SAMHOP Microelectronics Corp.
3W STEREO AUDIO POWER AMPLIFIER WITH ADVANCED DC VOLUME CONTROL
The SM7511 allows the user to specify a difference between BTL gain and SE gain. This is desirable to avoid
any listening discomfort when plugging in headphones. When switching to SE mode, the SEDIFF and SEMAX
pins control the single-ended gain proportional to the gain set by the voltage on the VOLUME pin. When
SEDIFF =0V, the difference between the BTL gain and the SE gain is 6dB. As the voltage on the SEDIFF
terminal is increased, the gain in SE mode decreases. The voltage on the SEDIFF terminal is subtracted from
the voltage on the VOLUME terminal and this value is used to determine the SE gain.
Some audio systems require that the gain be limited in the single-ended mode to a level that is comfortable for
headphone listening. Most volume control devices only have one terminal for setting the gain. For example, if
the speaker gain is 20 dB, the gain in the headphone channel is fixed at 14 dB. The level of gain could cause
discomfort to listeners and the SEMAX pin allows the designer to limit this discomfort when plugging in
headphones. The SEMAX terminal controls the maximum gain for single-ended mode.
The funtionality of the SEDIFF and SEMAX pin are combined to set the SE gain. A block diagram of the
combined funtionality is shown in Figure 6. The value obtained from the block diagram for SE_VOLUME is a dc
voltage that can be used in conjunction with Table 2 to determine the SE gain. Again, the voltage listed in the
table are for VDD=5V. The values must be scaled for other values of VDD.
Table 1 and Table 2 show a range of voltages for each gain step. There is a gap in the voltage between each
gain step. This gap represents the hysteresis about each trip point in the internal comparator. The hysteresis
ensures that the gain control is monotonic and does not oscillate from one gain step to another. If a
potentiometer is used to adjust the voltage on the control terminals, the gain increases as the potentiometer is
turned in one direction and decreases as it is turned back the other direction. The trip point, where the gain
actually changes, is different depending on whether the voltage is increased or decreased as a result of the
hysteresis about each trip point. The gaps in Table 1 and Table 2 can also be thought of as indeterminate
states where the gain could be in the next higher gain step or the lower gain step depending on the direction the
voltage is changing. If using a DAC to control the volume, set the volume, set the voltage in the middle of each
range to ensure that the desired gain is achieved.
A pictorial representation of the volume control can be found in Figure 7. The graph focuses on three gain steps
with the trip points defined in Table 1 for BTL gain. The dotted line represents the hysteresis about each gain
step.
SEDIFF(V)
SEMAX(V)
Is SEMAX>
(VOLUME-SEDIFF)?
VOLUME(V)
VOLUME-SEDIFF
YES
+
SE_VOLUME(V)=VOLUME(V)-SEDIFF(V)
SE_VOLUME(V)=SEMAX(V)
Figure 6. Block Diagram of SE Volume Control
Page 13
V.1.3 Revised April 02,2007
SAMHOP [ SAMHOP MICROELECTRONICS CORP. ]
