MAX9875ERP+T Datasheet MAX9875ERP+T | P25-P27

MAX9875

Low RF Susceptibility, Mono Audio

Subsystem with DirectDrive Headphone Amplifier

______________________________________________________________________________________ 25

ACKNOWLEDGE FROM MAX9875

1 BYTE

AUTOINCREMENT INTERNAL

ACKNOWLEDGE FROM MAX9875

NOT ACKNOWLEDGE FROM MASTER

ACKNOWLEDGE FROM MAX9875

R/W

R/WREPEATED START

Sr 1SLAVE ADDRESS REGISTER ADDRESS SLAVE ADDRESS DATA BYTE

Figure 11. Reading One Indexed Byte of Data from the MAX9875

1 BYTE

AUTOINCREMENT INTERNAL

ACKNOWLEDGE FROM MAX9875

B1 B0B3 B2B5 B4B7 B6

ACKNOWLEDGE FROM MAX9875

R/W

1 BYTE

ACKNOWLEDGE FROM MAX9875

B1 B0B3 B2B5 B4B7 B6

SLAVE ADDRESS

DATA BYTE 1

DATA BYTE n

Figure 10. Writing n-Bytes of Data to the MAX9875

The second byte transmitted from the master config-

ures the MAX9875’s internal register address pointer.

The pointer tells the MAX9875 where to write the next

byte of data. An acknowledge pulse is sent by the

MAX9875 upon receipt of the address pointer data.

The third byte sent to the MAX9875 contains the data

that will be written to the chosen register. An acknowl-

edge pulse from the MAX9875 signals receipt of the

data byte. The address pointer autoincrements to the

next register address after each received data byte.

This autoincrement feature allows a master to write to

sequential registers within one continuous frame. Figure

10 illustrates how to write to multiple registers with one

frame. The master signals the end of transmission by

issuing a STOP condition.

not write to these addresses.

Read Data Format

Send the slave address with the R/W bit set to 1 to initiate

a read operation. The MAX9875 acknowledges receipt of

its slave address by pulling SDA low during the 9th SCL

clock pulse. A START command followed by a read

command resets the address pointer to register 0x00.

The first byte transmitted from the MAX9875 will be the

contents of register 0x00. Transmitted data is valid on the

rising edge of SCL. The address pointer autoincrements

after each read data byte. This autoincrement feature

allows all registers to be read sequentially within one

continuous frame. A STOP condition can be issued after

any number of read data bytes. If a STOP condition is

issued followed by another read operation, the first data

byte to be read will be from register 0x00.

The address pointer can be preset to a specific register

before a read command is issued. The master presets

the address pointer by first sending the MAX9875‘s

slave address with the R/W bit set to 0 followed by the

sent followed by the slave address with the R/W bit set

to 1. The MAX9875 then transmits the contents of the

specified register. The address pointer autoincrements

after transmitting the first byte. The master acknowl-

edges receipt of each read byte during the acknowl-

edge clock pulse. The master must acknowledge all

correctly received bytes except the last byte. The final

byte must be followed by a not acknowledge from the

master and then a STOP condition. Figure 11 illustrates

the frame format for reading one byte from the

MAX9875. Figure 12 illustrates the frame format for

reading multiple bytes from the MAX9875.

MAX9875

Low RF Susceptibility, Mono Audio

Subsystem with DirectDrive Headphone Amplifier

26 ______________________________________________________________________________________

Applications Information

Filterless Class D Operation

Traditional Class D amplifiers require an output filter to

recover the audio signal from the amplifier’s output. The

filters add cost, increase the solution size of the amplifier,

and can decrease efficiency and THD+N performance.

The traditional PWM scheme uses large differential out-

put swings (2 x V

DD(P-P)

) and causes large ripple cur-

rents. Any parasitic resistance in the filter components

results in a loss of power, lowering the efficiency.

The MAX9875 does not require an output filter. The

device relies on the inherent inductance of the speaker

coil and the natural filtering of both the speaker and the

human ear to recover the audio component of the

square-wave output. Eliminating the output filter results

in a smaller, less costly, more efficient solution.

Because the frequency of the MAX9875 output is well

beyond the bandwidth of most speakers, voice coil

movement due to the square-wave frequency is very

small. Although this movement is small, a speaker not

designed to handle the additional power can be dam-

aged. For optimum results, use a speaker with a series

inductance > 10µH. Typical 8Ω speakers exhibit series

inductances in the 20µH to 100µH range.

Component Selection

Optional Ferrite Bead Filter

In applications where speaker leads exceed 20mm,

additional EMI suppression can be achieved by using a

filter constructed from a ferrite bead and a capacitor to

ground. A ferrite bead with low DC resistance, high-

frequency (> 1.176MHz) impedance of 100Ω to 600Ω,

and rated for at least 1A should be used. The capacitor

value varies based on the ferrite bead chosen and the

actual speaker lead length. Select a capacitor less than

1nF based on EMI performance.

Input Capacitor

An input capacitor, C

, in conjunction with the input

impedance of the MAX9875 forms a highpass filter that

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

coupling capacitor allows the amplifier to automatically

bias the signal to an optimum DC level. Assuming zero

source impedance, 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. 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.

BIAS Capacitor

BIAS is the output of the internally generated DC bias volt-

age. The BIAS bypass capacitor, C

BIAS

, reduces power

supply and other noise sources at the common-mode

bias node. Bypass BIAS with a 1µF capacitor to GND.

Charge-Pump Capacitor Selection

Use capacitors with an ESR less than 100mΩ for optimum

performance. Low-ESR ceramic capacitors minimize the

output resistance of the charge pump. Most surface-

mount ceramic capacitors satisfy the ESR requirement.

For best performance over the extended temperature

range, select capacitors with an X7R dielectric.

Flying Capacitor (C1)

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

IN IN

−

2π

MAX9875

OUT+

OUT-

Figure 13. Optional Ferrite Bead Filter

ACKNOWLEDGE FROM MAX9875

1 BYTE

AUTOINCREMENT INTERNAL

ACKNOWLEDGE FROM MAX9875

R/W

REPEATED START

Sr 1SLAVE ADDRESS REGISTER ADDRESS SLAVE ADDRESS DATA BYTE

Figure 12. Reading n-Bytes of Indexed Data from the MAX9875

MAX9875

Low RF Susceptibility, Mono Audio

Subsystem with DirectDrive Headphone Amplifier

______________________________________________________________________________________ 27

Increasing the value of C1 reduces the charge-pump out-

put resistance to an extent. Above 1µF, the on-resistance

of the switches and the ESR of C1 and C2 dominate.

Output Holding Capacitor (C2)

The output capacitor value and ESR directly affect the

ripple at V

. Increasing the value of C2 reduces output

ripple. Likewise, decreasing the ESR of C2 reduces both

ripple and output resistance. Lower capacitance values

can be used in systems with low maximum output power

levels. See the Output Power vs. Load Resistance graph

in the

Typical Operating Characteristics

PVDD Bulk Capacitor (C3)

In addition to the recommended PVDD bypass capaci-

tance, bulk capacitance equal to C3 should be used.

Place the bulk capacitor as close to the device as possible.

Supply Bypassing,

Layout, and Grounding

Proper layout and grounding are essential for optimum

performance. Use wide traces for the power-supply

inputs and amplifier outputs to minimize losses due to

parasitic trace resistance. Wide traces also aid in mov-

ing heat away from the package. Proper grounding

improves audio performance, minimizes crosstalk

between channels, and prevents any switching noise

from coupling into the audio signal. Connect PGND and

GND together at a single point on the PCB. Route all

traces that carry switching transients away from GND

and the traces/components in the audio signal path.

Connect PVDD to a 2.7V to 5.25V source. Bypass

PVDD to the PGND pin with a 1µF ceramic capacitor.

Additional bulk capacitance should be used to prevent

power-supply pumping. Place the bypass capacitors

as close to the MAX9875 as possible.

Connect V

to PVDD. Bypass V

to GND with a 1µF

capacitor. Place the bypass capacitors as close to the

MAX9875 as possible.

RF Susceptibility

GSM radios transmit using time-division multiple

access (TDMA) with 217Hz intervals. The result is an

RF signal with strong amplitude modulation at 217Hz

that is easily demodulated by audio amplifiers. Figure

14 shows the susceptibility of the MAX9875 to a trans-

mitting GSM radio placed in close proximity. Although

there is measurable noise at 217Hz and its harmonics,

the noise is well below the threshold of hearing using

typical headphones.

In RF applications, improvements to both layout and

component selection decreases the MAX9875’s sus-

ceptibility to RF noise and prevent RF signals from

being demodulated into audible noise. Trace lengths

should be kept below

the wavelength of the RF fre-

quency of interest. Minimizing the trace lengths pre-

vents them from functioning as antennas and coupling

RF signals into the MAX9875. The wavelength λ in

meters is given by:

λ = c/f

where c = 3 x 10

m/s, and f = the RF frequency of inter-

est.

Route audio signals on middle layers of the PCB to

allow ground planes above and below shield them from

RF interference. Ideally the top and bottom layers of the

PCB should primarily be ground planes to create effec-

tive shielding.

Additional RF immunity can also be obtained from rely-

ing on the self-resonant frequency of capacitors as it

exhibits the frequency response similar to a notch filter.

Depending on the manufacturer, 10pF to 20pF capaci-

tors typically exhibit self resonance at RF frequencies.

These capacitors, when placed at the input pins, can

effectively shunt the RF noise at the inputs of the

MAX9875. For these capacitors to be effective, they

must have a low-impedance, low-inductance path to

the ground plane. Do not use microvias to connect to

the ground plane as these vias do not conduct well at

RF frequencies.

RF SUSCEPTIBILITY

MAX9875 fig14

FREQUENCY (Hz)

EFFICIENCY (dBµ)

10k1k100

-130

-110

-90

-70

-50

-30

-10

-150

10 100k

THRESHOLD OF HEARING

MAX9875

NOISE FLOOR

Figure 14. MAX9875 Susceptibility to a GSM Cell Phone Radio

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

MAX9875ERP+T

Mfr. #:

Buy MAX9875ERP+T

Manufacturer:

Maxim Integrated

Description:

Audio Amplifiers Audio Subsystem w/DirectDrive

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MAX9875ERP+T