Datasheet

MAX9788
Hold Capacitor (C2)
The output capacitor value and ESR directly affect the
ripple at PV
SS
. Increasing C2 reduces output ripple.
Likewise, decreasing the ESR of C2 reduces both rip-
ple and output resistance. A 10µF capacitor is recom-
mended.
Charge-Pump Frequency Set Resistor (R
FS
)
The charge pump operates in two modes. When the
charge pump is loaded below 100mA, it operates in a
slow mode where the oscillation frequency is reduced to
1/4 of its normal operating frequency. Once loaded, the
charge-pump oscillation frequency returns to normal
operation. In applications where the design may be sen-
sitive to the operating charge-pump oscillation frequen-
cy, the value of the external resistor R
FS
can be changed
to adjust the charge-pump oscillation frequency shown
in Figure 4. A 100kΩ resistor is recommended.
Ceramic Speaker Impedance
Characteristics
A 1µF capacitor is a good model for the ceramic
speaker as it best approximates the impedance of a
ceramic speaker over the audio band. When selecting
a capacitor to simulate a ceramic speaker, the voltage
rating or the capacitor must be equal to or higher than
the expected output voltage swing. See Figure 5.
Series Load Resistor
The capacitive nature of the ceramic speaker results in
very low impedances at high frequencies. To prevent
the ceramic speaker from shorting the MAX9788 output
at high frequencies, a series load resistor must be
used. The output load resistor and the ceramic speaker
create a lowpass filter. To set the rolloff frequency of
the output filter, the approximate capacitance of the
speaker must be known. This information can be
obtained from bench testing or from the ceramic
speaker manufacturer. A series load resistor greater
than 10Ω is recommended. Set the lowpass filter cutoff
frequency with the following equation:
WLP Applications Information
For the latest application details on WLP construction,
dimensions, tape carrier information, PCB techniques,
bump-pad layout, and recommended reflow tempera-
ture profile, as well as the latest information on reliability
testing results, go to the Maxim website at www.maxim-
ic.com/ucsp for the application note, UCSP—A Wafer-
Level Chip-Scale Package .
f
RC
Hz
LP
L SPEAKER
=
××
()
1
2π
14V
P-P
, Class G Ceramic Speaker Driver
10 ______________________________________________________________________________________
CHARGE-PUMP OSCILLATION
FREQUENCY vs. R
FS
MAX9788 fig04
R
FS
(kΩ)
CHARGE-PUMP OSCILLATION FREQUENCY (kHz)
12510075
250
300
350
400
450
500
550
600
200
50 150
I
LOAD
> 100mA
IMPEDANCE vs. FREQUENCY
MAX9788 fig05
FREQUENCY (Hz)
IMPEDANCE (Ω)
1010.10.01
100
1k
10k
100k
1M
10
0.001 100
1μF CAPACITOR
CERAMIC
SPEAKER
Figure 4. Charge-Pump Oscillation Frequency vs. R
FS
Figure 5. Ceramic Speaker and Capacitor Impedance