MAX9720AEUE+ Datasheet MAX9720AEUE+, MAX9720AEUE+T, MAX9720AEBE+T | P13-P15

Mode Selection (MODE_)

SmartSense is controlled by the two mode select

inputs, MODE1 and MODE2. Table 1 shows the operat-

ing modes in relation to the status of the MODE_ inputs.

When MODE1 = MODE2 = low, the device is in low-

power shutdown mode. When MODE1 = high and

MODE2 = low, the device is in forced mono mode. The

right channel is disabled, OUTR goes high impedance,

and the stereo audio input is mixed, and the audio sig-

nal is reproduced on OUTL. SmartSense is disabled in

this mode. When MODE1 = low and MODE2 = high, the

device is in forced stereo mode, and SmartSense is

disabled. When the device detects the presence of a

short BEFORE forced stereo mode is selected, the

device remains in mono mode (Figure 5). When

MODE1 = MODE2 = high, the device is in automatic

detection mode; the operating mode of the device is

determined by SmartSense.

MODE1 is also used to execute a host-controlled

SmartSense routine and reset the ALERT output. On the

rising edge of MODE1, the device invokes a

SmartSense routine. The falling edge of MODE1 resets

the ALERT output to its idle state.

Automatic Detection Mode

A fault condition is defined as a short (under 4Ω) on

either amplifier output to ground. SmartSense automati-

cally detects and disables the shorted output. The

mixer/attenuator combines the two stereo inputs (INL

and INR), attenuates the resultant signal by a factor of

2, and redirects the audio playback to the remaining

active channel. This allows for full reproduction of a

stereo signal through a single headphone while main-

taining optimum headroom. The mixed mono signal is

output only on the properly loaded channel. If both out-

puts are shorted then both outputs go into a high-

impedance state and no audio playback occurs. In

automatic detection mode (MODE1 = MODE2 = high),

any of the following events trigger a SmartSense test

sequence:

• HPS rises above 0.8 x V

DD,

indicating a headphone

jack has been inserted into the socket.

• The 180mA high-side (sourcing) overcurrent thresh-

old is approached, and the output is near GND.

• The die temperature exceeds the thermal limit

(+140°C).

• Power or shutdown is cycled.

MAX9720

50mW, DirectDrive, Stereo Headphone

Amplifier with SmartSense and Shutdown

______________________________________________________________________________________ 13

M1 = L

M2 = L

M1 = H

M2 = L

M1 = L

M2 = H

SHORT

DETECTED

STATUS

CHANGE

SHDN

SmartSense

FORCED MONO

FORCED STEREO

MONO MODE

STATUS

CHANGE

STATUS

CHANGE

STATUS

CHANGE

STEREO MODE

STATUS

CHANGE

MAX9720

Figure 5. SmartSense Flow Diagram

MODE1 MODE2 SmartSense

OPERATING

MODE

High High Enabled

Automatic

detection mode

Low Low Disabled Shutdown

High Low Disabled Forced left mono

Low High Disabled Forced stereo

High Enabled Host controlled

X — Reset ALERT

Table 1. MAX9720 Operating Modes

MAX9720

For automatic headphone detection, connect HPS to the

control pin of a 3-wire headphone jack, as shown in

Figure 7. With no headphone present, the output imped-

ance of the amplifier pulls HPS to less than 0.8 x V

When a headphone plug is inserted into the jack, the

control pin is disconnected from the tip contact, and

HPS is pulled to V

through the internal 100kΩ pullup.

A debounce delay controls the time between HPS going

high and the initiation of the SmartSense test sequence.

This time is controlled by an external capacitor on the

TIME pin and allows the user to customize the de-

bounce time (see the TIME Capacitor section).

Shutdown

Driving MODE1 and MODE2 to GND shuts down the

MAX9720, disconnects the internal HPS pullup resistor,

disables the charge pump and amplifiers, sets the

amplifier output impedance to 1kΩ, and reduces sup-

ply current to less than 6µA.

Forced Mono Mode

In forced left mono mode (MODE1 = high, MODE2 =

low), the right channel is disabled and OUTR goes high

impedance. The stereo signal inputs are combined

through the mixer/attenuator and output on the left

channel. In forced mono mode, the SmartSense routine

is disabled.

Forced Stereo Mode

In forced stereo mode (MODE1 = low, MODE2 = high),

the device operates as a stereo headphone amplifier.

In forced stereo mode, the SmartSense routine is dis-

abled.

AALLEERRTT

Output

The MAX9720 includes an active-low, open-drain

ALERT output that indicates to the master device that

SmartSense has detected a fault condition. ALERT trig-

gers when an output short circuit is detected through

the SmartSense routine. During normal operation,

ALERT idles high. If a fault condition is detected,

ALERT pulls the line low. ALERT remains low until

MODE1 is toggled from high to low.

Click-and-Pop Suppression

In conventional single-supply audio amplifiers, the out-

put-coupling capacitor is a major contributor of audible

clicks and pops. Upon startup, the amplifier charges

the coupling capacitor to its bias voltage, typically half

the supply. Likewise, during shutdown, the capacitor is

discharged to GND. A DC shift across the capacitor

results, which in turn appears as an audible transient at

the speaker. Since the MAX9720 does not require out-

put-coupling capacitors, no audible transient occurs.

Additionally, the MAX9720 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 that there are minimal spectral components in

the audible range at the output upon startup and shut-

down.

In most applications, the preamplifier output driving the

MAX9720 has a DC bias of typically half the supply.

During startup, the input-coupling capacitor is charged

to the preamplifier’s DC bias voltage through the input

resistor of the MAX9720, resulting in a DC shift across

the capacitor and an audible click/pop. Delaying the

startup of the MAX9720 by 4 to 5 time constants (80ms

to 100ms) based on R

and C

, relative to the startup

of the preamplifier, eliminates this click/pop caused by

the input filter.

If the SmartSense routine occurs during normal opera-

tion, a low-level audible transient may be heard. To pre-

vent this, a host-controlled SmartSense routine should

only be executed when ALERT asserts.

50mW, DirectDrive, Stereo Headphone

Amplifier with SmartSense and Shutdown

14 ______________________________________________________________________________________

TIP

(SIGNAL)

SLEEVE

(GND)

Figure 6. Typical 2-Wire (Mono) Headphone Plug

OUTR

OUTL

MAX9720

HPS

100kΩ

Figure 7. HPS Configuration

Applications Information

Power Dissipation

Under 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 Ratings section under

Continuous Power Dissipation or can be calculated by

the following equation:

where T

J(MAX)

is +150°C, T

is the ambient temperature,

and θ

is the reciprocal of the derating factor in °C/W as

specified in the Absolute Maximum Ratings section. For

example, θ

of the TSSOP package is +106.38°C/W.

The MAX9720 has two power dissipation sources: the

charge pump and the two amplifiers. If the power dissipa-

tion for a given application exceeds the maximum

allowed for a given package, either reduce V

, increase

load impedance, decrease the ambient temperature, or

add heat sinking to the device. Large output traces

improve the maximum power dissipation in the package.

Thermal overload protection limits total power dissipa-

tion in the MAX9720. When the junction temperature

exceeds +140°C, the thermal protection circuitry dis-

ables the amplifier output stage. The amplifiers are

enabled once the junction temperature cools by 15°C,

resulting in a pulsing output under continuous thermal

overload conditions.

Output Power

The MAX9720 is specified for the worst-case condi-

tion—when both inputs are in phase. Under this condi-

tion, the amplifiers simultaneously draw current from

the charge pump, leading to a slight loss in headroom

of V

. In typical stereo audio applications, the left and

right signals present differences in both magnitude and

phase, subsequently leading to an increase in the max-

imum attainable output power. Figure 8 shows the two

extreme cases for in- and out-of-phase. In reality, the

available power lies between these extremes.

Powering Other Circuits from

a Negative Supply

An additional benefit of the MAX9720 is the internally

generated, negative supply voltage (PV

). PV

is the

negative supply for the MAX9720 headphone amplifiers.

can power other devices within a system. Limit the

current drawn from PV

to 5mA. Exceeding this affects

the operation of the headphone amplifiers. A typical

application is a negative supply to adjust the contrast of

LCD modules.

The charge-pump voltage at PV

is roughly propor-

tional to V

and is not a regulated voltage. Consider

the charge-pump output impedance when powering

other devices from PV

. See the Charge-Pump Output

Impedance graph in the Typical Operating

Characteristics. Use 2.2µF charge-pump capacitors for

the highest output power; 1µF or lower capacitors can

also be used for most applications. See the Output

Power vs. Load Resistance and Charge-Pump

Capacitance graph for details of the output power vs.

capacitor size.

Component Selection

Input Filtering

The input capacitor (C

), in conjunction with the

MAX9720 input impedance, forms a highpass filter that

removes the DC bias from an incoming signal (see

Typical Application Circuit). The AC-coupling capacitor

allows the amplifier to bias the signal to an optimum DC

level. Assuming zero-source impedance, the

-3dB point of the highpass filter is given by:

is the amplifier’s internal input impedance value

given in the Electrical Characteristics. Chose C

such

that f

-3dB

is well below the lowest frequency of interest.

Setting f

-3dB

too high affects the amplifier’s low-fre-

quency response. Use capacitors whose dielectrics

have low-voltage coefficients, such as tantalum or alu-

minum electrolytic. Capacitors with high-voltage coeffi-

cients, such as ceramics, may result in increased

distortion at low frequencies.

IN IN

−

DISSPKG MAX

J MAX A

()

−

MAX9720

50mW, DirectDrive, Stereo Headphone

Amplifier with SmartSense and Shutdown

______________________________________________________________________________________ 15

100

0.001

04020 80 120 160140

TOTAL HARMONIC DISTORTION PLUS

NOISE vs. OUTPUT POWER

0.01

0.1

OUTPUT POWER (mW)

THD+N (%)

60 100

= 3V

= -1V/V

f = 1kHz

= 16Ω

OUTPUTS

IN PHASE

SINGLE-

CHANNEL

OUTPUTS

OUT OF

PHASE

Figure 8. THD+N vs. Output Power with Inputs In-/Out-of-Phase

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21

MAX9720AEUE+

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MAX9720AEUE+

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