Mode Selection (MODE_)
SmartSense is controlled by the two mode select
inputs, MODE1 and MODE2. Table 1 shows the operat-
ing modes in relation to the status of the MODE_ inputs.
When MODE1 = MODE2 = low, the device is in low-
power shutdown mode. When MODE1 = high and
MODE2 = low, the device is in forced mono mode. The
right channel is disabled, OUTR goes high impedance,
and the stereo audio input is mixed, and the audio sig-
nal is reproduced on OUTL. SmartSense is disabled in
this mode. When MODE1 = low and MODE2 = high, the
device is in forced stereo mode, and SmartSense is
disabled. When the device detects the presence of a
short BEFORE forced stereo mode is selected, the
device remains in mono mode (Figure 5). When
MODE1 = MODE2 = high, the device is in automatic
detection mode; the operating mode of the device is
determined by SmartSense.
MODE1 is also used to execute a host-controlled
SmartSense routine and reset the
ALERT
output. On the
rising edge of MODE1, the device invokes a
SmartSense routine. The falling edge of MODE1 resets
the
ALERT
output to its idle state.
Automatic Detection Mode
A fault condition is defined as a short (under 4
) on
either amplifier output to ground. SmartSense automati-
cally detects and disables the shorted output. The
mixer/attenuator combines the two stereo inputs (INL
and INR), attenuates the resultant signal by a factor of
2, and redirects the audio playback to the remaining
active channel. This allows for full reproduction of a
stereo signal through a single headphone while main-
taining optimum headroom. The mixed mono signal is
output only on the properly loaded channel. If both out-
puts are shorted then both outputs go into a high-
impedance state and no audio playback occurs. In
automatic detection mode (MODE1 = MODE2 = high),
any of the following events trigger a SmartSense test
sequence:
HPS rises above 0.8 x V
DD,
indicating a headphone
jack has been inserted into the socket.
The 180mA high-side (sourcing) overcurrent thresh-
old is approached, and the output is near GND.
The die temperature exceeds the thermal limit
(+140
°
C).
Power or shutdown is cycled.
M
50mW, DirectDrive, Stereo Headphone
Amplifier with SmartSense and Shutdown
______________________________________________________________________________________
13
M1 = L
M2 = L
N
M1 = H
M2 = L
N
M1 = L
M2 = H
N
SHORT
DETECTED
N
STATUS
CHANGE
SHDN
SmartSense
FORCED MONO
FORCED STEREO
MONO MODE
STATUS
CHANGE
STATUS
CHANGE
STATUS
CHANGE
STEREO MODE
STATUS
CHANGE
N
N
N
N
N
Y
Y
Y
Y
Y
Y
Y
Y
Y
MAX9720
Figure 5. SmartSense Flow Diagram
MODE1
MODE2
SmartSense
OPERATING
MODE
High
High
Enabled
Automatic
detection mode
Low
High
Low
Low
Low
High
Disabled
Disabled
Disabled
Shutdown
Forced left mono
Forced stereo
High
Enabled
Host controlled
X
—
Reset
ALERT
Table 1. MAX9720 Operating Modes