LM4811
APPLICATION INFORMATION
Preliminary
CMOS IC
DIGITAL VOLUME CONTROL
The UTC
LM4811
is dual audio power amplifier with digital volume control and shutdown mode function. The
CLOCK pin is drived by an external clock. It’ s gain is controlled by the signals applied to the CLOCK and UP/DN
inputs.
The audio power amplifier’s gain is fixed by the signals, which are applied to the CLOCK and UP/DN inputs. The
gain is controlled by the logic voltage level applied to the UP/DN pin at each rising edge of the clock signal. At each
rising edge of the clock signal, when a logic high voltage level is applying to the UP/DN pin, 3dB is increased, in
verse when a logic low voltage is applying to the UP/DN pin, 3dB is decreased on the contrary. For both the CLOCK
and UP/DN inputs, the trigger point is 1.4V (MIN.) for a logic high level, and 0.4V (MAX.) for a logic low level.
The UTC
LM4811
contains16 discrete gain settings varying from +12dB (MAX.) to -33dB (MIN.), amplifier’s gain
is set to a default value of 0dB during power on condition. However, when coming out of shutdown mode, the UTC
LM4811
will revert back to its previous gain setting.
In order to avoid undesired state changes during transitions between V
IL
and V
IH
, the UTC
LM4811’s
CLOCK
and UP/DN pins should be debounced. This will ensure correct operation of the digital volume control. A
microcontroller or microprocessor output is expected to drive the CLOCK and UP/DN pins.
1
UP/DN
0
1
CLOCK
0
VOLUME
Default
Up
Up
Down
Down
Figure 2. Timing Diagram
POWER SUPPLY BYPASSING
To obtain low noise performance and high power supply rejection, proper supply bypassing is essentially
needed for any power amplifier. Capacitor on both the bypass and power supply pins should be placed closely as
possible. Bypass capacitor value is also critical as it directly affects the device’s half-supply voltage stability and
PSRR. The stability and supply rejection increase as the bypass capacitor’s value increases. In the typical
applications, employing a 5V regulator with 10μF and a 0.1μF bypass capacitors which aid in supply stability. This,
however, is not saving the need for bypassing the supply nodes of the UTC
LM4811.
The issue of selecting bypass
capacitors, especially C
B
, is thus dependent upon desired low frequency PSRR, click and pop performance, system
cost, and size constraints.
SHUTDOWN FUNCTION
The UTC
LM4811
contains a bias circuitry shutdown to reduce power consumption while not is use. By applying
a logic high to the SHUTDOWN pin, the shutdown function is active. For a logic high level, the trigger point is 1.4V
(MIN.), and 0.4V (MAX.) for a logic low. For a better shutdown performance, it is best to switch between ground and
V
DD
. By switching the SHUTDOWN pin to V
DD
, the UTC
LM4811
supply current draw will be minimized in idle mode.
Whereas the device will be disabled with shutdown voltages less than V
DD
, the idle current may be greater than
0.3μA (TYP.). In either case, the SHUTDOWN pin should be tied to a fixed voltage to avoid undesired state changes.
In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry. This
provides a quick, smooth shutdown transition. Another solution is to use a single-pole, single-throw switch in
conjunction with an external pull-up resistor. When the switch is closed, the SHUTDOWN pin is connected to ground
and enables the amplifier. If the switch is open, the external pull-up resistor, R
PU
, will disable the UTC
LM4811.
This
scheme guarantees that the SHUTDOWN pin will not float, thus preventing undesired state changes.
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
6 of 9
QW-R502-560.a
UTC-IC [ UNISONIC TECHNOLOGIES ]
