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LM4867 Fiches technique(PDF) 17 Page - National Semiconductor (TI) |
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LM4867 Fiches technique(HTML) 17 Page - National Semiconductor (TI) |
17 / 28 page Application Information (Continued) headphones are connected to the jack shown in Figure 8, and internal pull−up forces V DD on the internal comparator’s HP−IN inputs. This changes the comparator’s output state and enables the headphone function: it turns off Amp2B, switches Amp2A’s input signal from an audio signal to the V DD/2 voltage present on pin 14, and mutes the bridge- connected loads. Amp1A and Amp1B drive the headphones. Figure 8 also shows the suggested headphone jack electri- cal connections. The jack is designed to mate with a three−wire plug. The plug’s tip and ring should each carry one of the two stereo output signals, whereas the sleeve provides the return to Amp2A. A headphone jack with one control pin contact is sufficient to drive the HP−IN pin when connecting headphones. A switch can replace the headphone jack contact pin. When a switch shorts the HP−IN pin to V DD, bridge−connected speakers are muted and Amp1A and Amp2A drive a pair of headphones. When a switch shorts the HP−IN pin to GND, the LM4867 operates in bridge mode. If headphone drive is not needed, short the HP−IN pin to the −OUTB pin. Figure 7 shows an optional resistor connected between the amplifier output that drives the headphone jack sleeve and ground. This resistor provides a ground path that supressed power supply hum. This hum may occur in applications such as notebook computers in a shutdown condition and con- nected to an external powered speaker. The resistor’s 100 Ω value is a suggested starting point. Its final value must be determined based on the tradeoff between the amount of noise suppression that may be needed and minimizing the additional current drawn by the resistor (25mA for a 100 Ω resistor and a 5V supply). ESD PROTECTION As stated in the Absolute Maximum Ratings, pin 28 on the MT and MH packages have a maximum ESD susceptibility rating of 8000V. For higher ESD voltages, the addition of a PCDN042 dual transil (from California Micro Devices), as shown in Figure 7, will provide additional protection. SINGLE-ENDED OUTPUT POWER PERFORMANCE AND MEASUREMENT CONSIDERATIONS The LM4867 delivers clean, low distortion SE output power into loads that are greater than 10 Ω. As an example, output power for 16 Ω and 32Ω loads are shown in the Typical Performance Characteristic curves. For loads less than 10 Ω, the LM4876 can typically supply 180mW of low distor- tion power. However, when higher dissipation is desired in loads less than 10 Ω, a dramatic increase in THD+N may occur. This is normal operation and does not indicate that proper functionality has ceased. When a jump from moder- ate to excessively high distortion is seen, simply reducing the output voltage swing will restore the clean, low distortion SE operation. The dramatic jump in distortion for loads less than 10 Ω occurs when current limiting circuitry activates. During SE operation, AMP2A (refer to Figure 4) drives the headphone sleeve. An on-board circuit monitors this amplifier’s output current. The sudden increase in THD+N is caused by the current limit circuitry forcing AMP2A into a high−impedance output mode. When this occurs, the output waveform has discontinuities that produce large amounts of distortion. It has been observed that as the output power is steadily increased, the distortion may jump from 5% to greater than 35%. Indeed, 10% THD+N may not actually be achievable. USING THE SINGLE−ENDED OUTPUT FOR LINE LEVEL APPLICATIONS Some samples of the LM4867 may exhibit small amplitude, high frequency oscillation when the SE output is connected to a line-level input. This oscillation can be eliminated by connecting a 5%, 300 Ω resistor between Amp2A’s output pin and each amplifier, AMP1A and AMP1B, output. 20001371 FIGURE 7. The PCDN042 provides additional ESD protection beyond the 8000V shown in the Absolute Maximum Ratings for the AMP2A output www.national.com 17 |
Numéro de pièce similaire - LM4867_02 |
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Description similaire - LM4867_02 |
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