MAX9709ETN+D Datasheet MAX9709ETN+D, MAX9709ETN+TD | P13-P15

MAX9709

25W/50W, Filterless, Spread-Spectrum,

Stereo/Mono, Class D Amplifier

______________________________________________________________________________________ 13

TEMP returns high once the junction temperature cools

below the set threshold minus the thermal hysteresis. If

TEMP is connected to either MUTE or SS, the audio

output resumes. The temperature threshold is set by

the TH0, TH1, and TH2 inputs as shown in Table 1. An

RC filter may be used to eliminate any transient at the

TEMP output as shown in Figure 3.

If TH2 = TH1 = TH0 = HIGH, it is likely that the MAX9709

enters thermal shutdown without tripping the thermal

flag.

Gain Selection

The MAX9709 features four pin-selectable gain settings;

see Table 2.

Operating Modes

Fixed-Frequency Modulation (FFM) Mode

The MAX9709 features three switching frequencies in

the FFM mode (Table 3). In this mode, the frequency

spectrum of the Class D output consists of the funda-

mental switching frequency and its associated harmon-

ics (see the Wideband Output Spectrum graph in the

Typical Operating Characteristics

). Select one of the

three fixed switching frequencies such that the harmon-

ics do not fall in a sensitive band. The switching fre-

quency can be changed any time without affecting

audio reproduction.

Spread-Spectrum Modulation (SSM) Mode

The MAX9709 features a unique spread-spectrum

(SSM) mode that flattens the wideband spectral com-

ponents, improving EMI emissions that may be radiated

by the speaker and cables. This mode is enabled by

setting FS1 = FS2 = high. In SSM mode, the switching

frequency varies randomly by ±4% around the center

frequency (200kHz). The modulation scheme remains

the same, but the period of the triangle waveform

changes from cycle to cycle. Instead of a large amount

of spectral energy present at multiples of the switching

frequency, the energy is now spread over a bandwidth

that increases with frequency. Above a few megahertz,

the wideband spectrum looks like white noise for EMI

purposes. SSM mode reduces EMI compared to fixed-

frequency mode. This can also help to randomize visu-

al artifacts caused by radiated or supply borne

interference in displays.

Synchronous Switching Mode

The MAX9709 SYNCIN input allows the Class D amplifi-

er to switch at a frequency defined by an external clock

frequency. Synchronizing the amplifier with an external

clock source may confine the switching frequency to a

less sensitive band. The external clock frequency range

is from 600kHz to 1.2MHz and can have any duty cycle,

but the minimum pulse must be greater than 100ns.

SYNCOUT is an open-drain clock output for synchro-

nizing external circuitry. Its frequency is four times the

amplifier’s switching frequency and it is active in either

internal or external oscillator mode.

Figure 3. An RC Filter Eliminates Transient During Switching

Table 1. MAX9709 Junction Temperature

Threshold Setting

TEMP

0.1μF

10kΩ

DIGITAL

TO DIGITAL

INPUT

JUNCTION

TEMPERATURE

(°C)

TH2 TH1 TH0

80 Low Low Low

90 Low Low High

100 Low High Low

110 Low High High

120 High Low Low

129 High Low High

139 High High Low

158 High High High

Table 2. MAX9709 Gain Setting

G1 G2 GAIN (dB)

Low High 22

High High 25

High Low 29.5

Low Low 36

Table 3. Switching Frequencies

FS1

FS2

SYNCOUT

FREQUENCY (kHz)

MODULATION

0 0 200 Fixed-frequency

0 1 250 Fixed-frequency

1 0 160 Fixed-frequency

1 1 200 ±4 Spread-spectrum

MAX9709

25W/50W, Filterless, Spread-Spectrum,

Stereo/Mono, Class D Amplifier

14 ______________________________________________________________________________________

Linear Regulator (REG)

The supply voltage range for the MAX9709 is from 10V

to 22V to achieve high-output power. An internal linear

regulator reduces this voltage to 6.3V for use with

small-signal and digital circuitry that does not require

high-voltage supply. Bypass a 0.01µF capacitor from

REG to GND.

Applications Information

Logic Inputs

All of the digital logic inputs and output have an

absolute maximum rating of +12V. If the MAX9709 is

operating with a supply voltage between 10V and 12V,

digital inputs can be connected to PV

or V

. If

and V

are greater than 12V, digital inputs and

outputs must be connected to a digital system supply

lower than 12V.

Input Amplifier

Differential Input

The MAX9709 features a differential input structure,

making them compatible with many CODECs, and

offering improved noise immunity over a single-ended

input amplifier. In devices such as flat-panel displays,

noisy digital signals can be picked up by the amplifier’s

inputs. These signals appear at the amplifiers’ inputs as

common-mode noise. A differential input amplifier

amplifies only the difference of the two inputs, while any

signal common to both inputs is attenuated.

Single-Ended Input

The MAX9709 can be configured as a single-ended

input amplifier by capacitively coupling either input to

GND and driving the other input (Figure 4).

Component Selection

Input Filter

An input capacitor, C

, in conjunction with the input

impedance of the MAX9709, forms a highpass filter that

removes the DC bias from an incoming signal. The AC-

coupling capacitor allows the amplifier to bias the signal

to an optimum DC level. Assuming zero-source imped-

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

Choose C

so that f

-3dB

is well below the lowest fre-

quency of interest. Setting f

-3dB

too high affects the

low-frequency response of the amplifier. Use capaci-

tors with dielectrics that have low-voltage coefficients,

such as tantalum or aluminum electrolytic. Capacitors

with high-voltage coefficients, such as ceramics, may

result in increased distortion at low frequencies.

Output Filter

The MAX9709 does not require an output filter.

However, output filtering can be used if a design is fail-

ing radiated emissions due to board layout or cable

length, or the circuit is near EMI-sensitive devices. See

the MAX9709 evaluation kit for suggested filter topolo-

gies. The tuning and component selection of the filter

should be optimized for the load. A purely resistive load

(8Ω) used for lab testing requires different components

than a real, complex load-speaker load.

Charge-Pump Capacitor Selection

The MAX9709 has an internal charge-pump converter

that produces a voltage level for internal circuitry. It

requires a flying capacitor (C1) and a holding capacitor

(C2). Use capacitors with an ESR less than 100mΩ for

optimum performance. Low-ESR ceramic capacitors

minimize the output resistance of the charge pump. For

best performance over the extended temperature

range, select capacitors with an X7R dielectric. The

capacitors’ voltage rating must be greater than 36V.

IN IN

−

Figure 4. Single-Ended Input Connections

INR+

INR-

MAX9709

1μF

MAX9709

25W/50W, Filterless, Spread-Spectrum,

Stereo/Mono, Class D Amplifier

______________________________________________________________________________________ 15

Sharing Input Sources

In certain systems, a single audio source can be

shared by multiple devices (speaker and headphone

amplifiers). When sharing inputs, it is common to mute

the unused device, rather than completely shutting it

down. This prevents the unused device inputs from dis-

torting the input signal. Mute the MAX9709 by driving

MUTE low. Driving MUTE low turns off the Class D out-

put stage, but does not affect the input bias levels of

the MAX9709.

Frequency Synchronization

The MAX9709 outputs up to 27W on each channel in

stereo mode. If higher output power or a 2.1 solution is

needed, two MAX9709s can be used. Each MAX9709

is synchronized by connecting SYNCOUT from the first

MAX9709 to SYNCIN of the second MAX9709 (see

Figure 5).

Supply Bypassing/Layout

Proper power-supply bypassing ensures low distortion

operation. For optimum performance, bypass PV

PGND with a 0.1µF capacitor as close to each PV

pin as possible. A low-impedance, high-current power-

supply connection to PV

is assumed. Additional bulk

capacitance should be added, as required, depending

on the application and power-supply characteristics.

GND and PGND should be star-connected to system

ground. For the TQFN package, solder the exposed

paddle (EP) to the ground plane using multiple-plated

through-hole vias. The exposed paddle must be sol-

dered to the ground plane for rated power dissipation

and good ground return. Use wider PC board traces to

lower the parasitic resistance for the high-power output

pins (OUTR+, OUTR-, OUTL+, OUTL-). Refer to the

MAX9709 evaluation kit for layout guidance.

Thermal Considerations

Class D amplifiers provide much better efficiency and

thermal performance than a comparable Class AB

amplifier. However, the system’s thermal performance

must be considered with realistic expectations along

with its many parameters.

Continuous Sine Wave vs. Music

When a Class D amplifier is evaluated in the lab, often

a continuous sine wave is used as the signal source.

While this is convenient for measurement purposes, it

represents a worst-case scenario for thermal loading

on the amplifier. It is not uncommon for a Class D

amplifier to enter thermal shutdown if driven near maxi-

mum output power with a continuous sine wave. The

PC board must be optimized for best dissipation (see

the

PC Board Thermal Considerations

section).

Audio content, both music and voice, has a much lower

RMS value relative to its peak output power. Therefore,

while an audio signal may reach similar peaks as a

continuous sine wave, the actual thermal impact on the

Class D amplifier is highly reduced. If the thermal per-

formance of a system is being evaluated, it is important

to use actual audio signals instead of sine waves for

testing. If sine waves must be used, the thermal perfor-

mance is less than the system’s actual capability for

real music or voice.

PC Board Thermal Considerations

The exposed pad is the primary route for conducting

heat away from the IC. With a bottom-side exposed

pad, the PC board and its copper becomes the primary

heatsink for the Class D amplifier. Solder the exposed

pad to a copper polygon. Add as much copper as pos-

sible from this polygon to any adjacent pin on the Class

D amplifier as well as to any adjacent components, pro-

vided these connections are at the same potential.

These copper paths must be as wide as possible. Each

of these paths contributes to the overall thermal capa-

bilities of the system.

The copper polygon to which the exposed pad is

attached should have multiple vias to the opposite side

of the PC board, where they connect to another copper

polygon. Make this polygon as large as possible within

the system’s constraints for signal routing.

Additional improvements are possible if all the traces

from the device are made as wide as possible.

Although the IC pins are not the primary thermal path

out of the package, they do provide a small amount.

The total improvement would not exceed about 10%,

but it could make the difference between acceptable

performance and thermal problems.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P20

MAX9709ETN+D

Mfr. #:

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Manufacturer:

Maxim Integrated

Description:

Audio Amplifiers 20W/40W Filterless Class D Amplifier

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MAX9709ETN+D

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