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LM4840LQ bảng dữ liệu(PDF) 10 Page - National Semiconductor (TI) |
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LM4840LQ bảng dữ liệu(HTML) 10 Page - National Semiconductor (TI) |
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10 / 19 page  Application Information DIGITAL VOLUME CONTROL The LM4840 features a digital volume control which consists of the CLOCK, UP, and DOWN pins. An external clock may be fed to the CLOCK pin, or, by connecting a capacitor from the CLOCK pin to ground, the internal clock may be used. The internal clock frequency with respect to this capacitor value is determined from the following formula: f CLK = (7.338 x 10 -7 )/C When using an external clock, the clock is buffered and the internal clock frequency is that of the external clock divided by 2. Also, the maximum frequency should be kept below 100kHz. Volume changes are then effected by toggling either the UP or DOWN pins with a logic high. After a period of 4 clock pulses with either the UP or DOWN pins held high, the volume will change to the next specified step, either up or down. Volume levels for each step vary and are specified in Table 2. If either the UP or DOWN pin remains high after the first volume transition the volume will change again, but this time after 40 clock pulses. The next transition occurs at 20 clock pulses, then 12, then 8, and from then on 4 clock pulses for each volume transtition. This cycle is shown in the timing diagram shown in Figure 3. Releasing the held UP or DOWN pin to ground at any time re-starts the cycle. This is intended to provide the user with a volume control that pauses briefly after initial application, then slowly increases the rate of volume change as it is continuously applied. If both the UP and DOWN pins are held high, no volume change will occur. Trigger points for the UP and DOWN pins are at 60% of V DD minimum for a logic high, and 20% of VDD maximum for a logic low. It is recommended, however, to toggle UP and DOWN between V DD and GND for best performance. When using an external clock, clock pulses should be a minimum 0f 3V for a high and maximum of 0.9V for a low when using a 5V supply. Again, pulsing an external clock from V DD to GND ensures reliable performance. Fol- lowing these guidelines the volume may then be changed with a microcontroller or manually using switches. MEMORY FUNCTION The LM4840 features a volume memory that saves the last volume setting when power is turned off. This requires that an auxiliary power source be connected to V AUX through a diode as shown in Figure 1. Connecting the circuit as shown also provides that power to the V AUX pin is being drawn from V DD when VDD is on and is greater than VAUX.VAUX must be at a voltage of 2.3V or greater to maintain volume memory when V DD is absent. This feature is intended for such appli- cations as laptop computers, where V DD is the system power and V AUX is connected to the real time clock battery. The default volume setting for the LM4840 is -10dB in BTL mode, and -16dB in single-ended mode. This default setting is only achieved on power up when both V DD and VAUX had both been turned off, and the circuit had sufficient time to dis- charge (<500ms depending on capacitor value at V AUX). ELIMINATING OUTPUT COUPLING CAPACITORS Typical single-supply audio amplifiers that can switch be- tween driving bridge-tied-load (BTL) speakers and single-ended (SE) headphones use a coupling capacitor on each SE output. This capacitor blocks the half-supply volt- age to which the output amplifiers are typically biased and couples the audio signal to the headphones. The signal return to circuit ground is through the headphone jack’s sleeve. The LM4840 eliminates these coupling capacitors. Amp2A is internally configured to apply V DD/2 to a stereo headphone jack’s sleeve. This voltage matches the quiescent voltage present on the Amp1A and Amp1B outputs that drive the headphones. The headphones operate in a manner very similar to a bridge-tied-load (BTL). The same DC voltage is applied to both headphone speaker terminals. This results in no net DC current flow through the speaker. AC current flows through a headphone speaker as an audio signal’s output amplitude increases on the speaker’s terminal. When operating as a headphone amplifier, the headphone jack sleeve is not connected to circuit ground. Using the headphone output jack as a line-level output will place the LM4840’s one-half supply voltage on a plug’s sleeve con- nection. Driving a portable notebook computer or audio-visual display equipment is possible. This presents no difficulty when the external equipment uses capacitively coupled inputs. For the very small minority of equipment that is DC-coupled, the LM4840 monitors the current supplied by the amplifier that drives the headphone jack’s sleeve. If this current exceeds 500mA PK, the amplifier is shutdown, pro- tecting the LM4840 and the external equipment. For more information, see the section titled ’Single-Ended Output Power Performance and Measurement Considerations’. EXPOSED-DAP MOUNTING CONSIDERATIONS The LM4840’s exposed-DAP (die attach paddle) packages (MH, LQ) provide a low thermal resistance between the die and the PCB to which the part is mounted and soldered. This allows rapid heat transfer from the die to the surrounding PCB copper traces, ground plane and, finally, surrounding air. The result is a low voltage audio power amplifier that produces 2W at ≤ 1% THD with a 4Ω load. This high power is achieved through careful consideration of necessary ther- mal design. Failing to optimize thermal design may compro- mise the LM4840’s high power performance and activate unwanted, though necessary, thermal shutdown protection. The MH and LQ packages must have their exposed DAPs soldered to a grounded copper pad on the PCB. The DAP’s PCB copper pad is connected to a large plane of continuous unbroken copper. This plane forms a thermal mass and heat sink and radiation area. Place the heat sink area on either outside plane in the case of a two-sided PCB, or on an inner layer of a board with more than two layers. Connect the DAP copper pad to the inner layer or backside copper heat sink area with 32(4x8) (MH ) or 6(3x2) (LQ) vias. The via diam- eter should be 0.012in–0.013in with a 1.27mm pitch. Ensure efficient thermal conductivity by plating-through and solder- filling the vias. Best thermal performance is achieved with the largest prac- tical copper heat sink area. If the heatsink and amplifier share the same PCB layer, a nominal 2.5in2 (min) area is necessary for 5V operation with a 4 Ω load. Heatsink areas not placed on the same PCB layer as the should be 5in 2 (min) for the same supply voltage and load resistance. The last two area recommendations apply for 25˚C ambient tem- perature. Increase the area to compensate for ambient tem- peratures above 25˚C. In systems using cooling fans, the LM4840MH can take advantage of forced air cooling. With an air flow rate of 450 linear-feet per minute and a 2.5in 2 exposed copper or 5.0in 2 inner layer copper plane heatsink, the LM4840MH can continuously drive a 3 Ω load to full power. The LM4840LQ achieves the same output power www.national.com 10 |
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