max9788 Maxim Integrated Products, Inc., max9788 Datasheet - Page 9

• Manufacturer Part Number: max9788
• Description: Max9788 14vp-p, Class G Ceramic Speaker Driver
• Manufacturer: Maxim Integrated Products, Inc.

Applications that require thin cases, such as today’s
mobile phones, demand that external components
have a small form factor. Dynamic loudspeakers that
use a cone and voice coil typically cannot conform to
the height requirements. The option for these applica-
tions is to use a ceramic/piezoelectric loudspeaker.
Ceramic speakers are much more capacitive than a con-
ventional loudspeaker. Typical capacitance values for
such a speaker can be greater than 1µF. High peak-to-
peak voltage drive is required to achieve acceptable
sound pressure levels. The high output voltage require-
ment coupled with the capacitive nature of the speaker
demand that the amplifier supply much more current at
high frequencies than at lower frequencies. Above 10kHz,
the typical speaker impedance can be less than 16Ω.
The MAX9788 is ideal for driving a capacitive ceramic
speaker. The high charge-pump current limit allows for a
flat frequency response out to 20kHz while maintaining
high output voltage swings. See the Frequency Response
graph in the Typical Operating Characteristics . Figure 3
shows a typical circuit for driving a ceramic speaker.
A 10Ω series resistance is recommended between the
amplifier output and the ceramic speaker load to ensure
the output of the amplifier sees some fixed resistance at
high frequencies when the speaker is essentially an
electrical short.
Figure 3. Driving a Ceramic Speaker
MAX9788
CLASS G
OUTPUT
STAGE
_______________________________________________________________________________________
14V
Driving a Ceramic Speaker
OUT+
OUT-
P-P
R
, Class G Ceramic Speaker Driver
L
The AC-coupling capacitors (C
(R
from an input signal (see the Functional Diagram/
Typical Operating Circuit ). C
from the amplifier input. The -3dB point of the highpass
filter, assuming zero source impedance due to the
input signal source, is given by:
Ceramic speakers generally perform best at frequen-
cies greater than 1kHz. Low frequencies can deflect
the piezoelectric speaker element so that high frequen-
cies cannot be properly reproduced. This can cause
distortion in the speaker’s usable frequency band.
Select a C
quency response of the ceramic speaker. Use capaci-
tors with low-voltage coefficient dielectrics. Aluminum
electrolytic, tantalum, or film dielectric capacitors are
good choices for AC-coupling capacitors. Capacitors
with high-voltage coefficients, such as ceramics (non-
C0G dielectrics), can result in increased distortion at
low frequencies.
Use capacitors with an ESR less than 50mΩ 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.
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. Increasing the value
of C1 improves load regulation and reduces the charge-
pump output resistance to an extent. Above 1µF, the on-
resistance of the switches and the ESR of C1 and C2
dominate. A 4.7µF capacitor is recommended.
IN_
) form highpass filters that remove any DC bias
IN
f
so the f
−
3
dB
=
Charge-Pump Capacitor Selection
2π
-3dB
×
R
closely matches the low fre-
IN
Component Selection
Input-Coupling Capacitor
1
_
IN_
×
IN_
C
Flying Capacitor (C1)
IN
blocks DC voltages
) and input resistors
_
( )
Hz
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