Rainbow Electronics MAX9770 User Manual

Page 17

The MAX9770 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, and more efficient solution.

Because the frequency of the MAX9770 output is well
beyond the bandwidth of most speakers, voice coil
movement due to the square-wave frequency is mini-
mal. Although this movement is small, a speaker not
designed to handle the additional power may be dam-
aged. For optimum results, use a speaker with a series
inductance >10µH. Typical small 8

Ω speakers exhibit

series inductances in the range of 20µH to 100µH.

Output Offset

Unlike Class AB amplifiers, the output offset voltage of a
Class D amplifier does not noticeably increase quiescent
current draw when a load is applied. This is due to the
power conversion of the Class D amplifier. For example, a
15mV DC offset across an 8

Ω load results in 1.9mA extra

current consumption in a Class AB device. In the Class D
case, a 15mV offset into 8

Ω equates to an additional

power drain of 28µW. Due to the high efficiency of the
Class D amplifier, this represents an additional quiescent
current draw of 28µW/(V

/ 100 x

η), which is on the

order of a few microamps.

Power Supplies

The MAX9770 has different supplies for each portion of
the device, allowing for the optimum combination of
headroom and power dissipation and noise immunity.
The speaker amplifiers are powered from PV

. PV

ranges from 2.5V to 5.5V. The headphone amplifiers
are powered from V

and SV

. V

is the positive

supply of the headphone amplifiers and ranges from
2.5V to 5.5V. SV

is the negative supply of the head-

phone amplifiers. Connect SV

to CPV

. The charge

pump is powered by CPV

. CPV

ranges from 2.5V

to 5.5V and should be the same potential as V

. The

charge pump inverts the voltage at CPV

, and the

resulting voltage appears at CPV

. The remainder of

the device is powered by V

Component Selection

Input Filter

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
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 resistance value

given in the Electrical Characteristics. Be aware that
the MONO input has a higher input impedance than the
other inputs. Choose C

such that f

-3dB

is below the

lowest frequency of interest. Setting f

-3dB

too high

affects the amplifier’s low-frequency response. Setting
f

-3dB

too low can affect the click-and-pop performance.

Use capacitors with low-voltage coefficient dielectrics,
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 MAX9770 speaker amplifier does not require an out-
put filter for normal operation and audio reproduction. The
device passes FCC Class B radiated emissions stan-
dards with 100mm of unshielded speaker cables.
However, output filtering can be used if a design is failing
radiated emissions due to board layout or cable length,
or if the circuit is near EMI-sensitive devices. Use a com-
mon-mode choke connected in series with the speaker
outputs if board space is limited and emissions are a
concern. Use of an LC filter is necessary if excessive
speaker cable is used.

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 0.047µF capacitor to GND. Large values of
C

BIAS

result in poor click/pop performance, and small-

er values of C

BIAS

result in degradation of PSRR and

increased output noise.

R C

IN IN

−

MAX9770

1.2W Low-EMI, Filterless, Mono Class D Amplifier

with Stereo DirectDrive Headphone Amplifiers

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