Rainbow Electronics MAX9796 - Manual (Page 18)

MAX9796
Current-Limit and Thermal Protection
The MAX9796 features current limiting and thermal pro-
tection to protect the device from short circuits and
overcurrent conditions. The headphone amplifier puls-
es in the event of an overcurrent condition with a pulse
every 100µs as long as the condition is present. Should
the current still be high, the above cycle is repeated.
The speaker amplifier current-limit protection clamps
the output current without shutting down the output.
This can result in a distorted output. Current is limited
to 1.6A in the speaker amplifiers and 170mA in the
headphone and mono receiver amplifiers.
The MAX9796 has thermal protection that disables the
device at +150°C until the temperate decreases to
+120°C.
Click-and-Pop Suppression
In conventional single-supply headphone amplifiers,
the output-coupling capacitor is a major contributor of
audible clicks and pops. Upon startup, the amplifier
charges the coupling capacitor to its bias voltage, typi-
cally half the supply. Likewise, during shutdown, the
capacitor is discharged to GND. This results in a DC
shift across the capacitor, which in turn, appears as an
audible transient at the speaker. Since the MAX9796
headphone amplifier does not require output-coupling
capacitors, this problem does not arise.
In most applications, the output of the preamplifier dri-
ving the MAX9796 has a DC bias of typically half the
supply. During startup, the input-coupling capacitor is
charged to the preamplifier’s DC bias voltage, resulting
in a DC shift across the capacitor and an audible click-
and-pop. An internal delay of 30ms eliminates the click-
and-pop caused by the input filter.
Shutdown
The MAX9796 features a 0.1µA hard shutdown mode
that reduces power consumption to extend battery life
and a soft shutdown where current consumption is typi-
cally 8.5µA. Hard shutdown is controlled by connecting
SHDN to GND, disabling the amplifiers, bias circuitry,
charge pump, and I
2 C. In shutdown, the headphone
amplifier output impedance is 1.4kΩ and the speaker
output impedance is 300kΩ. Similarly, the MAX9796
enters soft shutdown when the SHDN bit = 0 (see Table
2). The I
2 C interface is active and the contents of the
command register are not affected when in soft shut-
down. This allows the master to write to the MAX9796
while in shutdown. The I
2 C interface is completely dis-
abled in hardware shutdown. When the MAX9796 is re-
enabled the default settings are applied (see Table 3).
2.3W, High-Power Class D Audio Subsystem
with DirectDrive Headphone Amplifiers
18 ______________________________________________________________________________________
INPUT
MIXER
INPUT
PREAMPLIFIER
INPUT A:
0dB, 6dB, 20dB
-75dB TO 0dB
-75dB TO 0dB 0dB OR 6dB
MVOL MONO+6dB
LVOL
-75dB TO 0dB
RVOL
INPUT B AND C:
0dB, 6dB
OUTPUT
MIXER
3dB
3dB
12dB
SPEAKER
HEADPHONE
RECEIVER MONO
Figure 6. Signal Path