MAX9750CETI+T Datasheet MAX9750CETI+T, MAX9750CETI+, MAX9750AETI+T | P19-P21

MAX9750/MAX9751/MAX9755

2.6W Stereo Audio Power Amplifiers and

DirectDrive Headphone Amplifiers

______________________________________________________________________________________ 19

Headphone Amplifier

In conventional single-supply headphone amplifiers,

the output-coupling capacitor is a major contributor of

audible clicks and pops. Upon startup, the amplifier

charges the coupling capacitor to its bias voltage, typi-

cally 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 MAX9750/MAX9751/

MAX9755 do not require output-coupling capacitors, no

audible transient occurs.

Additionally, the MAX9750/MAX9751/MAX9755 features

extensive click-and-pop suppression that eliminates

any audible transient sources internal to the device.

The Turn-On Response (Headphone Mode) and Turn-

Off Response (Headphone Mode) graphs in the

Typical

Operating Characteristics

shows that there are minimal

transient components in the audible range at the output

upon startup and shutdown.

Figure 7a. Volume Control Transfer Function

-80

-60

-70

-40

-50

-20

-30

-10

0 1.0 1.50.5 2.0 2.5 3.0 3.5 4.0

MAX9750A

VOLUME CONTROL TRANSFER FUNCTION

VOL

(V)

GAIN (dB)

AUDIO

TAPER POT

GAIN1 = GAIN2 = 0

SPEAKER MODE

HEADPHONE MODE

Figure 7b. Volume Control Transfer Function

-80

-60

-70

-40

-50

-20

-30

-10

0 1.0 1.50.5 2.0 2.5 3.0 3.5 4.0

MAX9750B

VOLUME CONTROL TRANSFER FUNCTION

VOL

(V)

GAIN (dB)

AUDIO

TAPER POT

GAIN1 = GAIN2 = 0

SPEAKER MODE

HEADPHONE MODE

Figure 7c. Volume Control Transfer Function

-80

-60

-70

-40

-50

-20

-30

-10

0 1.0 1.50.5 2.0 2.5 3.0 3.5 4.0

MAX9750C

VOLUME CONTROL TRANSFER FUNCTION

VOL

(V)

GAIN (dB)

AUDIO

TAPER POT

GAIN1 = GAIN2 = 0

SPEAKER MODE

HEADPHONE MODE

MAX9750

47kΩ

BEEP

0.47μF

SOURCE 3

47kΩ

0.47μF

SOURCE 2

47kΩ

0.47μF

SOURCE 1

INT

47kΩ

BIAS

WINDOW

DETECTOR

(0.8V

P-P

THRESHOLD)

SPEAKER/HEADPHONE

AMPLIFER INPUTS

OUT(BEEP)

FREQUENCY

DETECTOR

(300Hz THRESHOLD)

Figure 8. BEEP Input

MAX9750/MAX9751/MAX9755

2.6W Stereo Audio Power Amplifiers and

DirectDrive Headphone Amplifiers

20 ______________________________________________________________________________________

Applications Information

BTL Speaker Amplifiers

The MAX9750/MAX9751/MAX9755 feature speaker

amplifiers designed to drive a load differentially, a con-

figuration referred to as bridge-tied load (BTL). The BTL

configuration (Figure 9) offers advantages over the sin-

gle-ended configuration, where one side of the load is

connected to ground. Driving the load differentially

doubles the output voltage compared to a single-

ended amplifier under similar conditions. Thus, the

device’s differential gain is twice the closed-loop gain

of the input amplifier. The effective gain is given by:

Substituting 2 x V

OUT(P-P)

into the following equation

yields four times the output power due to double the

output voltage:

Since the differential outputs are biased at midsupply,

there is no net DC voltage across the load. This elimi-

nates the need for DC-blocking capacitors required for

single-ended amplifiers. These capacitors can be large

and expensive, can consume board space, and can

degrade low-frequency performance.

Power Dissipation and Heat Sinking

Under normal operating conditions, the MAX9750/

MAX9751/MAX9755 can dissipate a significant amount

of power. The maximum power dissipation for each

package is given in the

Absolute Maximum Ratings

under Continuous Power Dissipation, or can be calcu-

lated by the following equation:

where T

J(MAX)

is +150°C, T

is the ambient tempera-

ture, 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 thin QFN package is

+42°C/W. For optimum power dissipation, the exposed

paddle of the package should be connected to the

ground plane (see the

Layout and Grounding

section).

Output Power (Speaker Amplifier)

The increase in power delivered by the BTL configura-

tion directly results in an increase in internal power dis-

sipation over the single-ended configuration. The

maximum power dissipation for a given V

and load is

given by the following equation:

If the power dissipation for a given application exceeds

the maximum allowed for a given package, either reduce

, increase load impedance, decrease the ambient

temperature, or add heatsinking to the device. Large

output, supply, and ground PC board traces improve the

maximum power dissipation in the package.

DISS MAX

()

DISSPKG MAX

J MAX A

()

−

RMS

OUT P P

OUT

RMS

−()

=×2

OUT(P-P)

2 x V

OUT(P-P)

-1

Figure 9. Bridge-Tied Load Configuration

OUTPUT POWER (mW)

THD+N (%)

125100755025

0.01

0.1

100

1000

0.001

0 150

= 5V

= 16Ω

= 3dB

OUTPUTS IN PHASE

OUTPUTS 180° OUT OF PHASE

Figure 10. Total Harmonic Distortion Plus Noise vs. Output Power

with Inputs In/Out of Phase (Headphone Mode)

MAX9750/MAX9751/MAX9755

2.6W Stereo Audio Power Amplifiers and

DirectDrive Headphone Amplifiers

______________________________________________________________________________________ 21

Thermal-overload protection limits total power dissipa-

tion in these devices. When the junction temperature

exceeds +160°C, the thermal-protection circuitry dis-

ables the amplifier output stage. The amplifiers are

enabled once the junction temperature cools by 15°C.

This results in a pulsing output under continuous ther-

mal-overload conditions as the device heats and cools.

Output Power (Headphone Amplifier)

The headphone amplifiers have been specified for the

worst-case scenario—when both inputs are in phase.

Under this condition, the drivers 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 have differences in both magni-

tude and phase, subsequently leading to an increase in

the maximum attainable output power. Figure 10 shows

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

the available power lies between these extremes.

Power Supplies

The MAX9750/MAX9751/MAX9755 have different sup-

plies for each portion of the device, allowing for the opti-

mum combination of headroom and power dissipation

and noise immunity. The speaker amplifiers are pow-

ered from PV

. PV

ranges from 4.5V to 5.5V. The

headphone amplifiers are powered from HPV

and

. HPV

is the positive supply of the headphone

amplifiers and ranges from 3V to 5.5V. V

is the nega-

tive supply of the headphone amplifiers. Connect V

CPV

. The charge pump is powered by CPV

CPV

ranges from 3V to 5.5V and should be the same

potential as HPV

. The charge pump inverts the volt-

age at CPV

, and the resulting voltage appears at

CPV

. The remainder of the device is powered by V

Component Selection

Input Filtering

The input capacitor (C

), in conjunction with the ampli-

fier input resistance (R

), forms a highpass filter that

removes the DC bias from an incoming signal (see the

Block Diagrams

). 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 resistance value

given in the

Electrical Characteristics

table. Choose C

such that f

-3dB

is well below the lowest frequency of

interest. Setting f

-3dB

too high affects the amplifier’s

low-frequency response. Use capacitors with low-volt-

age coefficient dielectrics, such as tantalum or alu-

minum electrolytic. Capacitors with high-voltage

coefficients, such as ceramics, may result in increased

distortion at low frequencies.

BIAS Capacitor

BIAS is the output of the internally generated DC bias

voltage. The BIAS bypass capacitor, C

BIAS

, improves

PSRR and THD+N by reducing power supply and other

noise sources at the common-mode bias node, and

also generates the clickless/popless, startup/shutdown

DC bias waveforms for the speaker amplifiers. Bypass

BIAS with a 1µF capacitor to GND.

Charge-Pump Capacitor Selection

Use capacitors with an ESR less than 100mΩ for opti-

mum performance. Low-ESR ceramic capacitors mini-

mize the output resistance of the charge pump. For

best performance over the extended temperature

range, select capacitors with an X7R dielectric. Table 4

lists suggested manufacturers.

Flying Capacitor (C1)

The value of the flying capacitor (C1) affects the load

regulation and output resistance of the charge pump. 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

resistance to an extent. See the Output Power vs.

Charge-Pump Capacitance and Load Resistance

graph in the

Typical Operating Characteristics

. Above

2.2µF, the on-resistance of the switches and the ESR of

C1 and C2 dominate.

Output Capacitor (C2)

The output capacitor value and ESR directly affect the

ripple at CPV

. Increasing the value of C2 reduces

output ripple. Likewise, decreasing the ESR of C2

IN IN

−

2π

SUPPLIER PHONE FAX WEBSITE

Taiyo Yuden 800-348-2496 847-925-0899 www.t-yuden.com

TDK 807-803-6100 847-390-4405 www.component.tdk.com

Table 4. Suggested Capacitor Manufacturers

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P24 P25-P27 P28-P30

MAX9750CETI+T

Mfr. #:

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Maxim Integrated

Description:

Audio Amplifiers Integrated Circuits (ICs)

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MAX9750CETI+T

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