mW)
POWER DISSIPATION (mW)
VDD = 1.5V
fIN = 1kHz
POUT = POUTL + POUTR
OUTPUTS IN-PHASE
RL = 16Ω
RL = 32Ω
0
5
10
15
20
25
30
35
0 5 10 15 20
POWER DISSIPATION
vs. OUTPUT POWER
MAX9725 toc17
OUTPUT POWER (mW)
POWER DISSIPATION (mW)
VDD = 1V
fIN = 1kHz
POUT = POUTL + POUTR
OUTPUTS IN-PHASE
RL = 16Ω
RL = 32Ω
10 100 10k1k 100k
GAIN FLATNESS
vs. FREQUENCY
FREQUENCY (Hz)
AMPLITUDE (dB)
2
1
0
-1
-2
-3
-4
-5
-7
-6
-8
-9
-10
MAX9725 toc18
0
5
10
15
20
25
30
35
40
10 20 30 40 50
OUTPUT POWER vs. CHARGE-PUMP
CAPACITANCE AND LOAD RESISTANCE
MAX9725 toc19
LOAD RESISTANCE (Ω)
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
-160
-140
-120
-100
-80
-60
-40
-20
0
0 5 10 15 20
OUTPUT SPECTRUM
vs. FREQUENCY
MAX9725 toc20
FREQUENCY (kHz)
AMPLITUDE (dB)
fIN = 1kHz
RL = 32Ω
VOUT = -60dBV
VDD = 1.5V
0
1.5
1.0
0.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
0.9 1.11.0 1.2 1.3 1.4 1.5
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX9725 toc21
SUPPLY VOLTAGE (V)
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.
)
0
15
10
5
20
25
30
35
40
45
50
0.9 1.1 1.3 1.5
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%
80
70
0
10 100 1k
OUTPUT POWER
vs. LOAD RESISTANCE
20
MAX9725 toc14
LOAD RESISTANCE (Ω)
OUTPUT POWER (mW)
40
60
10
30
50
VDD = 1.5V
fIN = 1kHz
BOTH INPUTS
DRIVEN IN-PHASE
THD+N = 10%
THD+N = 1%
80
70
0
10 100 1k
OUTPUT POWER
vs. LOAD RESISTANCE
20
MAX9725 toc15
LOAD RESISTANCE (Ω)
OUTPUT POWER (mW)
40
60
10
30
50
VDD = 1V
fIN = 1kHz
BOTH INPUTS
DRIVEN IN-PHASE
THD+N = 10%
THD+N = 1%
MAX9725
1V, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
_______________________________________________________________________________________ 7
PIN BUMP
THIN
QFN UCSP NAME FUNCTION
1 A1 C1N Flying Capacitor Negative Terminal. Connect a 1µF capacitor from C1P to C1N.
2A2PV
SS Inverting Charge-Pump Output. Bypass with 1µF from PVSS to PGND. PVSS must be connected to
VSS.
3 A3 INL Left-Amplifier Inverting Input. Connect input resistor RIN from input capacitor C1N to INL (MAX9725E
only).
4 A4 INR Right-Amplifier Inverting Input. Connect input resistor RIN from input capacitor C1N to INR
(MAX9725E only).
5B4V
SS Amplifier Negative Power Supply. Must be connected to PVSS.
6 B3 SGND Signal Ground. SGND must be connected to PGND. SGND is the ground reference for the input and
output signal.
7 C4 OUTR Right-Channel Output. Connect feedback resistor RFB between OUTR and INR (MAX9725E only).
8 C3 OUTL Left-Channel Output. Connect feedback resistor RFB between OUTL and INL (MAX9725E only).
9C2V
DD Positive Power-Supply Input. Bypass with a 1µF capacitor to PGND.
10 C1 C1P Flying Capacitor Positive Terminal. Connect a 1µF capacitor from C1P to C1N.
11 B1 PGND Power Ground. Ground reference for the internal charge pump. PGND must be connected to SGND.
12 B2 SHDN Active-Low Shutdown. Connect to VDD for normal operation. Pull low to disable the amplifier and
charge pump.
EP — EP Exposed Paddle. Internally connected to VSS. Leave paddle unconnected or solder to VSS.
Pin Description
SHUTDOWN CURRENT
vs. SUPPLY VOLTAGE
MAX9725 toc22
SUPPLY VOLTAGE (V)
SHUTDOWN CURRENT (μA)
1.31.1
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0
0.9 1.5
EXITING SHUTDOWN
MAX9725 toc23
200μs/div
OUT_
1V/div
SHDN
500mV/div
POWER-UP/-DOWN WAVEFORM
MAX9725toc24
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
1V, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
8 _______________________________________________________________________________________
Detailed Description
The MAX9725 stereo headphone driver features Maxim’s
patented DirectDrive architecture, eliminating the large
output-coupling capacitors required by conventional sin-
gle-supply headphone drivers. The MAX9725 consists of
two 20mW class AB headphone drivers, shutdown con-
trol, inverting charge pump, internal gain-setting resistors,
and comprehensive click-and-pop suppression circuitry
(see the
Functional Diagram
). A negative power supply
(PVSS) is created by inverting the positive supply (VDD).
Powering the drivers from VDD and PVSS increases the
dynamic range of the drivers to almost twice that of other
1V single-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.
DirectDrive
Conventional 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
GND
VOUT
CONVENTIONAL DRIVER-BIASING SCHEME
DirectDrive BIASING SCHEME
VDD / 2
VDD
GND
VOUT
MAX9725
Low-Frequency Response
Large DC-blocking capacitors limit the amplifier’s low-
frequency response and can distort the audio signal:
1) 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
COUT is 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 COUT reduce this effect but result
in physically larger, more expensive capacitors.
Figure 2 shows the relationship between the size of
COUT and 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.
2) 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 Pump
The 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 Diagram
).
Extra noise attenuation is not typically required.
Shutdown
The MAX9725’s low-power shutdown mode reduces
supply current to 0.6µA. Driving SHDN low 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.
f2RC
-3dB L OUT
=1
π
Figure 2. Low-Frequency Attenuation for Common DC-Blocking
Capacitor Values
LF ROLLOFF (16Ω LOAD)
FREQUENCY (Hz)
ATTENUATION (dB)
100
-30
-25
-20
-10 -3dB CORNER FOR
100μF IS 100Hz
-15
-5
-3
0
-35
10 1k
33μF
330μF
220μF
100μF
Figure 3. Distortion Contributed By DC-Blocking Capacitors
ADDITIONAL THD+N DUE
TO DC-BLOCKING CAPACITORS
FREQUENCY (Hz)
THD+N (%)
10k1k100
0.001
0.01
0.1
1
10
0.0001
10 100k
TANTALUM
ALUM/ELEC
1V, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
_______________________________________________________________________________________ 9
MAX9725
1V, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
10 ______________________________________________________________________________________
Click-and-Pop Suppression
In conventional single-supply audio drivers, the output-
coupling capacitor is a major contributor of audible
clicks and pops. Upon startup, the driver charges the
coupling capacitor to its bias voltage, typically half the
supply. Likewise, on shutdown, the capacitor is dis-
charged to GND. This results in a DC shift across the
capacitor that appears as an audible transient at the
speaker. The MAX9725’s DirectDrive technology elimi-
nates the need for output-coupling capacitors.
The MAX9725 also features extensive click-and-pop
suppression that eliminates any audible transient
sources internal to the device. The Power-Up/-Down
Waveform in the
Typical Operating Characteristics
shows minimal DC shift and no spurious transients at
the output upon startup or shutdown.
In most applications, the output of the preamplifier dri-
ving the MAX9725 has a DC bias of typically half the
supply. At startup, the input-coupling capacitor is
charged to the preamplifier’s DC bias voltage through
the internal input resistor (25kΩfor MAX9725A-
MAX9725D, minimum 10kΩfor MAX9725E) causing an
audible click and pop. Delaying the rise of SHDN 4 or 5
time constants, based on RIN x CIN, relative to the start-
up of the preamplifier eliminates any click and pop
caused by the input filter (see the
Functional Diagram
).
Applications Information
Power Dissipation
Linear power amplifiers can dissipate a significant
amount of power under normal operating conditions.
The maximum power dissipation for each package is
given in the
Absolute Maximum Ratings
section under
Continuous Power Dissipation or can be calculated by
the following equation:
where TJ(MAX) is +150°C, TAis the ambient tempera-
ture, and θJA is the reciprocal of the derating factor in
°C/W as specified in the
Absolute Maximum Ratings
section. For example, θJA for the thin QFN package is
+59.3°C/W.
The MAX9725 has two power dissipation sources, the
charge pump and the two amplifiers. If the power dissi-
pation exceeds the rated package dissipation, 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
surrounding air.
Output Power
The MAX9725’s output power increases when the left
and right audio signals differ in magnitude and/or
phase. Figure 4 shows the two extreme cases for in-
and out-of-phase input signals. The output power of a
typical stereo application lies between the two extremes
shown in Figure 4. The MAX9725 is specified to output
20mW per channel when both inputs are in-phase.
Powering Other Circuits from
the Negative Supply
The MAX9725 internally generates a negative supply
voltage (PVSS) to provide the ground-referenced output
signal. Other devices can be powered from PVSS pro-
vided the current drawn from the charge pump does
not exceed 1mA. Headphone driver output power and
THD+N will be adversely affected if more than 1mA is
drawn from PVSS. Using PVSS as an LCD bias is a typi-
cal application for the negative supply.
PVSS is unregulated and proportional to VDD. Connect
a 1µF capacitor from C1P to C1N for best charge-pump
operation.
PT-T
DISSPKG(MAX)
J(MAX) A
JA
=θ
Figure 4. Output Power vs. Supply Voltage with Inputs In-/Out-
of-Phase
0
15
10
5
20
25
30
35
40
45
50
0.9 1.1 1.3 1.5
OUTPUT POWER vs. SUPPLY VOLTAGE
WITH INPUTS IN- AND OUT-OF-PHASE
SUPPLY VOLTAGE (V)
OUTPUT POWER (mW)
fIN = 1kHz
RL = 16Ω
THD+N = 1%
INPUTS 180° OUT-OF-PHASE
INPUTS IN-PHASE
MAX9725
Component Selection
Input Filtering
The AC-coupling capacitor (CIN) and an internal gain-
setting resistor form a highpass filter that removes any
DC bias from an input signal (see the
Functional
Diagram
). CIN allows the MAX9725A–MAX9725D to
bias the signal to an optimum DC level. The -3dB point
of the highpass filter, assuming zero source imped-
ance, is given by:
Choose CIN so f-3dB is well below the lowest frequency of
interest. RIN for the MAX9725A–MAX9725D is 25kΩand a
minimum of 10kΩfor the MAX9725E. Setting f-3dB too
high affects the amplifier’s low-frequency response. Use
capacitors with low-voltage coefficient dielectrics. Film or
C0G dielectric capacitors are good choices for AC-cou-
pling capacitors. Capacitors with high-voltage coeffi-
cients, such as ceramics, can result in increased
distortion at low frequencies.
Charge-Pump Capacitor Selection
Use capacitors with less than 100mΩof ESR. Low-ESR
ceramic capacitors minimize the output impedance of the
charge pump. Capacitors with an X7R dielectric provide
the best performance over the extended temperature
range. Table 1 lists suggested capacitor manufacturers.
Flying Capacitor (C1)
The value of C1 affects the charge pump’s load regula-
tion and output impedance. Choosing C1 too small
degrades the MAX9725’s ability to provide sufficient
current drive and leads to a loss of output voltage.
Increasing the value of C1 improves load regulation
and reduces the charge-pump output impedance. See
the Output Power vs. Charge-Pump Capacitance and
Load Resistance graph in the
Typical Operating
Characteristics
.
Hold Capacitor (C2)
The hold capacitor’s value and ESR directly affect the
ripple at PVSS. Increasing the value of C2 reduces rip-
ple. Choosing a capacitor with lower ESR reduces rip-
ple and output impedance. Lower capacitance values
can be used in systems with low maximum output
power levels. See the Output Power vs. Charge-Pump
Capacitance and Load Resistance graph in the
Typical
Operating Characteristics
.
Power-Supply Bypass Capacitor (C3)
The power-supply bypass capacitor (C3) lowers the
output impedance of the power supply and reduces the
impact of the MAX9725’s charge-pump switching tran-
sients. Bypass VDD to PGND with the same value as
C1. Place C3 as close to VDD as possible.
Layout and Grounding
Proper layout and grounding are essential for optimum
performance. Connect PGND and SGND together at a
single point on the PC board. Connect PVSS to SVSS
and bypass with C2 to PGND. Bypass VDD to PGND
with C3. Place capacitors C2 and C3 as close to the
MAX9725 as possible. Route PGND, and all traces that
carry switching transients, away from SGND and the
audio signal path.
The MAX9725 does not require additional heatsinking.
The thin QFN package features an exposed paddle that
improves thermal efficiency of the package. Ensure the
exposed paddle is electrically isolated from GND and
VDD. Connect the exposed paddle to VSS if necessary.
UCSP Applications Information
For the latest application details on UCSP construction,
dimensions, tape carrier information, printed circuit
board techniques, bump-pad layout , and recommend-
ed reflow temperature profile, as well as the latest infor-
mation on reliability testing results, go to Maxim’s
website at www.maxim-ic.com/ucsp for the Application
Note:
UCSP—A Wafer-Level Chip-Scale Package
.
f2C
-3dB IN
=××
1
πRIN
Table 1. Suggested Capacitor Manufacturers
SUPPLIER PHONE FAX WEBSITE
Murata 770-436-1300 — www.murata.com
Taiyo Yuden 800-348-2496 847-925-0899 www.t-yuden.com
TDK 847-803-6100 847-390-4405 www.component.tdk.com
1V, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
______________________________________________________________________________________ 11
MAX9725
1V, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
12 ______________________________________________________________________________________
OUTR
0.9V TO 1.8V
OUTL
SGND PGND
VDD
INR
INL
C1P
C1N
VSS
PVSS
MP3
DECODER
1μF
1μF
1μF
1μF
SHDN
1μF
MAX9725A–
MAX9725D
STEREO
DAC
MAX9725E
1μFRIN
RF
SGND PGND
CHARGE
PUMP
RF
INL
C1N
DIN
OUTL
1μFRIN INR
OUTR
PVSS
VSS
1μF1μF
VDD
SHDN
1μF
System Diagrams
MAX9725
CHARGE
PUMP
UVLO/
SHUTDOWN
CONTROL
CLICK-AND-POP
SUPPRESSION
C1N
C1P
PVSS VSS PGND SGND
*MAX9725A = 50kΩ.
MAX9725B = 37.5kΩ.
MAX9725C = 25kΩ.
MAX9725D = 100kΩ.
( ) DENOTE BUMPS FOR UCSP.
INR
VDD
VSS
VDD
SGND
OUTR
HEADPHONE
JACK
12
(B2)
9
(C2)
10
(C1)
11
(B1)
1
(A1)
2
(A2)
5
(B4)
8
(C3)
7
(C4)
6
(B3)
C1
1μF
C2
1μF
0.9V TO 1.8V
C3
1μF
CIN
0.47μF
RIN
25kΩ
LEFT-
CHANNEL
AUDIO IN
VSS
VDD
RF*
OUTL
SHDN
3
(A3)
INL
4
(A4)
SGND
MAX9725A–
MAX9725D
CIN
0.47μF
RF*
LEFT-
CHANNEL
AUDIO IN
RIN
25kΩ
Functional Diagrams
1V, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
______________________________________________________________________________________ 13
MAX9725
1V, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
14 ______________________________________________________________________________________
CHARGE
PUMP
UVLO/
SHUTDOWN
CONTROL
CLICK-AND-POP
SUPPRESSION
C1N
C1P
PVSS VSS PGND SGND
( ) DENOTE BUMPS FOR UCSP.
INR
VDD
VDD
VSS
SGND
OUTR
HEADPHONE
JACK
12
(B2)
9
(C2)
10
(C1)
11
(B1)
1
(A1)
2
(A2)
5
(B4)
8
(C3)
7
(C4)
RF
RIN
CIN
1μF
CIN
1μF
RF
6
(B3)
C1
1μF
C2
1μF
0.9V TO 1.8V
C3
1μF
VSS
VDD
OUTL
SHDN
3
(A3)
INL
4
(A4)
SGND
MAX9725E
RIGHT-
CHANNEL
AUDIO IN
LEFT-CHANNEL
AUDIO IN
RIN
Functional Diagrams (continued)
MAX9725
C1N
C1P
OUTL
SINGLE
1.5V CELL
AA OR AAA
BATTERY
OUTR
SGND
RFB PGND
MAX9725E
INVERTING
CHARGE PUMP
INR
INL
VDD
PVSS
C2
VSS
C3
RFB
DirectDrive OUTPUTS
ELIMINATE DC-BLOCKING
CAPACITORS.
Block Diagrams (continued) Ordering Information (continued)
PA RT PIN- PA CK A G ET O P
M A RK
G A IN
( V/V)
M AX9725BE TC+ 12 TQFN- EP* + AAE X - 1.5
MAX9725CEBC+TG45 12 U C S P+ ACM - 1
M AX9725C E TC+ 12 TQFN- EP* + AAE Y - 1
MAX9725D EBC +TG45 12 U C S P+ ACN - 4
M AX9725D E TC+ 12 TQFN- EP* + AAE Z - 4
MAX9725E EBC +TG45 12 U C S P+ AEF AD J
M AX9725E E TC+ ** 12 TQFN- EP* + AAGH AD J
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.
**
Future product—contact factory for availability.
123
C
B
A
UCSP
TOP VIEW
(BUMP-SIDE DOWN) 4
C1N PVSS INL INR
VSS
SGND
C1P VDD OUTL OUTR
PGND SHDN
TOP VIEW
THIN QFN
MAX9725
12 *EP
*EP = EXPOSED PAD.
11
+
10
4
5
PVSS
INL
6
C1N
OUTL
OUTR
VDD
12
PGND
3
987
SHDN
SGND
VSS
INR
MAX9725
C1P
Pin Configurations
1V, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
______________________________________________________________________________________ 15
MAX9725
1V, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
16 ______________________________________________________________________________________
PACKAGE TYPE PACKAGE CODE DOCUMENT NO.
12 UCSP B12-1 21-0104
12 TQFN-EP T1244-4 21-0139
Chip Information
PROCESS: BiCMOS
Package Information
For the latest package outline information and land patterns, go
to www.maxim-ic.com/packages. Note that a “+”, “#”, or “-” in
the package code indicates RoHS status only. Package draw-
ings may show a different suffix character, but the drawing per-
tains to the package regardless of RoHS status.
MAX9725
1V, Low-Power, DirectDrive, Stereo Headphone
Amplifier with Shutdown
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________
17
© 2009 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.
REVISION
NUMBER
REVISION
DATE DESCRIPTION PAGES
CHANGED
0 11/04 Initial release —
1 5/05 Removed future product asterisks for UCSP package, added EC table note 1–3, 13, 14
2 11/07 Added MAX9725E packages, MAX9725E EC table, block diagram, functional
diagram, and system diagram. Updated package outlines. 1–3, 6, 8–19
3 8/08 Corrected error in Functional Diagram 14
4 3/09 Updated Ordering Information, style changes 1, 15
Revision History