SSM2804CBZ-R7 datasheet(14/36 Pages) AD

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SSM2804

Rev. 0 | Page 14 of 36

HARDWARE-BASED HEADPHONE LIMITER

To provide fail-safe headphone level limiting independent of

the register values sent to the amplifier over the I2C bus, the

SSM2804 incorporates an optional hardware-based headphone

limiter feature. The user controls the limiter level by supplying

a voltage at the SD pin (see

). The hardware limiter is

activated by setting the LIM_MODE bit to 0 in the additional

control register (Bit D3 of Register 0x0E). After the desired

limiter value is set, the user can lock the limiter setting by

setting the LIMLOCK bit (Bit D7 of Register 0x0E).

Table 7

Table 7. Hardware Limiter Options

Limiter

Level

Power into

32 Ω (mW)

Power into

16 Ω (mW)

SD Pin Voltage

Shutdown

N/A

<0.87 V

±0.40 V

2.5

0.87 V < V

SD < 1.08 V

±8 V

1.08 V < V

SD < 1.29 V

±1.13 V

SD > 1.29 V

Note that after the hardware limiter lock bit is set, the locked

levels cannot be reset until the SSM2804 is powered down, the

SD pin is strobed low, or all eight bits of the software reset

In addition to the hardware-based limiter, several other limiter

levels can be selected using the I2C-based limiter function (set

the HPLIM bits of Register 0x0E; see Table 44). The effect of

the limiter function on the headphone output is shown in

Figure 29.

CH1 500mV BW

M20.0ms

A CH1

110mV

Figure 29. Limited Headphone Signal

ACTIVATING OR DEACTIVATING THE EMISSION

LIMITING CIRCUITRY

To activate or deactivate the emission limiting circuitry, change

the value of the EDGE bits in the additional control register

(Bits[D1:D0] of Register 0x0E). Four levels of emission control

are available, allowing the user to determine the best trade-off

between efficiency and EMI reduction.

In the default (fastest edge) mode, the user can pass FCC

Class-B emission testing with 10 cm twisted pair speaker wire

for loudspeaker connection. If longer speaker wire is desired,

change the EDGE setting to a slower edge rate mode.

The trade-off is slightly lower efficiency and noise performance.

The penalty for using the emission control circuitry is far less

than the decreased performance observed when using a ferrite

bead based EMI filter for emission limiting purposes.

AUTOMATIC LEVEL CONTROL (ALC)

Automatic level control (ALC) is a function that automatically

adjusts amplifier gain to generate the desired output amplitude

with reference to a particular input stimulus. The primary use for

the ALC is to protect an audio power amplifier or speaker load

from the damaging effects of clipping or current overloading.

This is accomplished by limiting the output amplitude of the

amplifier upon reaching a preset threshold voltage. Another

benefit of the ALC is that it makes sound sources with a wide

dynamic range more intelligible by boosting low level signals

and limiting very high level signals.

Before activating the ALC by setting the ALCEN bit (Bit D7

of Register 0x0B), the user has full control of the left and right

channel PGA gain. After the ALC is activated (ALCEN = 1),

the user has no control over the gain settings; the left channel

PGA gain is locked into the device and controls the gain for both

the left and right channels. To change the gain, the user must

reset the ALCEN bit to 0 and then load the new gain settings.

Figure 30 shows the response of the SSM2804 to a linearly

increasing input signal. When the output reaches the current

threshold value, the amplifier gain decreases by 0.5 dB so that

the output voltage remains under the threshold. As more atten-

uation is added to the system, the threshold increases according

to a profile determined by the compressor setting bits in the

ALC Control 2 register (Bits[D6:D5] of Register 0x0B), causing

a rounded “knee” as the output voltage approaches the output

limiter level. The effect of this compression curve is shown in

Figure 30.

5.6

5.2

4.8

4.4

4.0

3.6

3.2

2.8

2.4

1.6

1.2

2.0

0.8

0.4

100

120

140

160

180

200

TIME (ms)

INPUT

GAIN = 6dB

GAIN = 12dB

GAIN = 18dB

GAIN = 24dB

Figure 30. Output Response to Linearly Increasing Input Ramp Signal

When the input level is small and the output voltage is smaller

than the ALC threshold value, the gain of the amplifier stays at

the preset gain setting. When the input exceeds the ALC thresh-

old value, the ALC gradually reduces the gain from the preset

gain setting down to 1 dB.

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