MAX13301 Datasheet, PDF (29/37 Pages) Maxim Integrated Products – 4-Channel, Automotive Class D Audio Amplifier Feedback After the Filter

4-Channel, Automotive Class D Audio Amplifier
the status register only indicates which channel’s output
is shorted and not which of its differential outputs is short-
ed. The I²C status register can indicate, for example, that
output 1 is shorted to battery, but it cannot differentiate
between an OUT1+ and OUT1- short-to-battery.
Before running the short-to-ground/battery diagnostic,
perform steps 1 to 3 of the shutdown procedure out-
lined in the Startup and Shutdown section. Before set-
ting CTRL3.SDET to 1, discharge the output by set-
ting CTRL3.DIS to 1 for 200µs and reset CTRL3.DIS
to 0. Run the short-to-ground/battery diagnostic by
setting CTRL3.SDET (short-to-ground/battery diagnos-
tic enable bit) to 1. To test for short-to-ground, set
CTRL2.STBY to 0; to test short-to-battery, set
CTRL2.STBY to 1. With CTRL2.STBY = 0, 6V is developed
at each output. An output voltage > 6V is interpreted as
a short-to-battery. An output voltage < 150mV is inter-
preted as a short to ground. Results of the diagnostic
are reported in the OSTAT2.SBAT[3:0] (short-to-bat-
tery indicator) and OSTAT2.SGND[3:0] (short-to-ground
indicator) bits. Wait a minimum of 200Fs after setting
CTRL3.SDET for valid results. After running the short-to-
ground/battery diagnostic, clear the CTRL3.SDET bit to 0.
Because no latch is set, a short-to-ground or battery
does not prevent the device from powering up. Therefore,
the microcontroller can enable the device into a short
although it is discouraged. Should the device be enabled
into a short, the real-time overcurrent latches the shorted
channel off. The device offers real-time protection for
short-to-battery, short-to-ground, and shorted load to pre-
vent damage to the device.
Open-Load Diagnostic
This diagnostic detects an open between OUT_+ and
OUT_- of > 100I or > 300I, depending on the value of
CTRL3.LDM (line driver mode bit).
Before running the open-load diagnostic, perform steps
1 to 3 of the shutdown procedure outlined in the Startup
and Shutdown section. Run the open-load diagnos-
tic test by setting CTRL3.RDET (open-load diagnostic
enable bit) to 1, and in the same command, dis-
charge the output capacitors by setting CTRL3.DIS
(discharge bit) to 1. During the diagnostic, all low-side
FETs of the negative outputs (OUT_-) are turned on,
while all other FETs are turned off. The device sources
a 2mA current from OUT_+ to OUT_-. If a load is not
present, OUT_+ swings high and is interpreted as an
open output. Results of the diagnostic are reported in
OSTAT1.LDOK[3:0] (load OK indicator bit). Wait
a minimum of 200Fs for valid results after setting
CTRL3.RDET. After running the open-load diagnostic,
clear CTRL3.RDET and CTRL3.DIS to 0.
Shorted-Load Diagnostic
This diagnostic detects shorted loads on any of the
outputs. To detect shorted loads, the device should be
in play mode. Set CTRL3.TW (tweeter-detect current
threshold setting bit) to 0 and apply a low-frequency
(typically < 20Hz) sinusoidal signal to all the inputs.
The device compares the load current to the shorted-
load current threshold. If the load current exceeds the
threshold, the corresponding OSTAT1.LOAD[3:0] (load
indicator bit) is set to 1, indicating that there is a shorted
load. The shorted-load current threshold depends on the
programmed current limit as set by the CTRL3.HCL. See
the High-Current Threshold parameter in the Electrical
Characteristics table.
Note that the OSTAT1.LOAD[3:0] bits do not latch high
upon detecting a short. During zero crossings, the
load current does not exceed the threshold and the
OSTAT1.LOAD[3:0] bits are cleared to 0. There are two
ways to obtain the results of the shorted load diagnostic:
1) Continuously read the OSTAT1.LOAD[3:0] bits to
determine if any have been set high.
2) The open-drain CL0 output can also be monitored if
MAP.LCTM (low-current threshold map bit) is set to 1
(setting this bit to 1 maps the OSTAT1.LOAD[3:0] bits
to the CL0 output). Because CL0 is the NORed func-
tion of the OSTAT1.LOAD[3:0] bits, CL0 pulls low if a
short exists on any of the outputs.
After running the diagnostic, clear MAP.LCTM to 0 to
unmap OSTAT1.LOAD[3:0] to the CL0 output. Doing so
prevents CL0 from being asserted when the shorted-
load current threshold is exceeded during play.
Shorted-load diagnostic is done on all outputs. A shorted
load is traceable to the output on which it exists by
examining the OSTAT1.LOAD[3:0] bits. A shorted load
on output 3 causes LOAD3 to go low, a shorted load on
output 2 causes LOAD2 to go low, etc.
Missing Speaker Diagnostic
Using the same technique outlined above, it is possible
to detect the absence of a speaker. Set CTRL3.TW to 1
to decrease the shorted-load current threshold by a fac-
tor of 4. Apply a low-frequency sine wave (< 20Hz) to the
inputs and monitor the load indicator either through the
I²C registers or the CL0 output. By using the appropriate
input signal, a 2I, 4I, or 8I speaker can be detected.
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