MAX7348AEP-T Maxim Integrated, MAX7348AEP-T Datasheet - Page 24

• Manufacturer Part Number: MAX7348AEP-T
• Description: Peripheral Drivers & Components - PCIs Low-EMI Key Switch & Sounder Controller
• Manufacturer: Maxim Integrated

The sounder output is designed to drive a standard, low-
cost piezo transducer directly without further buffering.
Piezo transducers appear as a capacitive load of typical-
ly 10nF. If a resistive or inductive sounder is used, such
as a small loudspeaker, fit a coupling capacitor between
the sounder output and the transducer. For example, if a
32Ω speaker is used, connect the positive side of a 22µF
electrolytic capacitor to the sounder output, the negative
side of the capacitor to one end of the speaker, and the
other end of the speaker to GND.
2-Wire Interfaced Low-EMI Key Switch
and Sounder Controllers
Figure 10. Ghost-Key Phenomenon
Figure 12. Third-Order Chebyshev Lowpass Filter and Output Stage
24
______________________________________________________________________________________
SOUNDER
OUTPUT
KEY-SWITCH MATRIX
REGULAR KEY-PRESS
0.22μF
EVENT
2.61kΩ
68nF
Audio Transducers
21.5kΩ
22nF
GHOST-KEY
EVENT
32.4kΩ
SHDN
BIAS
IN+
IN-
SHUTDOWN
The sounder output can also drive a power amplifier for
higher sound levels. In this case, it is usually desirable
to include a lowpass filter before the speaker to convert
the square-wave tones to something closer to a sinu-
soid. The recommended cutoff frequency of this filter is
around 3kHz. An example circuit is shown in Figure 12,
which uses the uncommitted op amp of the MAX4366
bridge power amplifier to implement a third-order
Chebyshev lowpass filter.
Figure 11. Valid Three-Key Combinations
PROCESS: BiCMOS
50kΩ
50kΩ
21.5kΩ
KEY-SWITCH MATRIX
V
CC
EXAMPLES OF VALID THREE-KEY COMBINATIONS
220pF
10kΩ
MAX4366
10kΩ
Chip Information
OUT-
OUT+
KEY-SWITCH MATRIX
16Ω