LM1972M/NOPB National Semiconductor, LM1972M/NOPB Datasheet - Page 7

• Manufacturer Part Number: LM1972M/NOPB
• Description: IC AUDIO ATTENUATR W/MUTE 20SOIC
• Manufacturer: National Semiconductor
• Type: Audio Attenuatorr

Specifications of LM1972M/NOPB

• Applications: Consoles, MIDI
• Mounting Type: Surface Mount
• Package / Case: 20-SOIC (7.5mm Width)
• Attenuator Step Size: 0.5/1dB
• Number Of Bits: 16
• Number Of Channels: 2
• Power Dissipation: 150mW
• Supply Current: 4mA
• Operating Temperature (min): 0C
• Operating Temperature (max): 70C
• Operating Temperature Classification: Commercial
• Package Type: SOIC W
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM1972M; *LM1972M/NOPB; LM1972M

MUTE FUNCTION
One major feature of a μPot is its ability to mute the input
signal to an attenuation level of 104dB as shown in Figure 3.
This is accomplished internally by physically isolating the out-
put from the input while also grounding the output pin through
approximately 2 kΩ.
The mute function is obtained during power-up of the device
or by sending any binary data of 01111111 and above (to
11111111) serially to the device. The device may be placed
into mute from a previous attenuation setting by sending any
of the above data. This allows the designer to place a mute
button onto his system which could cause a microcontroller
to send the appropriate data to a μPot and thus mute any or
all channels. Since this function is achieved through software,
the designer has a great amount of flexibility in configuring the
system.
DC INPUTS
Although the μPot was designed to be used as an attenuator
for signals within the audio spectrum, the device is capable of
tracking an input DC voltage. The device will track DC volt-
ages to a diode drop above each supply rail.
One point to remember about DC tracking is that with a buffer
at the output of the μPot, the resolution of DC tracking will
depend upon the gain configuration of that output buffer and
its supply voltage. It should also be remembered that the out-
put buffer's supply voltage does not have to be the same as
the μPot's supply voltage. This could allow for more resolution
when DC tracking.
SERIAL DATA FORMAT
The LM1972 uses a 3-wire serial communication format that
is easily controlled by a microcontroller. The timing for the 3-
wire set, comprised of DATA-IN, CLOCK, and LOAD/SHIFT
is shown in Figure 2. Figure 9 exhibits in block diagram form
how the digital interface controls the tap switches which select
the appropriate attenuation level. As depicted in Figure 2, the
LOAD/SHIFT line is to go low at least 150 ns before the rising
edge of the first clock pulse and is to remain low throughout
the transmission of each set of 16 data bits. The serial data
is comprised of 8 bits for channel selection and 8 bits for at-
tenuation setting. For both address data and attenuation set-
ting data, the MSB is sent first and the 8 bits of address data
are to be sent before the 8 bits of attenuation data. Please
refer to Figure 7 to confirm the serial data format transfer pro-
cess.
μPot SYSTEM ARCHITECTURE
The μPot's digital interface is essentially a shift register, where
serial data is shifted in, latched, and then decoded. As new
data is shifted into the DATA-IN pin, the previously latched
data is shifted out the DATA-OUT pin. Once the data is shifted
in, the LOAD/SHIFT line goes high, latching in the new data.
The data is then decoded and the appropriate switch is acti-
vated to set the desired attenuation level for the selected
FIGURE 7. Serial Data Format Transfer Process
7
channel. This process is continued each and every time an
attenuation change is made. Each channel is updated, only,
when that channel is selected for an attenuator change or the
system is powered down and then back up again. When the
μPot is powered up, each channel is placed into the muted
mode.
MSB:
TABLE 1. LM1972 Micropot Attenuator
0000 0000
0000 0001
0000 0010
0000 0000
0000 0001
0000 0010
0000 0011
0001 1110
0001 1111
0010 0000
0010 0001
0010 0010
0101 1110
0101 1111
0110 0000
0110 0001
0110 0010
0111 1100
0111 1101
0111 1110
0111 1111
1000 0000
1111 1110
1111 1111
Contents
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Address Register (Byte 0)
Register Set Description
LSB
Data Register (Byte 1)
Attenuation Level dB
100.0 (Mute)
100.0 (Mute)
100.0 (Mute)
100.0 (Mute)
Channel 1
Channel 2
Channel 3
15.0
15.5
16.0
16.5
17.0
47.0
47.5
48.0
49.0
50.0
76.0
77.0
78.0
0.0
0.5
1.0
1.5
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