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MAX9720AEBE+T-MAX9720BEBE+T
50mW, DirectDrive®, Stereo Headphone Amplifier with SmartSense™ and Shutdown
General DescriptionThe MAX9720 stereo headphone amplifier combines
Maxim’s patented DirectDrive architecture and
SmartSense™, an automatic mono/stereo detection fea-
ture. Conventional headphone amplifiers require a bulky
DC-blocking capacitor between the headphone and the
amplifier. DirectDrive produces a ground-referenced out-
put from a single supply, eliminating the need for large
DC-blocking capacitors, saving cost, board space, and
component height.
SmartSense automatically detects the presence of a
short at either the left or right amplifier output. Under a
fault condition, the shorted output is automatically dis-
abled and the stereo input signal is automatically mixed
and routed to the remaining active channel. This feature
is useful in cell phone and PDA applications where a
variety of headphone jacks with unknown loads can be
inserted into the headphone jack socket. SmartSense
prevents both damage to the amplifier and eliminates
battery drain into a shorted load.
The MAX9720 delivers up to 50mW per channel into a
16Ωload and has an ultra-low 0.003% THD+N. A high
(92dB at 217kHz) power-supply rejection ratio (PSRR)
allows the device to operate from noisy digital supplies
without additional power conditioning. The gain of the
MAX9720 is set internally, further reducing component
count. Two gain options are available (-1V/V, MAX9720A
and -1.41V/V, MAX9720B). The headphone outputs
include a comprehensive click-and-pop circuitry that
eliminates audible glitches on startup and shutdown. A
shutdown mode provides a fast 250µs turn-on time.
The MAX9720 operates from a single 1.8V to 3.6V
supply and consumes only 5mA of supply current. The
MAX9720 also features thermal overload protection,
and is specified over the extended -40°C to +85°C tem-
perature range. The MAX9720 is available in a tiny
(2mm x 2mm x 0.6mm) 16-bump chip-scale package
(UCSP™) and a 16-pin TSSOP package.
Applications
FeaturesDirectDrive Eliminates Bulky DC-Blocking
CapacitorsSmartSense Automatic Short DetectionLow 5mA Quiescent Current Fixed Gain Eliminates External Feedback Network
MAX9720A: -1V/V
MAX9720B: -1.41V/V50mW per Channel Output PowerUltra-Low 0.003% THD+NHigh PSRR (92dB at 217Hz)Integrated Click-and-Pop Suppression1.8V to 3.6V Single-Supply OperationThermal Overload ProtectionAvailable in Space-Saving Packages
16-Bump UCSP (2mm x 2mm x 0.6mm)
16-Pin TSSOP
MAX9720
50mW, DirectDrive, Stereo Headphone
Amplifier with SmartSense and Shutdown
Ordering Information19-2859; Rev 0; 4/03
Pin Configuration and Typical Application Circuit appear at
end of data sheet.PDAs
Cellular Phones
MP3 Players
Notebook PCs
Smart Phones
Tablet PCs
Portable Audio Equipment
SmartSense and UCSP are trademarks of Maxim Integrated
Products, Inc.
Simplified Block Diagram
MAX9720
50mW, DirectDrive, Stereo Headphone
Amplifier with SmartSense and Shutdown
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(VDD= VMODE1= VMODE2= 3.0V, PGND = SGND = 0V, RL= ∞, C1 = C2 = 2.2µF. TA= TMINto TMAX,unless otherwise noted.
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.
PGND to SGND.....................................................-0.3V to +0.3V
PVSSto SVSS.........................................................-0.3V to +0.3V
VDDto PGND or SGND............................................-0.3V to +4V
PVSSand SVSSto PGND or SGND..........................-4V to +0.3V
IN_, OUT_, and HPS to SGND.......(SVSS- 0.3V) to (VDD+ 0.3V)
C1P to PGND...............................(PGND - 0.3V) to (VDD+ 0.3V)
C1N to PGND.............................(PVSS- 0.3V) to (PGND + 0.3V)ALERTto PGND.......................................................-0.3V to +4V
MODE_ to PGND........................................-0.3V to (VDD+ 0.3V)
TIME to SGND............................................-0.3V to (VDD+ 0.3V)
Output Short Circuit to GND or VDD...............................Continuous
Continuous Power Dissipation (TA= +70°C)
16-Bump UCSP (derate 8.2mW/°C above +70°C).......659mW
16-Pin TSSOP (derate 9.4mW/°C above +70°C).......754.7mW
Junction Temperature......................................................+150°C
Operating Temperature Range...........................-40°C to +85°C
Storage Temperature Range.............................-65°C to +150°C
Bump Temperature (soldering)
Reflow...........................................................................+235°C
Lead Temperature (soldering, 10s).................................+300°C
MAX9720
50mW, DirectDrive, Stereo Headphone
Amplifier with SmartSense and Shutdown
Note 1:All specifications are 100% tested at TA= +25oC; temperature limits are guaranteed by design.
Note 2:Inputs are AC-coupled to ground.
Note 3:Inputs are connected directly to ground.
ELECTRICAL CHARACTERISTICS (continued)(VDD= VMODE1= VMODE2= 3.0V, PGND = SGND = 0V, RL= ∞, C1 = C2 = 2.2µF. TA= TMINto TMAX,unless otherwise noted.
MAX9720
50mW, DirectDrive, Stereo Headphone
Amplifier with SmartSense and Shutdown
Typical Operating Characteristics(VDD= 3V, THD+N bandwidth = 22Hz to 22kHz, MODE1 = MODE2 = VDD.)
MAX9720
50mW, DirectDrive, Stereo Headphone
Amplifier with SmartSense and Shutdown
TOTAL HARMONIC DISTORTION
PLUS NOISE vs. OUTPUT POWERMAX9720 toc10
OUTPUT POWER (mW)
THD+N (%)
TOTAL HARMONIC DISTORTION
PLUS NOISE vs. OUTPUT POWER
MAX9720 toc11
OUTPUT POWER (mW)
THD+N (%)
TOTAL HARMONIC DISTORTION
PLUS NOISE vs. OUTPUT POWER
MAX9720 toc12
OUTPUT POWER (mW)
THD+N (%)604020
TOTAL HARMONIC DISTORTION
PLUS NOISE vs. OUTPUT POWER
MAX9720 toc13
OUTPUT POWER (mW)
THD+N (%)604020
TOTAL HARMONIC DISTORTION
PLUS NOISE vs. OUTPUT POWER
MAX9720 toc14
OUTPUT POWER (mW)
THD+N (%)604020
TOTAL HARMONIC DISTORTION
PLUS NOISE vs. OUTPUT POWER
MAX9720 toc15
OUTPUT POWER (mW)
THD+N (%)
TOTAL HARMONIC DISTORTION
PLUS NOISE vs. OUTPUT POWER
MAX9720 toc16
OUTPUT POWER (mW)
THD+N (%)
TOTAL HARMONIC DISTORTION
PLUS NOISE vs. OUTPUT POWER
MAX9720 toc17
OUTPUT POWER (mW)
THD+N (%)
TOTAL HARMONIC DISTORTION
PLUS NOISE vs. OUTPUT POWER
MAX9720 toc18
OUTPUT POWER (mW)
THD+N (%)
Typical Operating Characteristics (continued)
(VDD= 3V, THD+N bandwidth = 22Hz to 22kHz, MODE1 = MODE2 = VDD.)
MAX9720
50mW, DirectDrive, Stereo Headphone
Amplifier with SmartSense and Shutdown
Typical Operating Characteristics (continued)(VDD= 3V, THD+N bandwidth = 22Hz to 22kHz, MODE1 = MODE2 = VDD.)
MAX9720
50mW, DirectDrive, Stereo Headphone
Amplifier with SmartSense and Shutdown
TOTAL HARMONIC DISTORTION
PLUS NOISE vs. OUTPUT POWERMAX9720 toc28
OUTPUT POWER (mW)
THD+N (%)302010
TOTAL HARMONIC DISTORTION
PLUS NOISE vs. OUTPUT POWER
MAX9720 toc29
OUTPUT POWER (mW)
THD+N (%)302010
TOTAL HARMONIC DISTORTION
PLUS NOISE vs. OUTPUT POWER
MAX9720 toc30
OUTPUT POWER (mW)
THD+N (%)155302010
TOTAL HARMONIC DISTORTION
PLUS NOISE vs. OUTPUT POWER
MAX9720 toc31
OUTPUT POWER (mW)
THD+N (%)155302010
TOTAL HARMONIC DISTORTION
PLUS NOISE vs. OUTPUT POWER
MAX9720 toc32
OUTPUT POWER (mW)
THD+N (%)155302010
OUTPUT POWER vs. SUPPLY VOLTAGE
MAX9720 toc33
SUPPLY VOLTAGE (V)
OUTPUT POWER (mW)
OUTPUT POWER vs. SUPPLY VOLTAGE
MAX9720 toc34
SUPPLY VOLTAGE (V)
OUTPUT POWER (mW)
OUTPUT POWER vs. SUPPLY VOLTAGE
MAX9720 toc35
SUPPLY VOLTAGE (V)
OUTPUT POWER (mW)
OUTPUT POWER vs. SUPPLY VOLTAGE
MAX9720 toc36
SUPPLY VOLTAGE (V)
OUTPUT POWER (mW)
Typical Operating Characteristics (continued)
(VDD= 3V, THD+N bandwidth = 22Hz to 22kHz, MODE1 = MODE2 = VDD.)
MAX9720
50mW, DirectDrive, Stereo Headphone
Amplifier with SmartSense and Shutdown
Typical Operating Characteristics (continued)(VDD= 3V, THD+N bandwidth = 22Hz to 22kHz, MODE1 = MODE2 = VDD.)
MAX9720
50mW, DirectDrive, Stereo Headphone
Amplifier with SmartSense and Shutdown
CROSSTALK vs. FREQUENCYMAX9720 toc46
FREQUENCY (Hz)
CROSSTALK (dB)
10k1k100
-120100k
GAIN FLATNESS vs. FREQUENCYMAX9720 toc47
FREQUENCY (Hz)
GAIN (dB)1001010.1
0.0110k
CHARGE-PUMP OUTPUT IMPEDANCE
vs. SUPPLY VOLTAGEMAx9720 toc48
SUPPLY VOLTAGE (V)
OUTPUT IMPEDANCE (
OUTPUT POWER vs. LOAD RESISTANCE
AND CHARGE-PUMP CAPACITOR SIZE
MAx9720 toc49
LOAD RESISTANCE (Ω)
OUTPUT POWER (mW)302050
OUTPUT SPECTRUM vs. FREQUENCYMAX9720 toc50
FREQUENCY (Hz)
OUTPUT SPECTRUM (dB)
10k1k
100100k
SUPPLY CURRENT vs. SUPPLY VOLTAGEMAX9720 toc51
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (mA)
SHUTDOWN SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX9720 toc52
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (
EXITING SHUTDOWN
MAX9720 toc53
400µs/div
OUT_500mV/div
fIN = 1kHz
RL = 32Ω
MODE1 AND
MODE2
POWER-UP/DOWN WAVEFORMMAX9720 toc54
200ms/div
FFT: 25Hz/div
OUT_FFT
OUT_
20dB/div
10mV/div
VIN = GND
RL = 32Ω
VDD
Typical Operating Characteristics (continued)(VDD= 3V, THD+N bandwidth = 22Hz to 22kHz, MODE1 = MODE2 = VDD.)
MAX9720
Detailed DescriptionThe MAX9720 fixed-gain, stereo headphone amplifier
includes Maxim’s patented DirectDrive architecture and
SmartSense. DirectDrive eliminates the large output-
coupling capacitors required by conventional single-
supply headphone amplifiers. SmartSense automatically
detects the presence of a short at either output. Under a
fault condition, the shorted output is automatically
disabled and the stereo input signal is automatically
mixed and routed to the remaining active channel. This
prevents damage to the amplifier and optimizes power
savings by eliminating battery drain into a shorted load.
The device consists of two 50mW Class AB headphone
amplifiers, an internal feedback network (MAX9720A:
fixed -1V/V gain, MAX9720B: fixed -1.41V/V gain), a
mono mixer/attenuator, undervoltage lockout (UVLO)/
shutdown control, SmartSense, a charge pump, and
comprehensive click-and-pop suppression circuitry
(see Functional Diagram). 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 GND
(Figure 1). The amplifiers have almost twice the supply
range compared to other single-supply amplifiers,
nearly quadrupling the available output power. The
benefit of the GND bias is that the amplifier outputs do
not have a DC component (typically VDD/2). This elimi-
nates the large DC-blocking capacitors required with
conventional headphone amplifiers, conserving board
space, system cost, and improving frequency
response.
The noninvasive SmartSense feature of the MAX9720
detects a short on either output. The SmartSense routine
executes when the device is powered up or brought out
of shutdown (see the SmartSensesection). If a fault is
detected, the shorted channel is shut down, the output
goes high impedance, and the stereo audio input is
mixed/attenuated and fed to the remaining active chan-
nel. The device also features an ALERToutput that indi-
cates to a host µC that SmartSense has detected a
short-circuit condition on either amplifier output.
Forced stereo and forced mono modes can also be
selected through the two MODE_ inputs. In forced
operation mode, SmartSense is disabled and the
device operates as specified by the MODE_ inputs,
regardless of output load conditions. A fast low-power
shutdown mode is also selected through the MODE_
inputs (see the Mode_ Selectionsection).
The UVLO prevents operation from an insufficient
power supply and click-and-pop suppression, which
eliminates audible transients on startup and shutdown.
Additionally, the MAX9720 features thermal overload
protection and can withstand ±4kV ESD strikes on the
output.
50mW, DirectDrive, Stereo Headphone
Amplifier with SmartSense and Shutdown
Pin Description
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 patented DirectDrive architecture uses a
charge pump to create an internal negative supply volt-
age. This allows the MAX9720 output to be biased
about GND, almost doubling dynamic range while
operating from a single supply. With no DC component,
there is no need for the large DC-blocking capacitors.
Instead of two large capacitors (220µF typ), the
MAX9720 charge pump requires only two, small ceram-
ic capacitors (1µF typ), conserving board space,
reducing cost, and improving the frequency response
of the headphone amplifier. See the Output Power vs.
Charge-Pump Capacitance and Load Resistance
graph in the Typical Operating Characteristicsfor
details of the possible 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 raised 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 amplifier’s ESD structures
are the only path to system ground. The amplifier
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
equipment, resulting in large ground-loop current
and possible damage to the amplifiers.When using a combination microphone and speak-
er headset (in a cell phone or PDA application), the
microphone typically requires a GND return. Any
DC bias on the sleeve conflicts with the microphone
requirements (Figure 2).
Low-Frequency ResponseIn addition to the cost and size disadvantages, the DC-
blocking capacitors limit the low-frequency response of
the amplifier and distort the audio signal:The impedance of the headphone load and the DC-
blocking capacitor form a highpass filter with the
-3dB point determined 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-supply headphone amplifiers to block
the midrail DC component of the audio signal from the
headphones. Depending on the -3dB point, the filter
can attenuate low-frequency signals within the audio
band. Larger values of COUTreduce the attenuation,
but are physically larger, more expensive capacitors.
Figure 3 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 audio
band.
MAX9720
50mW, DirectDrive, Stereo Headphone
Amplifier with SmartSense and Shutdown
