Introduction to Automatic Level Control

Maxim's automatic level control (ALC) offers two important advantages ( Figures 1 and 2 ).

1. Protect speakers by limiting amplifier output power.
2. It boosts low-level signals without distorting high-level signals.

　　ALC technology is used in the MAX9756, MAX9757, and MAX9758 2.3W stereo speaker amplifiers and DirectDrive headphone amplifiers.

Figures 1 and 2. The MAX9756's automatic level control (ALC) feature protects the speaker without causing distortion.

Comparing ALC and Output Limiting

　　ALC is different from the traditional output limiting function. The output limiting function limits the output signal swing to a predetermined amplitude to protect the electroacoustic components from being damaged by excessive peak values. As a result, the output signal is clipped (distorted) ( Figure 3 ). The ALC function protects the electroacoustic components by reducing the gain. No distortion is generated ( Figure 4 ).

Figure 3. An output limiter causes the output signal to clip during an overvoltage condition, which produces audible distortion.

Figure 4. The MAX9756's ALC reduces amplifier gain during an overvoltage condition without distorting the output signal.

 How does ALC work?

　　There are three important timing specifications in Maxim's ALC technology ( Figures 5 and 6 ): attack time, hold time, and release time.

Figure 5. Important timing indicators of the ALC function.

Figure 6. Graphical representation of three important timing metrics for the ALC function. Data from the MAX9756.

Attack Time
　　This is the time it takes to reduce the gain after the output signal has exceeded the threshold level. The time constant is determined by 15,000 XC _CT (seconds) , where C _CT is the external timing capacitor. The time constant is chosen depending on the application. For example, some applications require rapid reduction of the volume of transient sounds (such as snare drum hits, gunshots, etc.) (DVD), in which case a shorter attack time must be selected. In other applications with longer attack times, the ALC will ignore short peaks and only reduce the gain when the loudness increases significantly.

Hold Time
　　: The delay after the signal falls below the threshold level (before the release phase begins). The hold time is not adjustable; it is internally set to 50ms. The hold time is set to prevent gain 'breathing' (where the ALC action in response to low frequency components can be heard). The hold time is overridden when the signal exceeds the preset threshold and re-enters the attack phase.

Release Time
　　: The time required for the gain to return to its normal level after the signal falls below the threshold level and the hold time has elapsed. More specifically, the release time is defined as the time it takes for the gain to release from the 6dB compression point to within 10% of normal gain after the input signal falls below the PREF (pin 29) threshold and the 50ms hold time has elapsed. The release time is determined by the attack/release time ratio, which is selected by setting the logic state of DR (pin 25) ( Table 1 ). The release time is adjustable from 95ms to 10s.

Table 1. Attack/Release Ratio Set by the Logic State of the DR Pin

Output power threshold

　　An external resistor connected between PREF and ground sets the threshold that clamps the speaker output ( Figure 7 ). Equation 1 relates this resistor value to the maximum expected output power and the speaker impedance.

Figure 7. Output power threshold set by an external resistor.

Maxim Application Recommendations

　　For applications such as notebooks that use music CDs and DVDs as the primary audio source, the recommended attack time is 495ms, the release time is 990ms, and the output power threshold is 1.2W for an RL = 8 load ( Figure 8 ).

Figure 8. Recommended ALC values ​​for laptop applications.