SAMHOP Microelectronics Corp.
3W STEREO AUDIO POWER AMPLIFIER WITH ADVANCED DC VOLUME CONTROL
After the channel gain is stepped down to the lowest gain, the amplifier begins discharging the bypass
capacitor from the nominal voltage of VDD/ 2 to ground. This time is dependent on the value of the bypass
capacitor. For a 0.47μF capacitor that is used in the application diagram in Figure 1, the time is approximately
500 ms. This time scales linearly with the value of bypass capacitor. For example, if a 1μF capacitor is used for
bypass, the time period to discharge the capacitor to ground is twice that of the 0.47μF capacitor, or 1 seconds.
Figure 3 below is a waveform captured at the output during the shutdown sequence when the part is in fade-on
mode. The gain is set to the highest level and the output is at VDD when the amplifier is shut down.
When a logic high is placed on the SHUTDOWN pin and the FADE pin is still held low, the device begins the
start-up process. The bypass capacitor will begin charging. Once the bypass voltage reaches the final value of
VDD/2, the gain increases in 2 dB steps from the lowest gain level to the gain level set by the dc voltage applied
to the VOLUME, SEDIFF and SEMAX pins.
In the fade-off mode, the amplifier stores the gain value prior to starting the shutdown sequences. The output of
the amplifier immediately drops to VDD/2 and the bypass capacitor begins a smooth discharge to ground. When
shutdown is released, the bypass capacitor charges up to VDD/2 and the channel gain returns immediately to
the value stored in memory. Figure 4 below is a waveform captured at the output during the shutdown
sequence when ths part is in the fade-off mode. The gain is set to the highest level, and the output is at VDD
when the amplifier is shut down.
The power-up sequence is different from the shutdown sequence and the voltage on the FADE pin does not
change the power-up sequence. Upon a power-up condition, the SM7511 begins in the lowest gain setting and
steps up 2 dB every 2 clock cycles until the final value is reached as determined by the dc voltage applied to
the VOLUME, SEDIFF and SEMAX pins.
Figure 5. Shutdown Sequence in the
Fade-off mode
Figure 4. Shutdown Sequence in the
Fade-on mode
VOLUME, SEDIFF AND SEMAX OPERATION
Three pins labeled VOLUME, SEDIFF and SEMAX control the BTL volume when driving speakers and the SE
volume when driving headphones. All of these pins are controlled with a dc voltage, which should not exceed
VDD.
When driving speakers in BTL mode, the VOLUME pin is the only pin that controls the gain. Table 1 shows the
gain for the BTL mode. The voltage listed in the table are for VDD=5V. For a different VDD, the values in the
table scale linearly. If VDD=4V, multiply all the voltages in the table by 4V/5V or 0.8.
V.1.3 Revised April 02,2007
Page 12
SAMHOP [ SAMHOP MICROELECTRONICS CORP. ]
