LM4840 datasheet(10/19 Pages) NSC

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:

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

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

portable

notebook

computer

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

(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

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

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