MAX6909 MAXIM [Maxim Integrated Products], MAX6909 Datasheet - Page 24

• Manufacturer Part Number: MAX6909
• Description: I2C-Compatible Real-Time Clocks with uP Supervisor and NV RAM Controller
• Manufacturer: MAXIM [Maxim Integrated Products]

The push-pull, piezo transducer drive output, PZT, is
selectable through the configuration register for fre-
quencies of 1.024kHz, 2.048kHz, 4.096kHz, or
8.192kHz (Table 6). Bits D0 and D1 control which fre-
quency outputs to PZT. If in battery backup mode
(when V
V
to prevent battery drain from the backup battery on the
BATT pin.
Table 7 lists the piezo transducer control bits.
Bit D3, the PZT SEL bit, selects whether the ALM, alarm
output, controls when the selected PZT frequency is
gated to PZT or whether control is given to the PZT
CNTL bit, bit D2. If D3 = 1, then the ALM controls gat-
ing of the selected PZT frequency to PZT. When the
alarm is triggered, the selected frequency stays on PZT
until the alarm is cleared by writing to or reading from
the alarm configuration register. If D3 = 0, then the PZT
CNTL bit, D2, determines when and for how long the
selected frequency appears at PZT. Bit D2, the PZT
CNTL bit, controls whether the selected frequency is
gated to PZT, provided D3 = 0. D2 = 1 gates the
selected frequency to PZT and D2 = 0 inhibits the
selected frequency (PZT remains low).
I
Supervisor and NV RAM Controller
Table 6. Piezo Transducer Selectable
Frequencies
Table 7. Piezo Transducer Control Bits
24
BATT
2
(PZT SEL)
(PZT FREQ)
C-Compatible Real-Time Clocks with µP
______________________________________________________________________________________
D3
0
0
1
1
), the PZT output is disabled to high impedance
D1
CC
0
0
1
1
falls below the reset threshold and below
(PZT CNTL)
Piezo Transducer Output Drive
D2
0
1
0
1
(PZT FREQ)
D0
0
1
0
1
PZT CNTL bit, D2, has control
PZT CNTL bit, D2, has control
ALM has control, D2 is ignored; assume alarm triggered
ALM has control, D2 is ignored; assume alarm cleared by reading the
alarm configuration register
FREQUENCY (kHz)
PZT TYPICAL
1.024
2.048
4.096
8.19
CONDITION
Anytime a frequency is selected to be gated through to
the PZT output, it is modulated by a 1Hz square wave.
The PZT output then turns on for 0.5s and off for 0.5s.
Since the human ear is particularly sensitive to changes
in condition, switching a sound on and off makes it
more noticeable than a continuous sound of the same
frequency.
The PZT output swings between V
the output stage’s on-resistance, R
ibility of the PZT output to work with many different types
of piezo buzzers, R
practical. To minimize peak currents into the piezo
buzzer, an external current-limiting resistor Rs may
be required. Ipeak is now equal to V
R
amplitude from the external piezo buzzer. The value of Rs
varies for each application and should be chosen at the
prototype design stage with the piezo buzzer installed
in a cavity approximating its final housing. The typical
value of R
I
Figure 16 is the piezo transducer functional diagram.
Figure 16. Piezo Transducer Functional Diagram
PZT
OUT_PZT
is the average of the sink and source currents.
OUT_PZT
). Rs can be adjusted to reduce the sound
MAX6909/
MAX6910
V
PZT
R
OUT_PZT
is calculated from V
OUT_PZT
PZT
is designed to be as low as
Rs
OUT_PZT
CC
Low
Selected frequency
Selected frequency
Low
and GND through
OUT
PIEZO
BUZZER
. To allow flex-
PZT
/ I
CC
PZT
/ (Rs +
, where